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Free Pascal : Reference guide. Reference guide for Free Pascal, version 0.99.12 1.6 July 1999 Micha¿el Van Canneyt Contents I The Pascal language 10 1 Pascal Tokens 11 1.1 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.2 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.3 Reserved words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Turbo Pascal reserved words . . . . . . . . . . . . . . . . . . . . . . 12 Delphi reserved words . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Free Pascal reserved words . . . . . . . . . . . . . . . . . . . . . . . . 13 Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.4 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.6 Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.7 Character strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2 Constants 16 2.1 Ordinary constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2 Typed constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3 Types 18 3.1 Base types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Ordinal types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Real types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2 Character types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Short strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Ansistrings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Constant strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 PChar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.3 Structured Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 1 CONTENTS Record types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Set types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 File types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.4 Pointers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.5 Procedural types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4 Objects 37 4.1 Declaration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.2 Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.3 Constructors and destructors . . . . . . . . . . . . . . . . . . . . . . 39 4.4 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.5 Method invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.6 Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5 Classes 45 5.1 Class definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 5.2 Class instantiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Virtual methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Message methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5.4 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 6 Expressions 53 6.1 Expression syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.2 Function calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 6.3 Set constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 6.4 Value typecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 6.5 The @ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6.6 Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Arithmetic operators . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Logical operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Boolean operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 String operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Set operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Relational operators . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 7 Statements 62 7.1 Simple statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Procedure statements . . . . . . . . . . . . . . . . . . . . . . . . . . 63 2 CONTENTS Goto statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 7.2 Structured statements . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Compound statements . . . . . . . . . . . . . . . . . . . . . . . . . . 65 The Case statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 The If..then..else statement . . . . . . . . . . . . . . . . . . . . . 67 The For..to/downto..do statement . . . . . . . . . . . . . . . . . . 68 The Repeat..until statement . . . . . . . . . . . . . . . . . . . . . 69 The While..do statement . . . . . . . . . . . . . . . . . . . . . . . . 69 The With statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Exception Statements . . . . . . . . . . . . . . . . . . . . . . . . . . 71 7.3 Assembler statements . . . . . . . . . . . . . . . . . . . . . . . . . . 71 8 Using functions and procedures 73 8.1 Procedure declaration . . . . . . . . . . . . . . . . . . . . . . . . . . 73 8.2 Function declaration . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 8.3 Parameter lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Value parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Variable parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Constant parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Open array parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 76 8.4 Function overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 8.5 Forward defined functions . . . . . . . . . . . . . . . . . . . . . . . . 77 8.6 External functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 8.7 Assembler functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 8.8 Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Public . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 cdecl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 popstack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Export . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 StdCall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Alias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 8.9 Unsupported Turbo Pascal modifiers . . . . . . . . . . . . . . . . . . 82 9 Programs, units, blocks 83 9.1 Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 9.2 Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 9.3 Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 9.4 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Block scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Record scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Class scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3 CONTENTS Unit scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 9.5 Libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 10 Exceptions 90 10.1 The raise statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 10.2 The try...except statement . . . . . . . . . . . . . . . . . . . . . . . . 91 10.3 The try...finally statement . . . . . . . . . . . . . . . . . . . . . . . . 92 10.4 Exception handling nesting . . . . . . . . . . . . . . . . . . . . . . . 93 10.5 Exception classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 11 Using assembler 94 11.1 Assembler statements . . . . . . . . . . . . . . . . . . . . . . . . . . 94 11.2 Assembler procedures and functions . . . . . . . . . . . . . . . . . . 94 II Reference : The System unit 96 12 The system unit 97 12.1 Types, Constants and Variables . . . . . . . . . . . . . . . . . . . . . 97 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 12.2 Functions and Procedures . . . . . . . . . . . . . . . . . . . . . . . . 99 Abs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Addr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Append . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Arctan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Assign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Assigned . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 BinStr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Blockread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Blockwrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Chdir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Chr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Close . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Concat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Continue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Cos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 CSeg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Dec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 4 CONTENTS Delete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Dispose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 DSeg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Eof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Eoln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Erase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Exp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Filepos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Filesize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Fillchar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Fillword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Flush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Frac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Freemem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Getdir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Getmem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Halt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 HexStr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Hi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 High . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Inc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Int . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 IOresult . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Ln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Lo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 LongJmp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Low . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Lowercase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Mark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Maxavail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Memavail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Mkdir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Move . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 New . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Odd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Ofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Ord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 5 CONTENTS Paramcount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Paramstr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Pi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Pos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Pred . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Ptr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Random . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Randomize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Readln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Rename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Rewrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Rmdir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Round . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Runerror . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Seek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 SeekEof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 SeekEoln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Seg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 SetJmp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 SetLength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 SetTextBuf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Sin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 SizeOf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 Sptr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 Sqr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Sqrt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 SSeg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Str . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Succ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Swap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Trunc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Truncate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Upcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Val . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Write . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 WriteLn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 6 List of Tables 3.1 Predefined ordinal types . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2 Predefined integer types . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.3 Boolean types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.4 Supported Real types . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.5 AnsiString memory structure . . . . . . . . . . . . . . . . . . . . . . 24 3.6 PChar pointer arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.7 Set Manipulation operators . . . . . . . . . . . . . . . . . . . . . . . 32 6.1 Precedence of operators . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.2 Binary arithmetic operators . . . . . . . . . . . . . . . . . . . . . . . 59 6.3 Unary arithmetic operators . . . . . . . . . . . . . . . . . . . . . . . 59 6.4 Logical operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 6.5 Boolean operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6.6 Set operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 6.7 Relational operators . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 7.1 Allowed C constructs in Free Pascal . . . . . . . . . . . . . . . . . . 63 8.1 Unsupported modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 82 7 LIST OF TABLES About this guide This document describes all constants, types, variables, functions and procedures as they are declared in the system unit. Furthermore, it describes all pascal constructs supported by Free Pascal, and lists all supported data types. It does not, however, give a detailed explanation of the pascal language. The aim is to list which Pascal constructs are supported, and to show where the Free Pascal implementation di ers from the Turbo Pascal implementation. Notations Throughout this document, we will refer to functions, types and variables with typewriter font. Functions and procedures have their own subsections, and for each function or procedure we have the following topics: Declaration The exact declaration of the function. Description What does the procedure exactly do ? Errors What errors can occur. See Also Cross references to other related functions/commands. The cross-references come in two flavours: * References to other functions in this manual. In the printed copy, a number will appear after this reference. It refers to the page where this function is explained. In the on-line help pages, this is a hyperlink, on which you can click to jump to the declaration. * References to Unix manual pages. (For linux related things only) they are printed in typewriter font, and the number after it is the Unix manual section. Syntax diagrams All elements of the pascal language are explained in syntax diagrams. Syntax diagrams are like flow charts. Reading a syntax diagram means that you must get from the left side to the right side, following the arrows. When you are at the right of a syntax diagram, and it ends with a single arrow, this means the syntax diagram is continued on the next line. If the line ends on 2 arrows pointing to each other, then the diagram is ended. Syntactical elements are written like this - - syntactical elements are like this - Keywords you must type exactly as in the diagram: - - keywords are like this - When you can repeat something there is an arrow around it: - - this can be repeated - 6 When there are di erent possibilities, they are listed in columns: 8 LIST OF TABLES - - First possibility - Second possibility Note, that one of the possibilities can be empty: - - - First possibility Second possibility This means that both the first or second possibility are optional. Of course, all these elements can be combined and nested. 9 Part I The Pascal language 10 Chapter 1 Pascal Tokens In this chapter we describe all the pascal reserved words, as well as the various ways to denote strings, numbers, identifiers etc. 1.1 Symbols Free Pascal allows all characters, digits and some special ASCII symbols in a Pascal source file. Recognised symbols - - letter A...Z - a...z - - digit 0...9 - - - hex digit 0...9 - A...F a...f The following characters have a special meaning: + - * / = < > [ ] . , ( ) : ^ @ { } $ # and the following character pairs too: <= >= := += -= *= /= (* *) (. .) // When used in a range specifier, the character pair (. is equivalent to the left square bracket [. Likewise, the character pair .) is equivalent to the right square bracket ]. When used for comment delimiters, the character pair (* is equivalent to the left brace { and the character pair *) is equivalent to the right brace }. These character pairs retain their normal meaning in string expressions. 11 1.2. COMMENTS 1.2 Comments Free Pascal supports the use of nested comments. The following constructs are valid comments: (* This is an old style comment *) { This is a Turbo Pascal comment } // This is a Delphi comment. All is ignored till the end of the line. The following are valid ways of nesting comments: { Comment 1 (* comment 2 *) } (* Comment 1 { comment 2 } *) { comment 1 // Comment 2 } (* comment 1 // Comment 2 *) // comment 1 (* comment 2 *) // comment 1 { comment 2 } The last two comments must be on one line. The following two will give errors: // Valid comment { No longer valid comment !! } and // Valid comment (* No longer valid comment !! *) The compiler will react with a 'invalid character' error when it encounters such constructs, regardless of the -So switch. 1.3 Reserved words Reserved words are part of the Pascal language, and cannot be redefined. They will be denoted as this throughout the syntax diagrams. Reserved words can be typed regardless of case, i.e. Pascal is case insensitive. We make a distinction between Turbo Pascal and Delphi reserved words, since with the -So switch, only the Turbo Pascal reserved words are recognised, and the Delphi ones can be redefined. By default, Free Pascal recognises the Delphi reserved words. Turbo Pascal reserved words The following keywords exist in Turbo Pascal mode absolute const else implementation and constructor end in array continue file inherited asm destructor for inline begin div function interface break do goto label case downto if mod 12 1.4. IDENTIFIERS nil packed shl until not procedure shr uses object program string var of record then while on repeat to with operator self type xor or set unit Delphi reserved words The Delphi (II) reserved words are the same as the pascal ones, plus the following ones: as finalization library try class finally on except initialization property exports is raise Free Pascal reserved words On top of the Turbo Pascal and Delphi reserved words, Free Pascal also considers the following as reserved words: dispose false true exit new Modifiers The following is a list of all modifiers. Contrary to Delphi, Free Pascal doesn't allow you to redefine these modifiers. absolute external pascal register abstract far popstack stdcall alias forward private virtual assembler index protected write cdecl name public default near published export override read Remark that predefined types such as Byte, Boolean and constants such as maxint are not reserved words. They are identifiers, declared in the system unit. This means that you can redefine these types. You are, however, not encouraged to do this, as it will cause a lot of confusion. 1.4 Identifiers Identifiers denote constants, types, variables, procedures and functions, units, and programs. All names of things that you define are identifiers. An identifier con- sists of 255 significant characters (letters, digits and the underscore character), from which the first must be an alphanumeric character, or an underscore ( ) The fol- lowing diagram gives the basic syntax for identifiers. 13 1.5. NUMBERS Identifiers - - identifier letter - 6 letter digit 1.5 Numbers Numbers are denoted in decimal notation. Real (or decimal) numbers are writ- ten using engeneering notation (e.g. 0.314E1). Free Pascal supports hexadecimal format the same way as Turbo Pascal does. To specify a constant value in hexadec- imal format, prepend it with a dollar sign ($). Thus, the hexadecimal $FF equals 255 decimal. In addition to the support for hexadecimal notation, Free Pascal also supports binary notation. You can specify a binary number by preceding it with a percent sign (%). Thus, 255 can be specified in binary notation as %11111111. The following diagrams show the syntax for numbers. Numbers - - hex digit sequence hex digit - 6 - - bin digit sequence 1 - 6 0 - - digit sequence digit - 6 - - unsigned integer digit sequence - $ hex digit sequence % bin digit sequence - - sign + - - - - unsigned real digit sequence - . digit sequence scale factor - - scale factor E digit sequence - e sign - - unsigned number unsigned real - unsigned integer - - signed number unsigned number - sign 14 1.6. LABELS 1.6 Labels Labels can be digit sequences or identifiers. Label - - label digit sequence - identifier 1.7 Character strings A character string (or string for short) is a sequence of zero or more characters from the ASCII character set, enclosed by single quotes, and on 1 line of the program source. A character set with nothing between the quotes ('') is an empty string. Character strings - - character string quoted string - 6 control string - - quoted string ' string character ' - 6 - - string character Any character except ' or CR - " - - control string # unsigned integer - 6 15 Chapter 2 Constants Just as in Turbo Pascal, Free Pascal supports both normal and typed constants. 2.1 Ordinary constants Ordinary constants declarations are not di erent from the Turbo Pascal or Delphi implementation. Constant declaration - - constant declaration identifier = expression ; - 6 The compiler must be able to evaluate the expression in a constant declaration at compile time. This means that most of the functions in the Run-Time library can- not be used in a constant declaration. Operators such as +, -, *, /, not, and, or, div(), mod(), ord(), chr(), sizeof can be used, however. For more in- formation on expressions, see chapter 6, page 53. You can only declare constants of the following types: Ordinal types, Real types, Char, and String. The following are all valid constant declarations: Const e = 2.7182818; { Real type constant. } a = 2; { Ordinal (Integer) type constant. } c = '4'; { Character type constant. } s = 'This is a constant string'; {String type constant.} s = chr(32) ls = SizeOf(Longint); Assigning a value to an ordinary constant is not permitted. Thus, given the previous declaration, the following will result in a compiler error: s := 'some other string'; 16 2.2. TYPED CONSTANTS 2.2 Typed constants Typed constants serve to provide a program with initialised variables. Contrary to ordinary constants, they may be assigned to at run-time. The di erence with normal variables is that their value is initialised when the program starts, whereas normal variables must be initialised explicitly. Typed constant declaration - - typed constant declaration identifier : type = typed constant ; - 6 - - - - typed constant constant - address constant array constant record constant procedural constant Given the declaration: Const S : String = 'This is a typed constant string'; The following is a valid assignment: S := 'Result : '+Func; Where Func is a function that returns a String. Typed constants also allow you to initialize arrays and records. For arrays, the initial elements must be specified, surrounded by round brackets, and separated by commas. The number of elements must be exactly the same as the number of elements in the declaration of the type. As an example: Const tt : array [1..3] of string[20] = ('ikke', 'gij', 'hij'); ti : array [1..3] of Longint = (1,2,3); For constant records, you should specify each element of the record, in the form Field : Value, separated by commas, and surrounded by round brackets. As an example: Type Point = record X,Y : Real end; Const Origin : Point = (X:0.0 , Y:0.0); The order of the fields in a constant record needs to be the same as in the type declaration, otherwise you'll get a compile-time error. 17 Chapter 3 Types All variables have a type. Free Pascal supports the same basic types as Turbo Pascal, with some extra types from Delphi. You can declare your own types, which is in essence defining an identifier that can be used to denote your custom type when declaring variables further in the source code. Type declaration - - type declaration identifier = type ; - There are 7 major type classes : Types - - type simple type - string type structured type pointer type procedural type type identifier The last class, type identifier, is just a means to give another name to a type. This gives you a way to make types platform independent, by only using your own types, and then defining these types for each platform individually. The programmer that uses your units doesn't have to worry about type size and so on. It also allows you to use shortcut names for fully qualified type names. You can e.g. define system.longint as Olongint and then redefine longint. 3.1 Base types The base or simple types of Free Pascal are the Delphi types. We will discuss each separate. 18 3.1. BASE TYPES Table 3.1: Predefined ordinal types Name Integer Shortint SmallInt Longint Byte Word Cardinal Boolean ByteBool LongBool Char Simple types - - simple type ordinal type - real type - - real type real type identifier - Ordinal types With the exception of Real types, all base types are ordinal types. Ordinal types have the following characteristics: 1. Ordinal types are countable and ordered, i.e. it is, in principle, possible to start counting them one bye one, in a specified order. This property allows the operation of functions as Inc (120), Ord (128), Dec (107) on ordinal types to be defined. 2. Ordinal values have a smallest possible value. Trying to apply the Pred (131) function on the smallest possible value will generate a range check error if range checking is enabled. 3. Ordinal values have a largest possible value. Trying to apply the Succ (144) function on the largest possible value will generate a range check error if range checking is enabled. Integers A list of pre-defined ordinal types is presented in table (3.1) The integer types, and their ranges and sizes, that are predefined in Free Pascal are listed in table (3.2). Free Pascal does automatic type conversion in expressions where di erent kinds of integer types are used. 19 3.1. BASE TYPES Table 3.2: Predefined integer types Type Range Size in bytes Byte 0 .. 255 1 Shortint -127 .. 127 1 Integer -32768 .. 32767 21 Word 0 .. 65535 2 Longint -2147483648 .. 2147483648 4 Cardinal 0..4294967296 4 Table 3.3: Boolean types Name Size Ord(True) Boolean 1 1 ByteBool 1 Any nonzero value WordBool 2 Any nonzero value LongBool 4 Any nonzero value Boolean types Free Pascal supports the Boolean type, with its two pre-defined possible values True and False. It also supports the ByteBool, WordBool and LongBool types. These are the only two values that can be assigned to a Boolean type. Of course, any expression that resolves to a boolean value, can also be assigned to a boolean type. Assuming B to be of type Boolean, the following are valid assignments: B := True; B := False; B := 1<>2; { Results in B := True } Boolean expressions are also used in conditions. Remark: In Free Pascal, boolean expressions are always evaluated in such a way that when the result is known, the rest of the expression will no longer be evaluated (Called short-cut evaluation). In the following example, the function Func will never be called, which may have strange side-e ects. ... B := False; A := B and Func; Here Func is a function which returns a Boolean type. Remark: The WordBool, LongBool and ByteBool types were not supported by Free Pascal until version 0.99.6. Enumeration types Enumeration types are supported in Free Pascal. On top of the Turbo Pascal implementation, Free Pascal allows also a C-style extension of the enumeration type, where a value is assigned to a particular element of the enumeration list. 20 3.1. BASE TYPES Enumerated types - - enumerated type ( identifier list ) - 6 assigned enum list , - - identifier list identifier - 6 , - - assigned enum list identifier := expression - 6 , (see chapter 6, page 53 for how to use expressions) When using assigned enumerated types, the assigned elements must be in ascending numerical order in the list, or the compiler will complain. The expressions used in assigned enumerated elements must be known at compile time. So the following is a correct enumerated type declaration: Type Direction = ( North, East, South, West ); The C style enumeration type looks as follows: Type EnumType = (one, two, three, forty := 40,fortyone); As a result, the ordinal number of forty is 40, and not 3, as it would be when the ':= 40' wasn't present. The ordinal value of fortyone is then 41, and not 4, as it would be when the assignment wasn't present. After an assignment in an enumerated definition the compiler adds 1 to the assigned value to assign to the next enumerated value. When specifying such an enumeration type, it is important to keep in mind that you should keep the enumerated elements in ascending order. The following will produce a compiler error: Type EnumType = (one, two, three, forty := 40, thirty := 30); It is necessary to keep forty and thirty in the correct order. When using enumer- ation types it is important to keep the following points in mind: 1. You cannot use the Pred and Succ functions on this kind of enumeration types. If you try to do that, you'll get a compiler error. 2. Enumeration types are by default stored in 4 bytes. You can change this behaviour with the {$PACKENUM n} compiler directive, which tells the compiler the minimal number of bytes to be used for enumeration types. For instance Type LargeEnum = ( BigOne, BigTwo, BigThree ); {$PACKENUM 1} SmallEnum = ( one, two, three ); Var S : SmallEnum; 21 3.1. BASE TYPES L : LargeEnum; begin WriteLn ('Small enum : ',SizeOf(S)); WriteLn ('Large enum : ',SizeOf(L)); end. will, when run, print the following: Small enum : 1 Large enum : 4 More information can be found in the Programmers' guide, in the compiler directives section. Subrange types A subrange type is a range of values from an ordinal type (the host type). To define a subrange type, one must specify it's limiting values: the highest and lowest value of the type. Subrange types - - subrange type constant .. constant - Some of the predefined integer types are defined as subrange types: Type Longint = $80000000..$7fffffff; Integer = -32768..32767; shortint = -128..127; byte = 0..255; Word = 0..65535; But you can also define subrange types of enumeration types: Type Days = (monday,tuesday,wednesday,thursday,friday, saturday,sunday); WorkDays = monday .. friday; WeekEnd = Saturday .. Sunday; Real types Free Pascal uses the math coprocessor (or an emulation) for all its floating-point calculations. The Real native type is processor dependant, but it is either Single or Double. Only the IEEE floating point types are supported, and these depend on the target processor and emulation options. The true Turbo Pascal compatible types are listed in table (3.4). Until version 0.9.1 of the compiler, all the Real types were mapped to type Double, meaning that they all have size 8. The SizeOf (142) function is your friend here. The Real type of turbo pascal is automatically mapped to Double. The Comp type is, in e ect, a 64-bit integer. 22 3.2. CHARACTER TYPES Table 3.4: Supported Real types Type Range Significant digits Size2 Single 1.5E-45 .. 3.4E38 7-8 4 Real 5.0E-324 .. 1.7E308 15-16 8 Double 5.0E-324 .. 1.7E308 15-16 8 Extended 1.9E-4951 .. 1.1E4932 19-20 10 Comp -2E64+1 .. 2E63-1 19-20 8 3.2 Character types Char Free Pascal supports the type Char. A Char is exactly 1 byte in size, and contains one character. You can specify a character constant by enclosing the character in single quotes, as follows : 'a' or 'A' are both character constants. You can also specify a character by their ASCII value, by preceding the ASCII value with the number symbol (#). For example specifying #65 would be the same as 'A'. Also, the caret character (^) can be used in combination with a letter to specify a character with ASCII value less than 27. Thus ^G equals #7 (G is the seventh letter in the alphabet.) If you want to represent the single quote character, type it two times successively, thus '''' represents the single quote character. Strings Free Pascal supports the String type as it is defined in Turbo Pascal and it supports ansistrings as in Delphi. To declare a variable as a string, use the following type specification: ShortString - - string type string - [ unsigned integer ] The meaning of a string declaration statement is interpreted di erently depending on the {$H} switch. The above declaration can declare an ansistrng or a short string. Whatever the actual type, ansistrings and short strings can be used interchangeably. The compiler always takes care of the necessary type coversions. Note, however, that the result of an expression that contains ansistrings and short strings will always be an ansistring. Short strings A string declaration declares a short string in the following cases: 1. If the switch is o : {$H-}, the string declaration will always be a short string declaration. 23 3.2. CHARACTER TYPES Table 3.5: AnsiString memory structure O set Contains -12 Longint with maximum string size. -8 Longint with actual string size. -4 Longint with reference count. 0 Actual string, null-terminated. 2. If the switch is on {$H+}, and there is a length specifier, the declaration is a short string declaration. The predefined type ShortString is defined as a string of length 255: ShortString = String[255]; For short strings Free Pascal reserves Size+1 bytes for the string S, and in the zeroeth element of the string (S[0]) it will store the length of the variable. If you don't specify the size of the string, 255 is taken as a default. For example in {$H-} TypeNameString = String[10]; StreetString = String; NameString can contain maximum 10 characters. While StreetString can contain 255 characters. The sizes of these variables are, respectively, 11 and 256 bytes. Ansistrings If the {$H} switch is on, then a string definition that doesn't contain a length specifier, will be regarded as an ansistring. Ansistrings are strings that have no length limit. They are reference counted. Internally, an ansistring is treated as a pointer. If the string is empty (''), then the pointer is nil. If the string is not empty, then the pointer points to a structure in heap memory that looks as in table (3.5). Because of this structure, it is possible to typecast an ansistring to a pchar. If the string is empty (so the pointer is nil) then the compiler makes sure that the typecasted pchar will point to a null byte. AnsiStrings can be unlimited in length. Since the length is stored, the length of an ansistring is available immediatly, providing for fast access. Assigning one ansistring to another doesn't involve moving the actual string. A statement S2:=S1; results in the reference count of S2 being decreased by one, The referece count of S1 is increased by one, and finally S1 (as a pointer) is copied to S2. This is a significant speed-up in your code. 24 3.2. CHARACTER TYPES If a reference count reaches zero, then the memory occupied by the string is deal- located automatically, so no memory leaks arise. When an ansistring is declared, the Free Pascal compiler initially allocates just memory for a pointer, not more. This pinter is guaranteed to be nil, meaning that the string is initially empty. This is true for local, global or part of a structure (arrays, records or objects). This does introduce an overhead. For instance, declaring VarA : Array[1..100000] of string; Will copy 1000000 times nil into A. When A goes out of scope, then the 100000 strings will be dereferenced one by one. All this happens invisibly for the program- mer, but when considering performance issues, this is important. Memory will be allocated only when the string is assigned a value. If the string goes out of scope, then it is automatically dereferenced. If you assign a value to a character of a string that has a reference count greater than 1, such as in the following statements: S:=T; { reference count for S and T is now 2 } S[I]:='@'; then a copy of the string is created before the assignment. This is known as copy- on-write semantics. It is impossible to access the length of an ansistring by referring to the zeroeth char- acter. The following statement will generate a compiler error if S is an ansistring: Len:=S[0]; Instead, you must use the Length (122) function to get the length of a string. To set the length of an ansistring, you can use the SetLength (140) function. Con- stant ansistrings have a reference count of -1 and are treated specially. Ansistrings are converted to short strings by the compiler if needed, this means that you can mix the use of ansistrings and short strings without problems. You can typecast ansistrings to PChar or Pointer types: Var P : Pointer; PC : PChar; S : AnsiString; begin S :='This is an ansistring'; PC:=Pchar(S); P :=Pointer(S); There is a di erence between the two typecasts. If you typecast an empty ansistring to a pointer, the pointer wil be Nil. If you typecast an empty ansistring to a PChar, then the result will be a pointer to a zero byte (an empty string). The result of such a typecast must be used with care. In general, it is best to consider the result of such a typecast as read-only, i.e. suitable for passing to a procedure that needs a constant pchar argument. It is therefore NOT advisable to typecast one of the following: 25 3.2. CHARACTER TYPES 1. expressions. 2. strings that have reference count larger than 0. (call uniquestring if you want to ensure a string has reference count 1) Constant strings To specify a constant string, you enclose the string in single-quotes, just as a Char type, only now you can have more than one character. Given that S is of type String, the following are valid assignments: S := 'This is a string.'; S := 'One'+', Two'+', Three'; S := 'This isn''t difficult !'; S := 'This is a weird character : '#145' !'; As you can see, the single quote character is represented by 2 single-quote characters next to each other. Strange characters can be specified by their ASCII value. The example shows also that you can add two strings. The resulting string is just the concatenation of the first with the second string, without spaces in between them. Strings can not be substracted, however. Whether the constant string is stored as an ansistring or a short string depends on the settings of the {$H} switch. PChar Free Pascal supports the Delphi implementation of the PChar type. PChar is defined as a pointer to a Char type, but allows additional operations. The PChar type can be understood best as the Pascal equivalent of a C-style null-terminated string, i.e. a variable of type PChar is a pointer that points to an array of type Char, which is ended by a null-character (#0). Free Pascal supports initializing of PChar typed constants, or a direct assignment. For example, the following pieces of code are equivalent: program one; var p : PChar; begin P := 'This is a null-terminated string.'; WriteLn (P); end. Results in the same as program two; const P : PChar = 'This is a null-terminated string.' begin WriteLn (P); end. These examples also show that it is possible to write the contents of the string to a file of type Text. The strings unit contains procedures and functions that manipulate the PChar type as you can do it in C. Since it is equivalent to a pointer to a type Char variable, it is also possible to do the following: 26 3.3. STRUCTURED TYPES Table 3.6: PChar pointer arithmetic Operation Result P + I Adds I to the address pointed to by P. I + P Adds I to the address pointed to by P. P - I Substracts I from the address pointed to by P. P - Q Returns, as an integer, the distance between 2 addresses (or the number of characters between P and Q) Program three; Var S : String[30]; P : PChar; begin S := 'This is a null-terminated string.'#0; P := @S[1]; WriteLn (P); end. This will have the same result as the previous two examples. You cannot add null-terminated strings as you can do with normal Pascal strings. If you want to concatenate two PChar strings, you will need to use the unit strings. However, it is possible to do some pointer arithmetic. You can use the operators + and - to do operations on PChar pointers. In table (3.6), P and Q are of type PChar, and I is of type Longint. 3.3 Structured Types A structured type is a type that can hold multiple values in one variable. Stuctured types can be nested to unlimited levels. Structured Types - - structured type array type - record type class type class reference type set type file type Unlike Delphi, Free Pascal does not support the keyword Packed for all structured types, as can be seen in the syntax diagram. It will be mentioned when a type supports the packed keyword. In the following, each of the possible structured types is discussed. Arrays Free Pascal supports arrays as in Turbo Pascal, multi-dimensional arrays and packed arrays are also supported: 27 3.3. STRUCTURED TYPES Array types - - array type array [ ordinal type ] of type - packed 6 , The following is a valid array declaration: Type RealArray = Array [1..100] of Real; As in Turbo Pascal, if the array component type is in itself an array, it is possible to combine the two arrays into one multi-dimensional array. The following declaration: TypeAPoints = array[1..100] of Array[1..3] of Real; is equivalent to the following declaration: TypeAPoints = array[1..100,1..3] of Real; The functions High (119) and Low (124) return the high and low bounds of the leftmost index type of the array. In the above case, this would be 100 and 1. Record types Free Pascal supports fixed records and records with variant parts. The syntax diagram for a record type is Record types - - record type record end - packed field list - - field list fixed fields - variant part ; fixed fields ; - - fixed fields identifier list : type - 6 ; - - variant part case ordinal type identifier of - identifier : - variant - 6 ; - - variant constant , : ( ) - 6 field list So the following are valid record types declarations: 28 3.3. STRUCTURED TYPES Type Point = Record X,Y,Z : Real; end; RPoint = Record Case Boolean of False : (X,Y,Z : Real); True : (R,theta,phi : Real); end; BetterRPoint = Record Case UsePolar : Boolean of False : (X,Y,Z : Real); True : (R,theta,phi : Real); end; The variant part must be last in the record. The optional identifier in the case statement serves to access the tag field value, which otherwise would be invisible to the programmer. It can be used to see which variant is active at a certain time. In e ect, it introduces a new field in the record. Remark that it is possible to nest variant parts, as in: Type MyRec = Record X : Longint; Case byte of 2 : (Y : Longint; case byte of 3 : (Z : Longint); ); end; The size of a record is the sum of the sizes of its fields, each size of a field is rounded up to two. If the record contains a variant part, the size of the variant part is the size of the biggest variant, plus the size of the tag field type if an identifier was declared for it. Here also, the size of each part is first rounded up to two. So in the above example, SizeOf (142) would return 24 for Point, 24 for RPoint and 26 for BetterRPoint. For MyRec, the value would be 12. If you want to read a typed file with records, produced by a Turbo Pascal program, then chances are that you will not succeed in reading that file correctly. The reason for this is that by default, elements of a record are aligned at 2-byte boundaries, for performance reasons. This default behaviour can be changed with the {$PackRecords n} switch. Possible values for n are 1, 2, 4, 16 or Default. This switch tells the compiler to align elements of a record or object or class that have size larger than n on n byte boundaries. Elements that have size smaller or equal than n are aligned on natural boundaries, i.e. to the first power of two that is larger than or equal to the size of the record element. The keyword Default selects the default value for the platform you're working on (currently, this is 2 on all platforms) Take a look at the following program: Program PackRecordsDemo; type{$PackRecords 2} Trec1 = Record A : byte; 29 3.3. STRUCTURED TYPES B : Word; end; {$PackRecords 1} Trec2 = Record A : Byte; B : Word; end; {$PackRecords 2} Trec3 = Record A,B : byte; end; {$PackRecords 1} Trec4 = Record A,B : Byte; end; {$PackRecords 4} Trec5 = Record A : Byte; B : Array[1..3] of byte; C : byte; end; {$PackRecords 8} Trec6 = Record A : Byte; B : Array[1..3] of byte; C : byte; end; {$PackRecords 4} Trec7 = Record A : Byte; B : Array[1..7] of byte; C : byte; end; {$PackRecords 8} Trec8 = Record A : Byte; B : Array[1..7] of byte; C : byte; end; Var rec1 : Trec1; rec2 : Trec2; rec3 : TRec3; rec4 : TRec4; rec5 : Trec5; rec6 : TRec6; rec7 : TRec7; rec8 : TRec8; begin Write ('Size Trec1 : ',SizeOf(Trec1)); 30 3.3. STRUCTURED TYPES Writeln (' Offset B : ',Longint(@rec1.B)-Longint(@rec1)); Write ('Size Trec2 : ',SizeOf(Trec2)); Writeln (' Offset B : ',Longint(@rec2.B)-Longint(@rec2)); Write ('Size Trec3 : ',SizeOf(Trec3)); Writeln (' Offset B : ',Longint(@rec3.B)-Longint(@rec3)); Write ('Size Trec4 : ',SizeOf(Trec4)); Writeln (' Offset B : ',Longint(@rec4.B)-Longint(@rec4)); Write ('Size Trec5 : ',SizeOf(Trec5)); Writeln (' Offset B : ',Longint(@rec5.B)-Longint(@rec5), ' Offset C : ',Longint(@rec5.C)-Longint(@rec5)); Write ('Size Trec6 : ',SizeOf(Trec6)); Writeln (' Offset B : ',Longint(@rec6.B)-Longint(@rec6), ' Offset C : ',Longint(@rec6.C)-Longint(@rec6)); Write ('Size Trec7 : ',SizeOf(Trec7)); Writeln (' Offset B : ',Longint(@rec7.B)-Longint(@rec7), ' Offset C : ',Longint(@rec7.C)-Longint(@rec7)); Write ('Size Trec8 : ',SizeOf(Trec8)); Writeln (' Offset B : ',Longint(@rec8.B)-Longint(@rec8), ' Offset C : ',Longint(@rec8.C)-Longint(@rec8)); end. The output of this program will be : Size Trec1 : 4 Offset B : 2 Size Trec2 : 3 Offset B : 1 Size Trec3 : 2 Offset B : 1 Size Trec4 : 2 Offset B : 1 Size Trec5 : 8 Offset B : 4 Offset C : 7 Size Trec6 : 8 Offset B : 4 Offset C : 7 Size Trec7 : 12 Offset B : 4 Offset C : 11 Size Trec8 : 16 Offset B : 8 Offset C : 15 And this is as expected. In Trec1, since B has size 2, it is aligned on a 2 byte boundary, thus leaving an empty byte between A and B, and making the total size 4. In Trec2, B is aligned on a 1-byte boundary, right after A, hence, the total size of the record is 3. For Trec3, the sizes of A,B are 1, and hence they are aligned on 1 byte boundaries. The same is true for Trec4. For Trec5, since the size of B ¡ 3 ¡ is smaller than 4, B will be on a 4-byte boundary, as this is the first power of two that is larger than it's size. The same holds for Trec6. For Trec7, B is aligned on a 4 byte boundary, since it's size ¡ 7 ¡ is larger than 4. However, in Trec8, it is aligned on a 8-byte boundary, since 8 is the first power of two that is greater than 7, thus making the total size of the record 16. As from version 0.9.3, Free Pascal supports also the 'packed record', this is a record where all the elements are byte-aligned. Thus the two following declarations are equivalent: {$PackRecords 1} Trec2 = Record A : Byte; B : Word; end; {$PackRecords 2} and Trec2 = Packed Record 31 3.3. STRUCTURED TYPES Table 3.7: Set Manipulation operators Operation Operator Union + Di erence - Intersection * Add element include Delete element exclude A : Byte; B : Word; end; Note the {$PackRecords 2} after the first declaration ! Set types Free Pascal supports the set types as in Turbo Pascal. The prototype of a set declaration is: Set Types - - set type set of ordinal type - Each of the elements of SetType must be of type TargetType. TargetType can be any ordinal type with a range between 0 and 255. A set can contain maximally 255 elements. The following are valid set declaration: TypeJunk = Set of Char; Days = (Mon, Tue, Wed, Thu, Fri, Sat, Sun); WorkDays : Set of days; Given this set declarations, the following assignment is legal: WorkDays := [ Mon, Tue, Wed, Thu, Fri]; The operators and functions for manipulations of sets are listed in table (3.7). You can compare two sets with the <> and = operators, but not (yet) with the < and > operators. As of compiler version 0.9.5, the compiler stores small sets (less than 32 elements) in a Longint, if the type range allows it. This allows for faster processing and decreases program size. Otherwise, sets are stored in 32 bytes. File types File types are types that store a sequence of some base type, which can be any type except another file type. It can contain (in principle) an infinite number of 32 3.4. POINTERS elements. File types are used commonly to store data on disk. Nothing stops you, however, from writing a file driver that stores it's data in memory. Here is the type declaration for a file type: File types - - file type file - of type If no type identifier is given, then the file is an untyped file; it can be considered as equivalent to a file of bytes. Untyped files require special commands to act on them (see Blockread (102), Blockwrite (103)). The following declaration declares a file of records: TypePoint = Record X,Y,Z : real; end; PointFile = File of Point; Internally, files are represented by the FileRec record, which is declared in the DOS unit. A special file type is the Text file type, represented by the TextRec record. A file of type Text uses special input-output routines. 3.4 Pointers Free Pascal supports the use of pointers. A variable of the pointer type contains an address in memory, where the data of another variable may be stored. Pointer types - - pointer type type identifier - As can be seen from this diagram, pointers are typed, which means that they point to a particular kind of data. The type of this data must be known at compile time. Dereferencing the pointer (denoted by adding ^ after the variable name) behaves then like a variable. This variable has the type declared in the pointer declaration, and the variable is stored in the address that is pointed to by the pointer variable. Consider the following example: Program pointers; type Buffer = String[255]; BufPtr = ^Buffer; Var B : Buffer; BP : BufPtr; PP : Pointer; etc.. 33 3.4. POINTERS In this example, BP is a pointer to a Buffer type; while B is a variable of type Buffer. B takes 256 bytes memory, and BP only takes 4 bytes of memory (enough to keep an adress in memory). Remark: Free Pascal treats pointers much the same way as C does. This means that you can treat a pointer to some type as being an array of this type. The pointer then points to the zeroeth element of this array. Thus the following pointer declaration Var p : ^Longint; Can be considered equivalent to the following array declaration: Var p : array[0..Infinity] of Longint; The di erence is that the former declaration allocates memory for the pointer only (not for the array), and the second declaration allocates memory for the entire array. If you use the former, you must allocate memory yourself, using the Getmem (117) function. The reference P^ is then the same as p[0]. The following program illustrates this maybe more clear: program PointerArray; var i : Longint; p : ^Longint; pp : array[0..100] of Longint; begin for i := 0 to 100 do pp[i] := i; { Fill array } p := @pp[0]; { Let p point to pp } for i := 0 to 100 do if p[i]<>pp[i] then WriteLn ('Ohoh, problem !') end. Free Pascal supports pointer arithmetic as C does. This means that, if P is a typed pointer, the instructions Inc(P); Dec(P); Will increase, respectively descrease the address the pointer points to with the size of the type P is a pointer to. For example Var P : ^Longint; ... Inc (p); will increase P with 4. You can also use normal arithmetic operators on pointers, that is, the following are valid pointer arithmetic operations: var p1,p2 : ^Longint; L : Longint; begin P1 := @P2; P2 := @L; L := P1-P2; P1 := P1-4; P2 := P2+4; end. 34 3.5. PROCEDURAL TYPES Here, the value that is added or substracted is not multiplied by the size of the type the pointer points to. 3.5 Procedural types Free Pascal has support for procedural types, although it di ers a little from the Turbo Pascal implementation of them. The type declaration remains the same, as can be seen in the following syntax diagram: Procedural types - - procedural type function header - procedure header of object - - ; call modifiers - - function header function formal parameter list : result type - - - procedure header procedure formal parameter list - - - call modifiers register - cdecl pascal stdcall popstack For a description of formal parameter lists, see chapter 8, page 73. The two following examples are valid type declarations: Type TOneArg = Procedure (Var X : integer); TNoArg = Function : Real; var proc : TOneArg; func : TNoArg; One can assign the following values to a procedural type variable: 1. Nil, for both normal procedure pointers and method pointers. 2. A variable reference of a procedural type, i.e. another variable of the same type. 3. A global procedure or function address, with matching function or procedure header and calling convention. 4. A method address. Given these declarations, the following assignments are valid: Procedure printit (Var X : Integer); begin WriteLn (x); end; ... P := @printit; Func := @Pi; 35 3.5. PROCEDURAL TYPES From this example, the di erence with Turbo Pascal is clear: In Turbo Pascal it isn't necessary to use the address operator (@) when assigning a procedural type variable, whereas in Free Pascal it is required (unless you use the -So switch, in which case you can drop the address operator.) Remark that the modifiers concerning the calling conventions (cdecl, pascal, stdcall and popstack stick to the declaration; i.e. the following code would give an error: Type TOneArgCcall = Procedure (Var X : integer);cdecl; var proc : TOneArgCcall; Procedure printit (Var X : Integer); begin WriteLn (x); end; begin P := @printit; end. Because the TOneArgCcall type is a procedure that uses the cdecl calling conven- tion. At the moment, the method procedural pointers (i.e. pointers that point to methods of objects, distinguished by the of object keywords in the declaration) are still in an experimental stage. 36 Chapter 4 Objects 4.1 Declaration Free Pascal supports object oriented programming. In fact, most of the compiler is written using objects. Here we present some technical questions regarding object oriented programming in Free Pascal. Objects should be treated as a special kind of record. The record contains all the fields that are declared in the objects definition, and pointers to the methods that are associated to the objects' type. An object is declared just as you would declare a record; except that you can now declare procedures and functions as if they were part of the record. Objects can "inherit" fields and methods from "parent" objects. This means that you can use these fields and methods as if they were included in the objects you declared as a "child" object. Furthermore, you can declare fields, procedures and functions as public or private. By default, fields and methods are public, and are exported outside the current unit. Fields or methods that are declared private are only accessible in the current unit. The prototype declaration of an object is as follows: object types - - object - packed heritage component list end 6object visibility specifier - - - - heritage ( object type identifier ) - - - component list - field definition method definition 6 6 - - field definition identifier list : type ; - - - method definition function header ; method directives - procedure header constructor header desctuctor header 37 4.2. FIELDS - - method directives - virtual ; abstract ; - - call modifiers ; - - object visibility specifier private - public As you can see, you can repeat as many private and public blocks as you want. Method definitions are normal function or procedure declarations. You cannot put fields after methods in the same block, i.e. the following will generate an error when compiling: Type MyObj = Object Procedure Doit; Field : Longint; end; But the following will be accepted: Type MyObj = Object Public Procedure Doit; Private Field : Longint; end; because the field is in a di erent section. Remark: Free Pascal also supports the packed object. This is the same as an object, only the elements (fields) of the object are byte-aligned, just as in the packed record. The declaration of a packed object is similar to the declaration of a packed record : Type TObj = packed object; Constructor init; ... end; Pobj = ^TObj; Var PP : Pobj; Similarly, the {$PackRecords } directive acts on objects as well. 4.2 Fields Object Fields are like record fields. They are accessed in the same way as you would access a record field : by using a qualified identifier. Given the following declaration: Type TAnObject = Object AField : Longint; 38 4.3. CONSTRUCTORS AND DESTRUCTORS Procedure AMethod; end; Var AnObject : TAnObject; then the following would be a valid assignment: AnObject.AField := 0; Inside methods, fields can be accessed using the short identifier: Procedure TAnObject.AMethod; begin ... AField := 0; ... end; Or, one can use the self identifier. The self identifier refers to the current instance of the object: Procedure TAnObject.AMethod; begin ... Self.AField := 0; ... end; You cannot access fields that are in a private section of an object from outside the objects' methods. If you do, the compiler will complain about an unknown identifier. It is also possible to use the with statement with an object instance: With AnObject do begin Afield := 12; AMethod; end; In this example, between the begin and end, it is as if AnObject was prepended to the Afield and Amethod identifiers. More about this in section 7.2, page 70 4.3 Constructors and destructors As can be seen in the syntax diagram for an object declaration, Free Pascal supports constructors and destructors. You are responsible for calling the constructor and the destructor explicitly when using objects. The declaration of a constructor or destructor is as follows: Constructors and destructors - - constructor declaration constructor header ; subroutine block - - - destructor declaration destructor header ; subroutine block - 39 4.4. METHODS - - constructor header constructor identifier - qualified method identifier - formal parameter list - - - desctructor header destructor identifier - qualified method identifier - formal parameter list - A constructor/destructor pair is required if you use virtual methods. In the dec- laration of the object type, you should use a simple identifier for the name of the constuctor or destructor. When you implement the constructor or destruc- tor, you should use a qulified method identifier, i.e. an identifier of the form objectidentifier.methodidentifier. Free Pascal supports also the extended syntax of the New and Dispose procedures. In case you want to allocate a dynamic variable of an object type, you can specify the constructor's name in the call to New. The New is implemented as a function which returns a pointer to the instantiated object. Consider the following declarations: Type TObj = object; Constructor init; ... end; Pobj = ^TObj; Var PP : Pobj; Then the following 3 calls are equivalent: pp := new (Pobj,Init); and new(pp,init); and also new (pp); pp^.init; In the last case, the compiler will issue a warning that you should use the extended syntax of new and dispose to generate instances of an object. You can ignore this warning, but it's better programming practice to use the extended syntax to create instances of an object. Similarly, the Dispose procedure accepts the name of a destructor. The destructor will then be called, before removing the object from the heap. In view of the compiler warning remark, the now following Delphi approach may be considered a more natural way of object-oriented programming. 4.4 Methods Object methods are just like ordinary procedures or functions, only they have an implicit extra parameter : self. Self points to the object with which the method was invoked. When implementing methods, the fully qualified identifier must be given in the function header. When declaring methods, a normal identifier must be given. 40 4.5. METHOD INVOCATION 4.5 Method invocation Methods are called just as normal procedures are called, only they have an object instance identifier prepended to them (see also chapter 7, page 62). To determine which method is called, it is necessary to know the type of the method. We treat the di erent types in what follows. Static methods Static methods are methods that have been declared without a abstract or virtual keyword. When calling a static method, the declared (i.e. compile time) method of the object is used. For example, consider the following declarations: Type TParent = Object ... procedure Doit; ... end; PParent = ^TParent; TChild = Object(TParent) ... procedure Doit; ... end; PChild = ^TChild; As it is visible, both the parent and child objects have a method called Doit. Consider now the following declarations and calls: Var ParentA,ParentB : PParent; Child : PChild; ParentA := New(PParent,Init); ParentB := New(PChild,Init); Child := New(PChild,Init); ParentA^.Doit; ParentB^.Doit; Child^.Doit; Of the three invocations of Doit, only the last one will call TChild.Doit, the other two calls will call TParent.Doit. This is because for static methods, the compiler determines at compile time which method should be called. Since ParentB is of type TParent, the compiler decides that it must be called with TParent.Doit, even though it will be created as a TChild. There may be times when you want the method that is actually called to depend on the actual type of the object at run- time. If so, the method cannot be a static method, but must be a virtual method. Virtual methods To remedy the situation in the previous section, virtual methods are created. This is simply done by appending the method declaration with the virtual modifier. Going back to the previous example, consider the following alternative declaration: 41 4.5. METHOD INVOCATION Type TParent = Object ... procedure Doit;virtual; ... end; PParent = ^TParent; TChild = Object(TParent) ... procedure Doit;virtual; ... end; PChild = ^TChild; As it is visible, both the parent and child objects have a method called Doit. Consider now the following declarations and calls : Var ParentA,ParentB : PParent; Child : PChild; ParentA := New(PParent,Init); ParentB := New(PChild,Init); Child := New(PChild,Init); ParentA^.Doit; ParentB^.Doit; Child^.Doit; Now, di erent methods will be called, depending on the actual run-time type of the object. For ParentA, nothing changes, since it is created as a TParent instance. For Child, the situation also doesn't change: it is again created as an instance of TChild. For ParentB however, the situation does change: Even though it was de- clared as a TParent, it is created as an instance of TChild. Now, when the program runs, before calling Doit, the program checks what the actual type of ParentB is, and only then decides which method must be called. Seeing that ParentB is of type TChild, TChild.Doit will be called. The code for this run-time checking of the ac- tual type of an object is inserted by the compiler at compile time. The TChild.Doit is said to override the TParent.Doit. It is possible to acces the TParent.Doit from within the varTChild.Doit, with the inherited keyword: Procedure TChild.Doit; begin inherited Doit; ... end; In the above example, when TChild.Doit is called, the first thing it does is call TParent.Doit. You cannot use the inherited keyword on static methods, only on virtual methods. Abstract methods An abstract method is a special kind of virtual method. A method can not be abstract if it is not virtual (this is not obvious from the syntax diagram). You cannot create an instance of an object that has an abstract method. The reason is obvious: there is no method where the compiler could jump to ! A method that is 42 4.6. VISIBILITY declared abstract does not have an implementation for this method. It is up to inherited objects to override and implement this method. Continuing our example, take a look at this: Type TParent = Object ... procedure Doit;virtual;abstract; ... end; PParent=^TParent; TChild = Object(TParent) ... procedure Doit;virtual; ... end; PChild = ^TChild; As it is visible, both the parent and child objects have a method called Doit. Consider now the following declarations and calls : Var ParentA,ParentB : PParent; Child : PChild; ParentA := New(PParent,Init); ParentB := New(PChild,Init); Child := New(PChild,Init); ParentA^.Doit; ParentB^.Doit; Child^.Doit; First of all, Line 3 will generate a compiler error, stating that you cannot generate instances of objects with abstract methods: The compiler has detected that PParent points to an object which has an abstract method. Commenting line 3 would allow compilation of the program. Remark that if you override an abstract method, you cannot call the parent method with inherited, since there is no parent method; The compiler will detect this, and complain about it, like this: testo.pp(32,3) Error: Abstract methods can't be called directly If, through some mechanism, an abstract method is called at run-time, then a run- time error will occur. (run-time error 211, to be precise) 4.6 Visibility For objects, only 2 visibility specifiers exist : private and public. If you don't specify a visibility specifier, public is assumed. Both methods and fields can be hid- den from a programmer by putting them in a private section. The exact visibility rule is as follows: Private All fields and methods that are in a private block, can only be accessed in the module (i.e. unit or program) that contains the object definition. They can be accessed from inside the object's methods or from outside them e.g. from other objects' methods, or global functions. 43 4.6. VISIBILITY Public sections are always accessible, from everywhere. Fields and metods in a public section behave as though they were part of an ordinary record type. 44 Chapter 5 Classes In the Delphi approach to Object Oriented Programming, everything revolves around the concept of 'Classes'. A class can be seen as a pointer to an object, or a pointer to a record. remark In earlier versions of Free Pascal it was necessary, in order to use classes, to put the objpas unit in the uses clause of your unit or program. This is no longer needed as of version 0.99.12. As of version 0.99.12 the system unit contains the basic definitions of TObject and TClass, as well as some auxiliary methods for using classes. The objpas unit still exists, and contains some redefinitions of basic types, so they coincide with Delphi types. The unit will be loaded automatically if you specify the -S2 or -Sd options. 5.1 Class definitions The prototype declaration of a class is as follows : Class types - - class - packed heritage component list end 6class visibility specifier - - heritage ( class type identifier ) - - - component list - field definition method definition 6 6 property definition - - field definition identifier list : type ; - - - method definition function header ; - class procedure header constructor header desctuctor header 45 5.2. CLASS INSTANTIATION - - virtual ; call modifiers ; ; abstract override message integer constant string constant - - class visibility specifier private - protected public published Again, You can repeat as many private, protected, published and public blocks as you want. Methods are normal function or procedure declarations. As you can see, the declaration of a class is almost identical to the declaration of an object. The real di erence between objects and classes is in the way they are created (see further in this chapter). The visibility of the di erent sections is as follows: Private All fields and methods that are in a private block, can only be accessed in the module (i.e. unit) that contains the class definition. They can be accessed from inside the classes' methods or from outside them (e.g. from other classes' methods) Protected Is the same as Private, except that the members of a Protected section are also accessible to descendent types, even if they are implemented in other modules. Public sections are always accessible. Published Is the same as a Public section, but the compiler generates also type information that is needed for automatic streaming of these classes. Fields defined in a published section must be of class type. Array peroperties cannot be in a published section. 5.2 Class instantiation Classes must be created using their constructor. Remember that a class is a pointer to an object, so when you declare a variable of some class, the compiler just allocates a pointer, not the entire object. The constructor of a class returns a pointer to an initialized instance of the object. So, to initialize an instance of some class, you would do the following : ClassVar := ClassType.ConstructorName; You cannot use the extended syntax of new and dispose to instantiate and destroy class instances. That construct is reserved for use with objects only. Calling the constructor will provoke a call to getmem, to allocate enough space to hold the class instance data. After that, the constuctor's code is executed. The constructor has a pointer to it's data, in self. Remarks: * The {$PackRecords } directive also a ects classes. i.e. the alignment in memory of the di erent fields depends on the value of the {$PackRecords } directive. 46 5.3. METHODS * Just as for objects and records, you can declare a packed class. This has the same e ect as on an object, or record, namely that the elements are aligned on 1-byte boundaries. i.e. as close as possible. * SizeOf(class) will return 4, since a class is but a pointer to an object. To get the size of the class instance data, use the TObject.InstanceSize method. 5.3 Methods invocation Method invocaticn for classes is no di erent than for objects. The following is a valid method invocation: Var AnObject : TAnObject; begin AnObject := TAnObject.Create; ANobject.AMethod; Virtual methods Classes have virtual methods, just as objects do. There is however a di erence between the two. For objects, it is su cient to redeclare the same method in a descendent object with the keyword virtual to override it. For classes, the situation is di erent: you must override virtual methods with the override keyword. Failing to do so, will start a new batch of virtual methods, hiding the previous one. The Inherited keyword will not jump to the inherited method, if virtual was used. The following code is wrong: Type ObjParent = Class Procedure MyProc ; v i r t u a l ; end ; ObjChild = Class ( ObjPArent ) Procedure MyProc ; v i r t u a l ; end ; The compiler will produce a warning: Warning: An inherited method is hidden by OBJCHILD.MYPROC The compiler will compile it, but using Inherited can produce strange e ects. The correct declaration is as follows: Type ObjParent = Class Procedure MyProc ; v i r t u a l ; end ; ObjChild = Class ( ObjPArent ) Procedure MyProc ; override ; end ; This will compile and run without warnings or errors. 47 5.3. METHODS Message methods New in classes are message methods. Pointers to message methods are stored in a special table, together with the integer or string cnstant that they were declared with. They are primarily intended to ease programming of callback functions in several GUI toolkits, such as Win32 or GTK. In di erence with Delphi, Free Pascal also accepts strings as message identifiers. Message methods that are declared with an integer constant can take only one var argument (typed or not): Procedure TMyObject . MyHandler ( Var Msg ) ; Message 1 ; The method implementation of a message function is no di erent from an ordinary method. It is also possible to call a message method directly, but you should not do this. Instead use the TObject.Dispatch method. The TOBject.Dispatch method can be used to call a message handler. It is de- clared in the system unit and will accept a var parameter which must have at the first position a cardinal with the message ID that should be called. For example: Type TMsg = Record MSGID : C a r d i n a l Data : Pointer ; VarMsg : TMSg; MyObject . Dispatch ( Msg ) ; In this example, the Dispatch method will look at the object and all it's ancestors (starting at the object, and searching up the class tree), to see if a message method with message MSGID has been declared. If such a method is found, it is called, and passed the Msg parameter. If no such method is found, DefaultHandler is called. DefaultHandler is a vir- tual method of TObject that doesn't do anything, but which can be overridden to provide any processing you might need. DefaultHandler is declared as follows: procedure d e f a u l t h a n d l e r ( var message ) ; v i r t u a l ; In addition to the message method with a Integer identifier, Free Pascal also supports a messae method with a string identifier: Procedure TMyObject . MyStrHandler ( Var Msg ) ; Message ' OnClick ' ; The working of the string message handler is the same as the ordinary integer message handler: The TOBject.DispatchStr method can be used to call a message handler. It is declared in the system unit and will accept one parameter which must have at the first position a string with the message ID that should be called. For example: Type TMsg = Record MsgStr : String [ 1 0 ] ; / / A r b i t r a r y length up to 255 c h a r a c t e r s . Data : Pointer ; VarMsg : TMSg; MyObject . DispatchStr ( Msg ) ; In this example, the DispatchStr method will look at the object and all it's an- 48 5.4. PROPERTIES cestors (starting at the object, and searching up the class tree), to see if a message method with message MsgStr has been declared. If such a method is found, it is called, and passed the Msg parameter. If no such method is found, DefaultHandlerStr is called. DefaultHandlerStr is a virtual method of TObject that doesn't do anything, but which can be overridden to provide any processing you might need. DefaultHandlerStr is declared as follows: procedure D e f a u l t H a n d l e r S t r ( var message ) ; v i r t u a l ; In addition to this mechanism, a string message method accepts a self parameter: TMyObject . StrMsgHandler ( Data : Pointer ; S e l f : TMyObject ) ; Message ' OnClick ' ; When encountering such a method, the compiler will generate code that loads the Self parameter into the object instance pointer. The result of this is that it is possible to pass Self as a parameter to such a method. remark: The type of the Self parameter must be of the same class as the class you define the method for. 5.4 Properties Classes can contain properties as part of their fields list. A property acts like a normal field, i.e. you can get or set it's value, but allows to redirect the access of the field through functions and procedures. They provide a means to associate an action with an assignment of or a reading from a class 'field'. This allows for e.g. checking that a value is valid when assigning, or, when reading, it allows to construct the value on the fly. Moreover, properties can be read-only or write only. The prototype declaration of a property is as follows: Properties - - property definition property identifier - property interface - property specifiers - - - property interface : type identifier - property parameter list - - index integerconstant - - property parameter list [ parameter declaration ] - 6 ; - - property specifiers - read specifier write specifier - - default specifier - - read specifier read field or method - - - write specifier write field or method - - - default specifier default - constant nodefault 49 5.4. PROPERTIES - - field or method field identifier - method identifier A read specifier is either the name of a field that contains the property, or the name of a method function that has the same return type as the property type. In the case of a simple type, this function must not accept an argument. A read specifier is optional, making the property write-only. A write specifier is op- tional: If there is no write specifier, the property is read-only. A write specifier is either the name of a field, or the name of a method procedure that accepts as a sole argument a variable of the same type as the property. The section (private, published) in which the specified function or procedure resides is irrelevant. Usu- ally, however, this will be a protected or private method. Example: Given the following declaration: Type MyClass = Class Private Field1 : Longint; Field2 : Longint; Field3 : Longint; Procedure Sety (value : Longint); Function Gety : Longint; Function Getz : Longint; Public Property X : Longint Read Field1 write Field2; Property Y : Longint Read GetY Write Sety; Property Z : Longint Read GetZ; end; Var MyClass : TMyClass; The following are valid statements: WriteLn ('X : ',MyClass.X); WriteLn ('Y : ',MyClass.Y); WriteLn ('Z : ',MyClass.Z); MyClass.X := 0; MyClass.Y := 0; But the following would generate an error: MyClass.Z := 0; because Z is a read-only property. What happens in the above statements is that when a value needs to be read, the compiler inserts a call to the various getNNN methods of the object, and the result of this call is used. When an assignment is made, the compiler passes the value that must be assigned as a paramater to the various setNNN methods. Because of this mechanism, properties cannot be passed as var arguments to a function or procedure, since there is no known address of the property (at least, not always). If the property definition contains an index, then the read and write specifiers must be a function and a procedure. Moreover, these functions require an additional parameter : An integer parameter. This allows to read or write several properties with the same function. For this, the properties must have the same type. The following is an example of a property with an index: 50 5.4. PROPERTIES uses objpas; Type TPoint = Class(TObject) Private FX,FY : Longint; Function GetCoord (Index : Integer): Longint; Procedure SetCoord (Index : Integer; Value : longint); Public Property X : Longint index 1 read GetCoord Write SetCoord; Property Y : Longint index 2 read GetCoord Write SetCoord; Property Coords[Index : Integer] Read GetCoord; end; Procedure TPoint.SetCoord (Index : Integer; Value : Longint); begin Case Index of 1 : FX := Value; 2 : FY := Value; end; end; Function TPoint.GetCoord (INdex : Integer) : Longint; begin Case Index of 1 : Result := FX; 2 : Result := FY; end; end; Var P : TPoint; begin P := TPoint.create; P.X := 2; P.Y := 3; With P do WriteLn ('X=',X,' Y=',Y); end. When the compiler encounters an assignment to X, then SetCoord is called with as first parameter the index (1 in the above case) and with as a second parameter the value to be set. Conversely, when reading the value of X, the compiler calls GetCoord and passes it index 1. Indexes can only be integer values. You can also have array properties. These are properties that accept an index, just as an array does. Only now the index doesn't have to be an ordinal type, but can be any type. A read specifier for an array property is the name method function that has the same return type as the property type. The function must accept as a sole arguent a variable of the same type as the index type. For an array property, you cannot specify fields as read specifiers. A write specifier for an array property is the name of a method procedure that accepts two arguments: The first argument has the same type as the index, and the second argument is a parameter of the same type as the property type. As an example, see the following declaration: Type TIntList = Class Private Function GetInt (I : Longint) : longint; Function GetAsString (A : String) : String; 51 5.4. PROPERTIES Procedure SetInt (I : Longint; Value : Longint;); Procedure SetAsString (A : String; Value : String); Public Property Items [i : Longint] : Longint Read GetInt Write SetInt; Property StrItems [S : String] : String Read GetAsString Write SetAsstring; end; Var AIntList : TIntList; Then the following statements would be valid: AIntList.Items[26] := 1; AIntList.StrItems['twenty-five'] := 'zero'; WriteLn ('Item 26 : ',AIntList.Items[26]); WriteLn ('Item 25 : ',AIntList.StrItems['twenty-five']); While the following statements would generate errors: AIntList.Items['twenty-five'] := 1; AIntList.StrItems[26] := 'zero'; Because the index types are wrong. Array properties can be declared as default properties. This means that it is not necessary to specify the property name when assigning or reading it. If, in the previous example, the definition of the items property would have been Property Items[i : Longint]: Longint Read GetInt Write SetInt; Default; Then the assignment AIntList.Items[26] := 1; Would be equivalent to the following abbreviation. AIntList[26] := 1; You can have only one default property per class, and descendent classes cannot redeclare the default property. 52 Chapter 6 Expressions Expressions occur in assignments or in tests. Expressions produce a value, of a certain type. Expressions are built with two components: Operators and their operands. Usually an operator is binary, i.e. it requires 2 operands. Binary op- erators occur always between the operands (as in X/Y). Sometimes an operator is unary, i.e. it requires only one argument. A unary operator occurs always before the operand, as in -X. When using multiple operands in an expression, the precedence rules of table (6.1) are used. When determining the precedence, the compiler uses the following rules: 1. Operators with equal precedence are executed from left to right. 2. In operations with unequal precedences the operands belong to the operater with the highest precedence. For example, in 5*3+7, the multiplication is higher in precedence than the addition, so it is executed first. The result would be 22. 3. If parentheses are used in an epression, their contents is evaluated first. Thus, 5*(3+7) would result in 50. 6.1 Expression syntax An expression applies relational operators to simple expressions. Simple expressions are a series of terms (what a term is, is explained below), joined by adding operators. Expressions Table 6.1: Precedence of operators Operator Precedence Category Not, @ Highest (first) Unary operators * / div mod and shl shr as Second Multiplying operators + - or xor Third Adding operators < <> < > <= >= in is Lowest (Last) relational operators 53 6.1. EXPRESSION SYNTAX - - expression simple expression - * simple expression <= > >= = <> in is - - simple expression term - 6 +-orxor The following are valid expressions: GraphResult<>grError (DoItToday=Yes) and (DoItTomorrow=No); Day in Weekend And here are some simple expressions: A + B -Pi ToBe or NotToBe Terms consist of factors, connected by multiplication operators. Terms - - term factor - 6 */divmodandshlshras Here are some valid terms: 2 * Pi A Div B (DoItToday=Yes) and (DoItTomorrow=No); Factors are all other constructions: Factors 54 6.2. FUNCTION CALLS - - factor ( expression ) - variable reference function call unsigned constant not factor sign factor set constructor value typecast address factor - - unsigned constant unsigned number - character string constant identifier Nil 6.2 Function calls Function calls are part of expressions (although, using extended syntax, they can be statements too). They are constructed as follows: Function calls - - function call function identifier - method designator actual parameter list qualified method designator variable reference - - - - actual parameter list ( ) - expression 6 , The variable reference must be a procedural type variable reference. A method designator can only be used inside the method of an object. A qualified method designator can be used outside object methods too. The function that will get called is the function with a declared parameter list that matches the actual parameter list. This means that 1. The number of actual parameters must equal the number of declared param- eters. 2. The types of the parameters must be compatible. For variable reference pa- rameters, the parameter types must be exactly the same. If no matching function is found, then the compiler will generate an error. Depend- ing on the fact of the function is overloaded (i.e. multiple functions with the same name, but di erent parameter lists) the error will be di erent. There are cases when the compiler will not execute the function call in an expression. This is the case when you are assigning a value to a procedural type variable, as in the following example: 55 6.3. SET CONSTRUCTORS Type FuncType = Function: Integer; Var A : Integer; Function AddOne : Integer; begin A := A+1; AddOne := A; end; Var F : FuncType; N : Integer; begin A := 0; F := AddOne; { Assign AddOne to F, Don't call AddOne} N := AddOne; { N := 1 !!} end. In the above listing, the assigment to F will not cause the function AddOne to be called. The assignment to N, however, will call AddOne. A problem with this syntax is the following construction: If F = AddOne Then DoSomethingHorrible; Should the compiler compare the addresses of F and AddOne, or should it call both functions, and compare the result ? Free Pascal solves this by deciding that a procedural variable is equivalent to a pointer. Thus the compiler will give a type mismatch error, since AddOne is considered a call to a function with integer result, and F is a pointer, Hence a type mismatch occurs. How then, should one compare whether F points to the function AddOne ? To do this, one should use the address operator @: If F = @AddOne Then WriteLn ('Functions are equal'); The left hand side of the boolean expression is an address. The right hand side also, and so the compiler compares 2 addresses. How to compare the values that both functions return ? By adding an empty parameter list: If F()=Addone then WriteLn ('Functions return same values '); Remark that this behaviour is not compatible with Delphi syntax. 6.3 Set constructors When you want to enter a set-type constant in an expression, you must give a set constructor. In essence this is the same thing as when you define a set type, only you have no identifier to identify the set with. A set constructor is a comma separated list of expressions, enclosed in square brackets. Set constructors 56 6.4. VALUE TYPECASTS - - set constructor [ ] - set group 6 , - - set group expression - .. expression All set groups and set elements must be of the same ordinal type. The empty set is denoted by [], and it can be assigned to any type of set. A set group with a range [A..Z] makes all values in the range a set element. If the first range specifier has a bigger ordinal value than the second the set is empty, e.g., [Z..A] denotes an empty set. The following are valid set constructors: [today,tomorrow] [Monday..Friday,Sunday] [ 2, 3*2, 6*2, 9*2 ] ['A'..'Z','a'..'z','0'..'9'] 6.4 Value typecasts Sometimes it is necessary to change the type of an expression, or a part of the expression, to be able to be assignment compatible. This is done through a value typecast. The syntax diagram for a value typecast is as follows: Typecasts - - value typecast type identifier ( expression ) - Value typecasts cannot be used on the left side of assignments, as variable typecasts. Here are some valid typecasts: Byte('A') Char(48) boolean(1) longint(@Buffer) The type size of the expression and the size of the type cast must be the same. That is, the following doesn't work: Integer('A') Char(4875) boolean(100) Word(@Buffer) This is di erent from Delphi or Turbo Pascal behaviour. 57 6.5. THE @ OPERATOR 6.5 The @ operator The address operator @ returns the address of a variable, procedure or function. It is used as follows: Address factor - - addressfactor @ variable reference - procedure identifier function identifier qualified method identifier The @ operator returns a typed pointer if the $T switch is on. If the $T switch is o then the address operator returns an untyped pointer, which is assigment compatible with all pointer types. The type of the pointer is ^T, where T is the type of the variable reference. For example, the following will compile Program tcast; {$T-} { @ returns untyped pointer } Type art = Array[1..100] of byte; Var Buffer : longint; PLargeBuffer : ^art; begin PLargeBuffer := @Buffer; end. Changing the {$T-} to {$T+} will prevent the compiler from compiling this. It will give a type mismatch error. By default, the address operator returns an untyped pointer. Applying the address operator to a function, method, or procedure identi- fier will give a pointer to the entry point of that function. The result is an untyped pointer. By default, you must use the address operator if you want to assign a value to a procedural type variable. This behaviour can be avoided by using the -So or -S2 switches, which result in a more compatible Delphi or Turbo Pascal syntax. 6.6 Operators Operators can be classified according to the type of expression they operate on. We will discuss them type by type. Arithmetic operators Arithmetic operators occur in arithmetic operations, i.e. in expressions that con- tain integers or reals. There are 2 kinds of operators : Binary and unary arithmetic operators. Binary operators are listed in table (6.2), unary operators are listed in table (6.3). With the exception of Div and Mod, which accept only integer expres- sions as operands, all operators accept real and integer expressions as operands. For binary operators, the result type will be integer if both operands are integer type expressions. If one of the operands is a real type expression, then the result is real. 58 6.6. OPERATORS Table 6.2: Binary arithmetic operators Operator Operation + Addition - Subtraction * Multiplication / Division Div Integer division Mod Remainder Table 6.3: Unary arithmetic operators Operator Operation + Sign identity - Sign inversion As an exception : division (/) results always in real values. For unary operators, the result type is always equal to the expression type. The division (/) and Mod operator will cause run-time errors if the second argument is zero. The sign of the result of a Mod operator is the same as the sign of the left side operand of the Mod operator. In fact, the Mod operator is equivalent to the following operation : I mod J = I - (I div J) * J but it executes faster than the right hand side expression. Logical operators Logical operators act on the individual bits of ordinal expressions. Logical operators require operands that are of an integer type, and produce an integer type result. The possible logical operators are listed in table (6.4). The following are valid logical expressions: A shr 1 { same as A div 2, but faster} Not 1 { equals -2 } Not 0 { equals -1 } Not -1 { equals 0 } B shl 2 { same as B * 2 for integers } Table 6.4: Logical operators Operator Operation not Bitwise negation (unary) and Bitwise and or Bitwise or xor Bitwise xor shl Bitwise shift to the left shr Bitwise shift to the right 59 6.6. OPERATORS Table 6.5: Boolean operators Operator Operation not logical negation (unary) and logical and or logical or xor logical xor 1 or 2 { equals 3 } 3 xor 1 { equals 2 } Boolean operators Boolean operators can be considered logical operations on a type with 1 bit size. Therefore the shl and shr operations have little sense. Boolean operators can only have boolean type operands, and the resulting type is always boolean. The possible operators are listed in table (6.5) Remark that boolean expressions are ALWAYS evaluated with short-circuit evaluation. This means that from the moment the result of the complete expression is known, evaluation is stopped and the result is returned. For instance, in the following expression: B := True or MaybeTrue; The compiler will never look at the value of MaybeTrue, since it is obvious that the expression will always be true. As a result of this strategy, if MaybeTrue is a function, it will not get called ! (This can have surprising e ects when used in conjunction with properties) String operators There is only one string operator : +. It's action is to concatenate the contents of the two strings (or characters) it stands between. You cannot use + to concatenate null-terminated (PChar) strings. The following are valid string operations: 'This is ' + 'VERY ' + 'easy !' Dirname+'\' The following is not: Var Dirname = Pchar; ...Dirname := Dirname+'\'; Because Dirname is a null-terminated string. Set operators The following operations on sets can be performed with operators: Union, di erence and intersection. The operators needed for this are listed in table (6.6). The set type of the operands must be the same, or an error will be generated by the compiler. 60 6.6. OPERATORS Table 6.6: Set operators Operator Action + Union - Di erence * Intersection Table 6.7: Relational operators Operator Action = Equal <> Not equal < Stricty less than > Strictly greater than <= Less than or equal >= Greater than or equal in Element of Relational operators The relational operators are listed in table (6.7) Left and right operands must be of the same type. You can only mix integer and real types in relational expressions. Comparing strings is done on the basis of their ASCII code representation. When comparing pointers, the addresses to which they point are compared. This also is true for PChar type pointers. If you want to compare the strings the Pchar points to, you must use the StrComp function from the strings unit. The in returns True if the left operand (which must have the same ordinal type as the set type) is an element of the set which is the right operand, otherwise it returns False 61 Chapter 7 Statements The heart of each algorithm are the actions it takes. These actions are contained in the statements of your program or unit. You can label your statements, and jump to them (within certain limits) with Goto statements. This can be seen in the following syntax diagram: Statements - - statement - label : simple statement structured statement asm statement A label can be an identifier or an integer digit. 7.1 Simple statements A simple statement cannot be decomposed in separate statements. There are basi- cally 4 kinds of simple statements: Simple statements - - siple statement assignment statement - procedure statement goto statement raise statement Of these statements, the raise statement will be explained in the chapter on Excep- tions (chapter 10, page 90) Assignments Assignments give a value to a variable, replacing any previous value the variable might have had: 62 7.1. SIMPLE STATEMENTS Table 7.1: Allowed C constructs in Free Pascal Assignment Result a += b Adds b to a, and stores the result in a. a -= b Substracts b from a, and stores the result in a. a *= b Multiplies a with b, and stores the result in a. a /= b Divides a through b, and stores the result in a. Assignments - - assignment statement variable reference := expression - function identifier += -= *= /= In addition to the standard Pascal assignment operator ( := ), which simply re- places the value of the varable with the value resulting from the expression on the right of the := operator, Free Pascal supports some c-style constructions. All available constructs are listed in table (7.1). For these constructs to work, you should specify the -Sc command-line switch. Remark: These constructions are just for typing convenience, they don't generate di erent code. Here are some examples of valid assignment statements: X := X+Y; X+=Y; { Same as X := X+Y, needs -Sc command line switch} X/=2; { Same as X := X/2, needs -Sc command line switch} Done := False; Weather := Good; MyPi := 4* Tan(1); Procedure statements Procedure statements are calls to subroutines. There are di erent possibilities for procedure calls: A normal procedure call, an object method call (fully qualified or not), or even a call to a procedural type variable. All types are present in the following diagram. Procedure statements - - procedure statement procedure identifier - method identifier qualified method identifier variable reference - - actual parameter list 63 7.2. STRUCTURED STATEMENTS The Free Pascal compiler will look for a procedure with the same name as given in the procedure statement, and with a declared parameter list that matches the actual parameter list. The following are valid procedure statements: Usage; WriteLn('Pascal is an easy language !'); Doit(); Goto statements Free Pascal supports the goto jump statement. Its prototype syntax is Goto statement - - goto statement goto label - When using goto statements, you must keep the following in mind: 1. The jump label must be defined in the same block as the Goto statement. 2. Jumping from outside a loop to the inside of a loop or vice versa can have strange e ects. 3. To be able to use the Goto statement, you need to specify the -Sg compiler switch. Goto statements are considered bad practice and should be avoided as much as possible. It is always possible to replace a goto statement by a construction that doesn't need a goto, although this construction may not be as clear as a goto statement. For instance, the following is an allowed goto statement: label jumpto; ... Jumpto : Statement; ... Goto jumpto; ... 7.2 Structured statements Structured statements can be broken into smaller simple statements, which should be executed repeatedly, conditionally or sequentially: Structured statements - - structured statement compound statement - repetitive statement conditional statement exception statement with statement 64 7.2. STRUCTURED STATEMENTS Conditional statements come in 2 flavours : Conditional statements - - conditional statement if statement - case statement Repetitive statements come in 3 flavours: Repetitive statements - - repetitive statement for statament - repeat statement while statement The following sections deal with each of these statements. Compound statements Compound statements are a group of statements, separated by semicolons, that are surrounded by the keywords Begin and End. The Last statement doesn't need to be followed by a semicolon, although it is allowed. A compound statement is a way of grouping statements together, executing the statements sequentially. They are treated as one statement in cases where Pascal syntax expects 1 statement, such as in if ... then statements. Compound statements - - compound statement begin statement end - 6 ; The Case statement Free Pascal supports the case statement. Its syntax diagram is Case statement - - case statement case expression of case - 6; else part ; - end - - - case constant : statement - 6 .. constant , 65 7.2. STRUCTURED STATEMENTS - - else part else statement - The constants appearing in the various case parts must be known at compile- time, and can be of the following types : enumeration types, Ordinal types (ex- cept boolean), and chars. The expression must be also of this type, or a compiler error will occur. All case constants must have the same type. The compiler will evaluate the expression. If one of the case constants values matches the value of the expression, the statement that follows this constant is executed. After that, the program continues after the final end. If none of the case constants match the expression value, the statement after the else keyword is executed. This can be an empty statement. If no else part is present, and no case constant matches the expression value, program flow continues after the final end. The case statements can be compound statements (i.e. a begin..End block). Remark: Contrary to Turbo Pascal, duplicate case labels are not allowed in Free Pascal, so the following code will generate an error when compiling: Var i : integer; ... Case i of 3 : DoSomething; 1..5 : DoSomethingElse; end; The compiler will generate a Duplicate case label error when compiling this, because the 3 also appears (implicitly) in the range 1..5. This is similar to Delhpi syntax. The following are valid case statements: Case C of 'a' : WriteLn ('A pressed'); 'b' : WriteLn ('B pressed'); 'c' : WriteLn ('C pressed'); else WriteLn ('unknown letter pressed : ',C); end; Or Case C of 'a','e','i','o','u' : WriteLn ('vowel pressed'); 'y' : WriteLn ('This one depends on the language'); else WriteLn ('Consonant pressed'); end; Case Number of 1..10 : WriteLn ('Small number'); 11..100 : WriteLn ('Normal, medium number'); else WriteLn ('HUGE number'); end; 66 7.2. STRUCTURED STATEMENTS The If..then..else statement The If .. then .. else.. prototype syntax is If then statements - - if statement if expression then statement - else statement - - The expression between the if and then keywords must have a boolean return type. If the expression evaluates to True then the statement following then is executed. If the expression evaluates to False, then the statement following else is executed, if it is present. Be aware of the fact that the boolean expression will be short-cut evaluated. (Mean- ing that the evaluation will be stopped at the point where the outcome is known with certainty) Also, before the else keyword, no semicolon (;) is allowed, but all statements can be compound statements. In nested If.. then .. else con- structs, some ambiguity may araise as to which else statement pairs with which if statement. The rule is that the else keyword matches the first if keyword not already matched by an else keyword. For example: If exp1 Then If exp2 then Stat1 else stat2; Despite it's appearance, the statement is syntactically equivalent to If exp1 Then begin If exp2 then Stat1 elsestat2 end; and not to { NOT EQUIVALENT } If exp1 Then begin If exp2 then Stat1 end elsestat2 If it is this latter construct you want, you must explicitly put the begin and end keywords. When in doubt, add them, they don't hurt. The following is a valid statement: 67 7.2. STRUCTURED STATEMENTS If Today in [Monday..Friday] then WriteLn ('Must work harder') else WriteLn ('Take a day off.'); The For..to/downto..do statement Free Pascal supports the For loop construction. A for loop is used in case one wants to calculated something a fixed number of times. The prototype syntax is as follows: For statement - - for statement for control variable := initial value to - downto - final value do statement - - - control variable variable identifier - - - initial value expression - - - final value expression - Statement can be a compound statement. When this statement is encountered, the control variable is initialized with the initial value, and is compared with the final value. What happens next depends on whether to or downto is used: 1. In the case To is used, if the initial value larger than the final value then Statement will never be executed. 2. In the case DownTo is used, if the initial value larger than the final value then Statement will never be executed. After this check, the statement after Do is executed. After the execution of the state- ment, the control variable is increased or decreased with 1, depending on whether To or Downto is used. The control variable must be an ordinal type, no other types can be used as counters in a loop. Remark: Contrary to ANSI pascal specifications, Free Pascal first initializes the counter variable, and only then calculates the upper bound. The following are valid loops: For Day := Monday to Friday do Work; For I := 100 downto 1 do WriteLn ('Counting down : ',i); For I := 1 to 7*dwarfs do KissDwarf(i); If the statement is a compound statement, then the Break (103) and Continue (105) reserved words can be used to jump to the end or just after the end of the For statement. 68 7.2. STRUCTURED STATEMENTS The Repeat..until statement The repeat statement is used to execute a statement until a certain condition is reached. The statement will be executed at least once. The prototype syntax of the Repeat..until statement is Repeat statement - - repeat statement repeat statement until expression - 6 ; This will execute the statements between repeat and until up to the moment when Expression evaluates to True. Since the expression is evaluated after the execution of the statements, they are executed at least once. Be aware of the fact that the boolean expression Expression will be short-cut evaluated. (Meaning that the evaluation will be stopped at the point where the outcome is known with certainty) The following are valid repeat statements repeat WriteLn ('I =',i); I := I+2; until I>100; repeat X := X/2 until x<10e-3 The Break (103) and Continue (105) reserved words can be used to jump to the end or just after the end of the repeat .. until statement. The While..do statement A while statement is used to execute a statement as long as a certain condition holds. This may imply that the statement is never executed. The prototype syntax of the While..do statement is While statements - - while statement while expression do statement - This will execute Statement as long as Expression evaluates to True. Since Expression is evaluated before the execution of Statement, it is possible that Statement isn't executed at all. Statement can be a compound statement. Be aware of the fact that the boolean expression Expression will be short-cut evalu- ated. (Meaning that the evaluation will be stopped at the point where the outcome is known with certainty) The following are valid while statements: I := I+2; while i<=100 do begin 69 7.2. STRUCTURED STATEMENTS WriteLn ('I =',i); I := I+2; end; X := X/2; while x>=10e-3 do X := X/2; They correspond to the example loops for the repeat statements. If the statement is a compound statement, then the Break (103) and Continue (105) reserved words can be used to jump to the end or just after the end of the While statement. The With statement The with statement serves to access the elements of a record1 or object or class, without having to specify the name of the each time. The syntax for a with state- ment is With statement - - with statement variable reference do statement - 6 , The variable reference must be a variable of a record, object or class type. In the with statement, any variable reference, or method reference is checked to see if it is a field or method of the record or object or class. If so, then that field is accessed, or that method is called. Given the declaration: Type Passenger = Record Name : String[30]; Flight : String[10]; end; Var TheCustomer : Passenger; The following statements are completely equivalent: TheCustomer.Name := 'Michael'; TheCustomer.Flight := 'PS901'; and With TheCustomer do begin Name := 'Michael'; Flight := 'PS901'; end; The statement 1 The with statement does not work correctly when used with objects or classes until version 0.99.6 70 7.3. ASSEMBLER STATEMENTS With A,B,C,D do Statement; is equivalent to With A do With B do With C do With D do Statement; This also is a clear example of the fact that the variables are tried last to first, i.e., when the compiler encounters a variable reference, it will first check if it is a field or method of the last variable. If not, then it will check the last-but-one, and so on. The following example shows this; Program testw; Type AR = record X,Y : Longint; end; Var S,T : Ar; begin S.X := 1;S.Y := 1; T.X := 2;T.Y := 2; With S,T do WriteLn (X,' ',Y); end. The output of this program is 2 2 Showing thus that the X,Y in the WriteLn statement match the T record variable. Exception Statements As of version 0.99.7, Free Pascal supports exceptions. Exceptions provide a conve- nient way to program error and error-recovery mechanisms, and are closely related to classes. Exception support is explained in chapter 10, page 90 7.3 Assembler statements An assembler statement allows you to insert assembler code right in your pascal code. Assembler statements - - asm statement asm assembler code end - registerlist - - registerlist [ stringconstant ] - 6 , 71 7.3. ASSEMBLER STATEMENTS More information about assembler blocks can be found in the Programmers' guide. The register list is used to indicate the registers that are modified by an assembler statement in your code. The compiler stores certain results in the registers. If you modify the registers in an assembler statement, the compiler should, sometimes, be told about it. The registers are denoted with their Intel names for the I386 processor, i.e., 'EAX', 'ESI' etc... As an example, consider the following assembler code: asmMovl $1,%ebx Movl $0,%eax addl %eax,%ebx end; ['EAX','EBX']; This will tell the compiler that it should save and restore the contents of the EAX and EBX registers when it encounters this asm statement. 72 Chapter 8 Using functions and procedures Free Pascal supports the use of functions and procedures, but with some extras: Function overloading is supported, as well as Const parameters and open arrays. Remark: In many of the subsequent paragraphs the words procedure and function will be used interchangeably. The statements made are valid for both, except when indicated otherwise. 8.1 Procedure declaration A procedure declaration defines an identifier and associates it with a block of code. The procedure can then be called with a procedure statement. Procedure declaration - - procedure declaration procedure header ; subroutine block ; - - - procedure header procedure identifier - qualified method identifier - formal parameter list - modifiers - - subroutine block block - external directive asm block forward See section 8.3, page 74 for the list of parameters. A procedure declaration that is followed by a block implements the action of the procedure in that block. The following is a valid procedure : Procedure DoSomething (Para : String); begin Writeln ('Got parameter : ',Para); 73 8.2. FUNCTION DECLARATION Writeln ('Parameter in upper case : ',Upper(Para)); end; Note that it is possible that a procedure calls itself. 8.2 Function declaration A function declaration defines an identifier and associates it with a block of code. The block of code will return a result. The function can then be called inside an expression, or with a procedure statement, if extended syntax is on. Function declaration - - function declaration function header ; subroutine block ; - - - function header function identifier - qualified method identifier - formal parameter list : result type - modifiers - - subroutine block block - external directive asm block forward The result type of a function can be any previously declared type. contrary to Turbo pascal, where only simple types could be returned. 8.3 Parameter lists When you need to pass arguments to a function or procedure, these parameters must be declared in the formal parameter list of that function or procedure. The parameter list is a declaration of identifiers that can be referred to only in that procedure or function's block. Parameters - - formal parameter list ( parameter declaration ) - 6 ; - - parameter declaration value parameter - variable parameter constant parameter Constant parameters and variable parameters can also be untyped parameters if they have no type identifier. 74 8.3. PARAMETER LISTS Value parameters Value parameters are declared as follows: Value parameters - - value parameter identifier list : parameter type - array of When you declare parameters as value parameters, the procedure gets a copy of the parameters that the calling block passes. Any modifications to these parameters are purely local to the procedure's block, and do not propagate back to the calling block. A block that wishes to call a procedure with value parameters must pass assignment compatible parameters to the procedure. This means that the types should not match exactly, but can be converted (conversion code is inserted by the compiler itself) Take care that using value parameters makes heavy use of the stack, especially if you pass large parameters. The total size of all parameters in the formal parameter list should be below 32K for portability's sake (the Intel version limits this to 64K). You can pass open arrays as value parameters. See section 8.3, page 76 for more information on using open arrays. Variable parameters Variable parameters are declared as follows: Variable parameters - - variable parameter var identifier list - : parameter type array of - - When you declare parameters as variable parameters, the procedure or function accesses immediatly the variable that the calling block passed in its parameter list. The procedure gets a pointer to the variable that was passed, and uses this pointer to access the variable's value. From this, it follows that any changes that you make to the parameter, will proagate back to the calling block. This mechanism can be used to pass values back in procedures. Because of this, the calling block must pass a parameter of exactly the same type as the declared parameter's type. If it does not, the compiler will generate an error. Variable parameters can be untyped. In that case the variable has no type, and hence is incompatible with all other types. However, you can use the address operator on it, or you can pass it to a function that has also an untyped parameter. If you want to use an untyped parameter in an assigment, or you want to assign to it, you must use a typecast. File type variables must always be passed as variable parameters. You can pass open arrays as variable parameters. See section 8.3, page 76 for more information on using open arrays. 75 8.3. PARAMETER LISTS Constant parameters In addition to variable parameters and value parameters Free Pascal also supports Constant parameters. You can specify a constant parameter as follows: Constant parameters - - constant parameter const identifier list - - - : parameter type array of A constant argument is passed by reference if it's size is larger than a longint. It is passed by value if the size equals 4 or less. This means that the function or procedure receives a pointer to the passed argument, but you are not allowed to assign to it, this will result in a compiler error. Likewise, you cannot pass a const parameter on to another function that requires a variable parameter. The main use for this is reducing the stack size, hence improving performance, and still retaining the semantics of passing by value... Constant parameters can also be untyped. See section 8.3, page 75 for more infor- mation about untyped parameters. You can pass open arrays as constant parameters. See section 8.3, page 76 for more information on using open arrays. Open array parameters Free Pascal supports the passing of open arrays, i.e. you can declare a procedure with an array of unspecified length as a parameter, as in Delphi. Open array parameters can be accessed in the procedure or function as an array that is declared with starting index 0, and last element index High(paremeter). For example, the parameter Row : Array of Integer; would be equivalent to Row : Array[0..N-1] of Integer; Where N would be the actual size of the array that is passed to the function. N-1 can be calculated as High(Row). Open parameters can be passed by value, by reference or as a constant parameter. In the latter cases the procedure receives a pointer to the actual array. In the former case, it receives a copy of the array. In a function or procedure, you can pass open arrays only to functions which are also declared with open arrays as parameters, not to functions or procedures which accept arrays of fixed length. The following is an example of a function using an open array: Function Average (Row : Array of integer) : Real; Var I : longint; Temp : Real; begin Temp := Row[0]; 76 8.4. FUNCTION OVERLOADING For I := 1 to High(Row) do Temp := Temp + Row[i]; Average := Temp / (High(Row)+1); end; 8.4 Function overloading Function overloading simply means that you can define the same function more than once, but each time with a di erent formal parameter list. The parameter lists must di er at least in one of it's elements type. When the compiler encounters a function call, it will look at the function parameters to decide which one of the defined functions it should call. This can be useful if you want to define the same function for di erent types. For example, in the RTL, the Dec procedure is is defined as: ... Dec(Var I : Longint;decrement : Longint); Dec(Var I : Longint); Dec(Var I : Byte;decrement : Longint); Dec(Var I : Byte); ... When the compiler encounters a call to the dec function, it will first search which function it should use. It therefore checks the parameters in your function call, and looks if there is a function definition which matches the specified parameter list. If the compiler finds such a function, a call is inserted to that function. If no such function is found, a compiler error is generated. You cannot have overloaded functions that have a cdecl or export modifier (Technically, because these two modifiers prevent the mangling of the function name by the compiler). 8.5 Forward defined functions You can define a function without having it followed by it's implementation, by having it followed by the forward procedure. The e ective implementation of that function must follow later in the module. The function can be used after a forward declaration as if it had been implemented already. The following is an example of a forward declaration. Program testforward; Procedure First (n : longint); forward; Procedure Second; begin WriteLn ('In second. Calling first...'); First (1); end; Procedure First (n : longint); begin WriteLn ('First received : ',n); end; begin Second; end. 77 8.6. EXTERNAL FUNCTIONS You cannot define a function twice as forward (nor is there any reason why you would want to do that). Likewise, in units, you cannot have a forward declared function of a function that has been declared in the interface part. The interface declaration counts as a forward declaration. The following unit will give an error when compiled: Unit testforward; interface Procedure First (n : longint); Procedure Second; implementation Procedure First (n : longint); forward; Procedure Second; begin WriteLn ('In second. Calling first...'); First (1); end; Procedure First (n : longint); begin WriteLn ('First received : ',n); end; end. 8.6 External functions The external modifier can be used to declare a function that resides in an external object file. It allows you to use the function in your code, and at linking time, you must link the object file containing the implementation of the function or procedure. External directive - - external directive external - string constant name string constant index integer constant - - It replaces, in e ect, the function or procedure code block. As such, it can be present only in an implementation block of a unit, or in a program. As an example: program CmodDemo; {$Linklib c} Const P : PChar = 'This is fun !'; Function strlen (P : PChar) : Longint; cdecl; external; begin WriteLn ('Length of (',p,') : ',strlen(p)) end. Remark The parameters in our declaration of the external function should match exactly the ones in the declaration in the object file. If the external modifier is followed by a string constant: 78 8.7. ASSEMBLER FUNCTIONS external 'lname'; Then this tells the compiler that the function resides in library 'lname'. The com- piler will then automatically link this library to your program. You can also specify the name that the function has in the library: external 'lname' name Fname; This tells the compiler that the function resides in library 'lname', but with name 'Fname'. The compiler will then automatically link this library to your program, and use the correct name for the function. Under Windows and os/2, you can also use the following form: external 'lname' Index Ind; This tells the compiler that the function resides in library 'lname', but with index Ind. The compiler will then automatically link this library to your program, and use the correct index for the function. 8.7 Assembler functions Functions and procedures can be completely implemented in assembly language. To indicate this, you use the assembler keyword: Assembler functions - - asm block assembler ; declaration part asm statement - Contrary to Delphi, the assembler keyword must be present to indicate an assembler function. For more information about assembler functions, see the chapter on using assembler in the Programmers' guide. 8.8 Modifiers A function or procedure declaration can contain modifiers. Here we list the various possibilities: Modifiers - - modifiers ; public - 6 alias : string constant interrupt call modifiers - - call modifiers register - pascal cdecl stdcall popstack 79 8.8. MODIFIERS Free Pascal doesn't support all Turbo Pascal modifiers, but does support a number of additional modifiers. They are used mainly for assembler and reference to C object files. More on the use of modifiers can be found in the Programmers' guide. Public The Public keyword is used to declare a function globally in a unit. This is useful if you don't want a function to be accessible from the unit file, but you do want the function to be accessible from the object file. as an example: Unit someunit; interface Function First : Real; Implementation Function First : Real; begin First := 0; end; Function Second : Real; [Public]; begin Second := 1; end; end. If another program or unit uses this unit, it will not be able to use the function Second, since it isn't declared in the interface part. However, it will be possible to access the function Second at the assembly-language level, by using it's mangled name (see the Programmers' guide). cdecl The cdecl modifier can be used to declare a function that uses a C type calling convention. This must be used if you wish to acces functions in an object file generated by a C compiler. It allows you to use the function in your code, and at linking time, you must link the object file containing the C implementation of the function or procedure. As an example: program CmodDemo; {$LINKLIB c} Const P : PChar = 'This is fun !'; Function strlen (P : PChar) : Longint; cdecl; external; begin WriteLn ('Length of (',p,') : ',strlen(p)) end. When compiling this, and linking to the C-library, you will be able to call the strlen function throughout your program. The external directive tells the compiler that the function resides in an external object filebrary (see 8.6). Remark The parameters in our declaration of the C function should match exactly the ones in the declaration in C. Since C is case sensitive, this means also that the name of the function must be exactly the same. the Free Pascal compiler will use the name exactly as it is typed in the declaration. 80 8.8. MODIFIERS popstack Popstack does the same as cdecl, namely it tells the Free Pascal compiler that a function uses the C calling convention. In di erence with the cdecl modifier, it still mangles the name of the function as it would for a normal pascal function. With popstack you could access functions by their pascal names in a library. Export Sometimes you must provide a callback function for a C library, or you want your routines to be callable from a C program. Since Free Pascal and C use di erent calling schemes for functions and procedures1, the compiler must be told to generate code that can be called from a C routine. This is where the Export modifier comes in. Contrary to the other modifiers, it must be specified separately, as follows: function DoSquare (X : Longint) : Longint; export; begin ... end; The square brackets around the modifier are not allowed in this case. Remark: as of version 0.9.8, Free Pascal supports the Delphi cdecl modifier. This modifier works in the same way as the export modifier. More information about these modifiers can be found in the Programmers' guide, in the section on the calling mechanism and the chapter on linking. StdCall As of version 0.9.8, Free Pascal supports the Delphi stdcall modifier. This modifier does actually nothing, since the Free Pascal compiler by default pushes parameters from right to left on the stack, which is what the modifier does under Delphi (which pushes parameters on the stack from left to right). More information about this modifier can be found in the Programmers' guide, in the section on the calling mechanism and the chapter on linking. Alias The Alias modifier allows you to specify a di erent name for a procedure or func- tion. This is mostly useful for referring to this procedure from assembly language constructs. As an example, consider the following program: Program Aliases; Procedure Printit; [Alias : 'DOIT']; begin WriteLn ('In Printit (alias : "DOIT")'); end; begin asm call DOIT end; end. 1More techically: In C the calling procedure must clear the stack. In Free Pascal, the subroutine clears the stack. 81 8.9. UNSUPPORTED TURBO PASCAL MODIFIERS Table 8.1: Unsupported modifiers Modifier Why not supported ? Near Free Pascal is a 32-bit compiler. Far Free Pascal is a 32-bit compiler. Remark: the specified alias is inserted straight into the assembly code, thus it is case sensitive. The Alias modifier, combined with the Public modifier, make a powerful tool for making externally accessible object files. 8.9 Unsupported Turbo Pascal modifiers The modifiers that exist in Turbo pascal, but aren't supported by Free Pascal, are listed in table (8.1). 82 Chapter 9 Programs, units, blocks A Pascal program consists of modules called units. A unit can be used to group pieces of code together, or to give someone code without giving the sources. Both programs and units consist of code blocks, which are mixtures of statements, pro- cedures, and variable or type declarations. 9.1 Programs A pascal program consists of the program header, followed possibly by a 'uses' clause, and a block. Programs - - program program header ; block . - uses clause - - program header program identifier - ( program parameters ) - - - - program parameters identifier list - - - uses clause uses identifier ; - 6 , The program header is provided for backwards compatibility, and is ignored by the compiler. The uses clause serves to identify all units that are needed by the program. The system unit doesn't have to be in this list, since it is always loaded by the compiler. The order in which the units appear is significant, it determines in which order they are initialized. Units are initialized in the same order as they appear in the uses clause. Identifiers are searched in the opposite order, i.e. when the compiler searches for an identifier, then it looks first in the last unit in the uses clause, then the last but one, and so on. This is important in case two units declare di erent types with the same identifier. When the compiler looks for unit files, it adds the extension .ppu (.ppw for Win32 platforms) to the name of the unit. On linux, unit names are converted to all lowercase when looking for a unit. 83 9.2. UNITS If a unit name is longer than 8 characters, the compiler will first look for a unit name with this length, and then it will truncate the name to 8 characters and look for it again. For compatibility reasons, this is also true on platforms that suport long file names. 9.2 Units A unit contains a set of declarations, procedures and functions that can be used by a program or another unit. The syntax for a unit is as follows: Units - - unit unit header interface part implementation part - - end . - initialization part finalization part begin statement 6 ; - - unit header unit unit identifier ; - - - interface part interface - 6 constant declaration part type declaration part procedure headers part - - procedure headers part procedure header ; - function header call modifiers ; - - - - implementation part implementation - uses clause - declaration part - - - initialization part initialization statement - 6 ; - - finalization part finalization statement - 6 ; The interface part declares all identifiers that must be exported from the unit. This can be constant, type or variable identifiers, and also procedure or function identifier declarations. Declarations inside the implementation part are not accessible outside the unit. The implementation must contain a function declaration for each function or procedure that is declared in the interface part. If a function is declared in the interface part, but no declaration of that function is present in the implementation part, then the compiler will give an error. When a program uses a unit (say unitA) and this units uses a second unit, say unitB, then the program depends indirectly also on unitB. This means that the compiler must have access to unitB when trying to compile the program. If the unit is not present at compile time, an error occurs. 84 9.3. BLOCKS Note that the identifiers from a unit on which a program depends indirectly, are not accessible to the program. To have access to the identifiers of a unit, you must put that unit in the uses clause of the program or unit where you want to yuse the identifier. Units can be mutually dependent, that is, they can reference each other in their uses clauses. This is allowed, on the condition that at least one of the references is in the implementation section of the unit. This also holds for indirect mutually dependent units. If it is possible to start from one interface uses clause of a unit, and to return there via uses clauses of interfaces only, then there is circular unit dependence, and the compiler will generate an error. As and example : the following is not allowed: Unit UnitA; interface Uses UnitB; implementation end. Unit UnitB interface Uses UnitA; implementation end. But this is allowed : Unit UnitA; interface Uses UnitB; implementation end. Unit UnitB implementation Uses UnitA; end. Because UnitB uses UnitA only in it's implentation section. In general, it is a bad idea to have circular unit dependencies, even if it is only in implementation sections. 9.3 Blocks Units and programs are made of blocks. A block is made of declarations of labels, constants, types variables and functions or procedures. Blocks can be nested in certain ways, i.e., a procedure or function declaration can have blocks in themselves. A block looks like the following: Blocks - - block declaration part statement part - 85 9.4. SCOPE - - declaration part - 6 label declaration part constant declaration part type declaration part variable declaration part procedure/function declaration part - - label declaration part label label ; - 6 , - - constant declaration part const constant declaration - 6 typed constant declaration - - type declaration part type type declaration - 6 - - variable declaration part var variable declaration - 6 - - procedure/function declaration part procedure declaration - 6 function declaration constructor declaration destructor declaration - - statement part compound statement - Labels that can be used to identify statements in a block are declared in the label declaration part of that block. Each label can only identify one statement. Con- stants that are to be used only in one block should be declared in that block's constant declaration part. Variables that are to be used only in one block should be declared in that block's constant declaration part. Types that are to be used only in one block should be declared in that block's constant declaration part. Lastly, functions and procedures that will be used in that block can be declared in the procedure/function declaration part. After the di erent declaration parts comes the statement part. This contains any actions that the block should execute. All identifiers declared before the statement part can be used in that statement part. 9.4 Scope Identifiers are valid from the point of their declaration until the end of the block in which the declaration occurred. The range where the identifier is known is the scope of the identifier. The exact scope of an identifier depends on the way it was defined. Block scope The scope of a variable declared in the declaration part of a block, is valid from the point of declaration until the end of the block. If a block contains a second block, in which the identfier is redeclared, then inside this block, the second declaration will 86 9.4. SCOPE be valid. Upon leaving the inner block, the first declaration is valid again. Consider the following example: Program Demo; Var X : Real; { X is real variable } Procedure NewDeclaration Var X : Integer; { Redeclare X as integer} begin // X := 1.234; {would give an error when trying to compile} X := 10; { Correct assigment} end; { From here on, X is Real again} begin X := 2.468; end. In this example, inside the procedure, X denotes an integer variable. It has it's own storage space, independent of the variable X outside the procedure. Record scope The field identifiers inside a record definition are valid in the following places: 1. to the end of the record definition. 2. field designators of a variable of the given record type. 3. identifiers inside a With statement that operates on a variable of the given record type. Class scope A component identifier is valid in the following places: 1. From the point of declaration to the end of the class definition. 2. In all descendent types of this class. 3. In all method declaration blocks of this class and descendent classes. 4. In a with statement that operators on a variable of the given class's definition. Note that method designators are also considered identifiers. Unit scope All identifiers in the interface part of a unit are valid from the point of declaration, until the end of the unit. Furthermore, the identifiers are known in programs or units that have the unit in their uses clause. Identifiers from indirectly dependent units are not available. Identifiers declared in the implementation part of a unit are valid from the point of declaration to the end of the unit. The system unit is automatically used in all units and programs. It's identifiers are therefore always known, in each program or unit you make. The rules of unit scope implie that you 87 9.5. LIBRARIES can redefine an identifier of a unit. To have access to an identifier of another unit that was redeclared in the current unit, precede it with that other units name, as in the following example: unit unitA; interface Type MyType = Real; implementation end. Program prog; Uses UnitA; { Redeclaration of MyType} Type MyType = Integer; Var A : Mytype; { Will be Integer } B : UnitA.MyType { Will be real } begin end. This is especially useful if you redeclare the system unit's identifiers. 9.5 Libraries Free Pascal supports making of dynamic libraries (DLLs under Win32 and os/2) trough the use of the Library keyword. A Library is just like a unit or a program: Libraries - - library library header ; block . - uses clause - - library header library identifier - By default, functions and procedures that are declared and implemented in library are not available to a programmer that wishes to use your library. In order to make functions or procedures available from the library, you must export them in an export clause: Exports clause - - exports clause exports exports list ; - - - exports list exports entry - 6 , - - exports entry identifier - index integer constant - - name string constant 88 9.5. LIBRARIES Under Win32, an index clause can be added to an exports entry. an index entry must be a positive number larger or equal than 1. It is best to use low index values, although nothing forces you to do this. Optionally, an exports entry can have a name specifier. If present, the name specifier gives the exact name (case sensitive) of the function in the library. If neither of these constructs is present, the functions or procedures are exported with the exact names as specified in the exports clause. 89 Chapter 10 Exceptions As of version 0.99.7, Free Pascal supports exceptions. Exceptions provide a conve- nient way to program error and error-recovery mechanisms, and are closely related to classes. Exception support is based on 3 constructs: Raise statements. To raise an exeption. This is usually done to signal an error condition. Try ... Except blocks. These block serve to catch exceptions raised within the scope of the block, and to provide exception-recovery code. Try ... Finally blocks. These block serve to force code to be executed irrespective of an exception occurrence or not. They generally serve to clean up memory or close files in case an exception occurs. The compiler generates many implicit Try ... Finally blocks around procedure, to force memory consistence. 10.1 The raise statement The raise statement is as follows: Raise statement - - raise statement - exception instance at address expression This statement will raise an exception. If it is specified, the exception instance must be an initialized instance of a class, which is the raise type. The address exception is optional. If itis not specified, the compiler will provide the address by itself. If the exception instance is omitted, then the current exception is re-raised. This construct can only be used in an exception handling block (see further). Remark: Control never returns after an exception block. The control is transferred to the first try...finally or try...except statement that is encountered when unwinding the stack. If no such statement is found, the Free Pascal Run-Time Library will generate a run-time error 217 (see also section 10.5, page 93). As an example: The following division checks whether the denominator is zero, and if so, raises an exception of type EDivException 90 10.2. THE TRY...EXCEPT STATEMENT Type EDivException = Class(Exception); Function DoDiv (X,Y : Longint) : Integer; begin If Y=0 then Raise EDivException.Create ('Division by Zero would occur'); Result := X Div Y; end; The class Exception is defined in the Sysutils unit of the rtl. (section 10.5, page 93) 10.2 The try...except statement A try...except exception handling block is of the following form : Try..except statement - - try statement try statement list except exceptionhandlers end - - - statement list statement - 6 ; - - exceptionhandlers - exception handler 6 ; else statement list statement list - - exception handler on class type identifier do statement - identifier : If no exception is raised during the execution of the statement list, then all statements in the list will be executed sequentially, and the except block will be skipped, transferring program flow to the statement after the final end. If an exception occurs during the execution of the statement list, the program flow will be transferred to the except block. Statements in the statement list between the place where the exception was raised and the exception block are ignored. In the exception handling block, the type of the exception is checked, and if there is an exception handler where the class type matches the exception object type, or is a parent type of the exception object type, then the statement following the corresponding Do will be executed. The first matching type is used. After the Do block was executed, the program continues after the End statement. The identifier in an exception handling statement is optional, and declares an ex- ception object. It can be used to manipulate the exception object in the exception handling code. The scope of this declaration is the statement block foillowing the Do keyword. If none of the On handlers matches the exception object type, then the statement list after else is executed. If no such list is found, then the exception is automatically re-raised. This process allows to nest try...except blocks. 91 10.3. THE TRY...FINALLY STATEMENT If, on the other hand, the exception was caught, then the exception object is de- stroyed at the end of the exception handling block, before program flow continues. The exception is destroyed through a call to the object's Destroy destructor. As an example, given the previous declaration of the DoDiv function, consider the following TryZ := DoDiv (X,Y); Except On EDivException do Z := 0; end; If Y happens to be zero, then the DoDiv function code will raise an exception. When this happens, program flow is transferred to the except statement, where the Exception handler will set the value of Z to zero. If no exception is raised, then program flow continues past the last end statement. To allow error recovery, the Try ... Finally block is supported. A Try...Finally block ensures that the statements following the Finally keyword are guaranteed to be executed, even if an exception occurs. 10.3 The try...finally statement A Try..Finally statement has the following form: Try...finally statement - - trystatement try statement list finally finally statements end - - - finally statements statementlist - If no exception occurs inside the statement List, then the program runs as if the Try, Finally and End keywords were not present. If, however, an exception occurs, the program flow is immediatly transferred from the point where the excepion was raised to the first statement of the Finally statements. All statements after the finally keyword will be executed, and then the exception will be automatically re-raised. Any statements between the place where the exception was raised and the first statement of the Finally Statements are skipped. As an example consider the following routine: Procedure Doit (Name : string); Var F : Text; begin TryAssign (F,Name); Rewrite (name); ... File handling ... Finally Close(F); end; 92 10.4. EXCEPTION HANDLING NESTING If during the execution of the file handling an execption occurs, then program flow will continue at the close(F) statement, skipping any file operations that might follow between the place where the exception was raised, and the Close statement. If no exception occurred, all file operations will be executed, and the file will be closed at the end. 10.4 Exception handling nesting It is possible to nest Try...Except blocks with Try...Finally blocks. Program flow will be done according to a lifo (last in, first out) principle: The code of the last encountered Try...Except or Try...Finally block will be executed first. If the exception is not caught, or it was a finally statement, program flow will be transferred to the last-but-one block, ad infinitum. If an exception occurs, and there is no exception handler present, then a runerror 217 will be generated. If you use the sysutils unit, a default handler is installed which will show the exception object message, and the address where the exception occurred, after which the program will exit with a Halt instruction. 10.5 Exception classes The sysutils unit contains a great deal of exception handling. It defines the following exception types: Exception = class(TObject) private fmessage : string; fhelpcontext : longint; public constructor create(const msg : string); constructor createres(indent : longint); property helpcontext : longint read fhelpcontext write fhelpcontext; property message : string read fmessage write fmessage; end; ExceptClass = Class of Exception; { mathematical exceptions } EIntError = class(Exception); EDivByZero = class(EIntError); ERangeError = class(EIntError); EIntOverflow = class(EIntError); EMathError = class(Exception); The sysutils unit also installs an exception handler. If an exception is unhandled by any exception handling block, this handler is called by the Run-Time library. Basically, it prints the exception address, and it prints the message of the Exception object, and exits with a exit code of 217. If the exception object is not a descen- dent object of the Exception object, then the class name is printed instead of the exception message. It is recommended to use the Exception object or a descendant class for all raise statements, since then you can use the message field of the exception object. 93 Chapter 11 Using assembler Free Pascal supports the use of assembler in your code, but not inline assembler macros. To have more information on the processor specific assembler syntax and its limitations, see the Programmers' guide. 11.1 Assembler statements The following is an example of assembler inclusion in your code. ... Statements; ... Asmyour asm code here ... end; ... Statements; The assembler instructions between the Asm and end keywords will be inserted in the assembler generated by the compiler. You can still use conditionals in your assembler, the compiler will recognise it, and treat it as any other conditionals. Remark: Before version 0.99.1, Free Pascal did not support reference to variables by their names in the assembler parts of your code. 11.2 Assembler procedures and functions Assembler procedures and functions are declared using the Assembler directive. The Assembler keyword is supported as of version 0.9.7. This permits the code generator to make a number of code generation optimizations. The code generator does not generate any stack frame (entry and exit code for the routine) if it contains no local variables and no parameters. In the case of functions, ordinal values must be returned in the accumulator. In the case of floating point values, these depend on the target processor and emulation options. Remark: From version 0.99.1 to 0.99.5 (excluding FPC 0.99.5a), the Assembler directive did not have the same e ect as in Turbo Pascal, so beware! The stack 94 11.2. ASSEMBLER PROCEDURES AND FUNCTIONS frame would be omitted if there were no local variables, in this case if the assembly routine had any parameters, they would be referenced directly via the stack pointer. This was NOT like Turbo Pascal where the stack frame is only omitted if there are no parameters and no local variables. As stated earlier, starting from version 0.99.5a, Free Pascal now has the same behaviour as Turbo Pascal. 95 Part II Reference : The System unit 96 Chapter 12 The system unit The system unit contains the standard supported functions of Free Pascal. It is the same for all platforms. Basically it is the same as the system unit provided with Borland or Turbo Pascal. Functions are listed in alphabetical order. Arguments of functions or procedures that are optional are put between square brackets. The pre-defined constants and variables are listed in the first section. The second section contains the supported functions and procedures. 12.1 Types, Constants and Variables Types The following integer types are defined in the System unit: shortint = -128..127; Longint = $80000000..$7fffffff; integer = -32768..32767; byte = 0..255; word = 0..65535; And the following pointer types: PChar = ^char; pPChar = ^PChar; For the SetJmp (139) and LongJmp (124) calls, the following jump bufer type is defined (for the I386 processor): jmp_buf = record ebx,esi,edi : Longint; bp,sp,pc : Pointer; end; PJmp_buf = ^jmp_buf; Constants The following constants for file-handling are defined in the system unit: 97 12.1. TYPES, CONSTANTS AND VARIABLES Const fmclosed = $D7B0; fminput = $D7B1; fmoutput = $D7B2; fminout = $D7B3; fmappend = $D7B4; filemode : byte = 2; Further, the following non processor specific general-purpose constants are also defined: const e r r o r a d d r : p o i n t e r = n i l ; e r r o r c o d e : word = 0; { max l e v e l in dumping on e r r o r } max frame dump : word = 20; Remark: Processor specific global constants are named Testxxxx where xxxx represents the processor number (such as Test8086, Test68000), and are used to determine on what generation of processor the program is running on. Variables The following variables are defined and initialized in the system unit: varoutput,input,stderr : text; exitproc : pointer; exitcode : word; stackbottom : Longint; loweststack : Longint; The variables ExitProc, exitcode are used in the Free Pascal exit scheme. It works similarly to the one in Turbo Pascal: When a program halts (be it through the call of the Halt function or Exit or through a run-time error), the exit mechanism checks the value of ExitProc. If this one is non-Nil, it is set to Nil, and the procedure is called. If the exit procedure exits, the value of ExitProc is checked again. If it is non-Nil then the above steps are repeated. So if you want to install your exit procedure, you should save the old value of ExitProc (may be non-Nil, since other units could have set it before you did). In your exit procedure you then restore the value of ExitProc, such that if it was non-Nil the exit-procedure can be called. The ErrorAddr and ExitCode can be used to check for error-conditions. If ErrorAddr is non-Nil, a run-time error has occurred. If so, ExitCode contains the error code. If ErrorAddr is Nil, then ExitCode contains the argument to Halt or 0 if the program terminated normally. ExitCode is always passed to the operating system as the exit-code of your process. Under GO32, the following constants are also defined : const seg0040 = $0040; segA000 = $A000; segB000 = $B000; segB800 = $B800; 98 12.2. FUNCTIONS AND PROCEDURES These constants allow easy access to the bios/screen segment via mem/absolute. 12.2 Functions and Procedures Abs Declaration: Function Abs (X : Every numerical type) : Every numerical type; Description: Abs returns the absolute value of a variable. The result of the function has the same type as its argument, which can be any numerical type. Errors: None. See also: Round (136) Program Example1 ; { Program to demonstrate the Abs f u n c t i o n . } Varr : real ; i : i n t e g e r ; begin r := abs ( - 1 . 0 ) ; { r :=1.0 } i := abs (-21); { i :=21 } end . Addr Declaration: Function Addr (X : Any type) : Pointer; Description: Addr returns a pointer to its argument, which can be any type, or a function or procedure name. The returned pointer isn't typed. The same result can be obtained by the @ operator, which can return a typed pointer (Programmers' guide). Errors: None See also: SizeOf (142) Program Example2 ; { Program to demonstrate the Addr f u n c t i o n . } Const Zero : i n t e g e r = 0; Var p : p o i n t e r ; i : I n t e g e r ; begin p:=Addr( p ) ; { P p o i n t s to i t s e l f } p:=Addr( I ) ; { P p o i n t s to I } p:=Addr( Zero ) ; { P p o i n t s to ' Zero ' } end . 99 12.2. FUNCTIONS AND PROCEDURES Append Declaration: Procedure Append (Var F : Text); Description: Append opens an existing file in append mode. Any data written to F will be appended to the file. If the file didn't exist, it will be created, contrary to the Turbo Pascal implementation of Append, where a file needed to exist in order to be opened by Append. Only text files can be opened in append mode. Errors: If the file can't be created, a run-time error will be generated. See also: Rewrite (135),Close (104), Reset (134) Program Example3 ; { Program to demonstrate the Append f u n c t i o n . } Var f : text ; begin Assign ( f , ' t e s t . txt ' ) ; Rewrite ( f ) ; { f i l e i s opened fo r w r i t e , and emptied } Writeln ( F, ' This i s the f i r s t l i n e of text . txt ' ) ; c l o s e ( f ) ; Append( f ) ; { f i l e i s opened f o r w r i t e , but NOT emptied . any text w r i t t e n to i t i s appended .} Writeln ( f , ' This i s the second l i n e of text . txt ' ) ; c l o s e ( f ) ; end . Arctan Declaration: Function Arctan (X : Real) : Real; Description: Arctan returns the Arctangent of X, which can be any Real type. The resulting angle is in radial units. Errors: None See also: Sin (141), Cos (107) Program Example4 ; { Program to demonstrate the ArcTan f u n c t i o n . } Var R : Real ; begin R:=ArcTan ( 0 ) ; { R:=0 } R:=ArcTan (1)/ pi ; { R:=0.25 } end . Assign Declaration: Procedure Assign (Var F; Name : String); 100 12.2. FUNCTIONS AND PROCEDURES Description: Assign assigns a name to F, which can be any file type. This call doesn't open the file, it just assigns a name to a file variable, and marks the file as closed. Errors: None. See also: Reset (134), Rewrite (135), Append (100) Program Example5 ; { Program to demonstrate the Assign f u n c t i o n . } Var F : text ; begin Assign ( F, ' ' ) ; Rewrite ( f ) ; { The f o l l o w i n g can be put in any f i l e by r e d i r e c t i n g i t from the command l i n e .} Writeln ( f , ' This goes to standard output ! ' ) ; Close ( f ) ; Assign ( F, ' Test . txt ' ) ; rewrite ( f ) ; writeln ( f , ' This doesn ' ' t go to standard output ! ' ) ; c l o s e ( f ) ; end . Assigned Declaration: Function Assigned (P : Pointer) : Boolean; Description: Assigned returns True if P is non-nil and retuns False of P is nil. The main use of Assigned is that Procedural variables, method variables and class-type variables also can be passed to Assigned. Errors: None See also: New (127) BinStr Declaration: Function BinStr Value : longint; cnt : byte) : String; Description: BinStr returns a string with the binary representation of Value. The string has at most cnt characters. (i.e. only the cnt rightmost bits are taken into account) To have a complete representation of any longint-type value, you need 32 bits, i.e. cnt=32 Errors: None. See also: Str (144),Val (146),HexStr (118) Program example81 ; { Program to demonstrate the BinStr f u n c t i o n } 101 12.2. FUNCTIONS AND PROCEDURES Const Value = 45678; Var I : l o n g i n t ; begin For I :=8 to 20 do Writeln ( BinStr ( Value , I ) : 2 0 ) ; end . Blockread Declaration: Procedure Blockread (Var F : File; Var Buffer; Var Count : Longint [; var Result : Longint]); Description: Blockread reads count or less records from file F. A record is a block of bytes with size specified by the Rewrite (135) or Reset (134) statement. The result is placed in Buffer, which must contain enough room for Count records. The function cannot read partial records. If Result is specified, it contains the number of records actually read. If Result isn't specified, and less than Count records were read, a run-time error is generated. This behavior can be controlled by the {$i} switch. Errors: If Result isn't specified, then a run-time error is generated if less than count records were read. See also: Blockwrite (103), Close (104), Reset (134), Assign (100) Program Example6 ; { Program to demonstrate the BlockRead and BlockWrite f u n c t i o n s . } Var Fin , fout : File ; NumRead, NumWritten : Word ; Buf : Array [ 1 . . 2 0 4 8 ] of byte ; Total : Longint ; begin Assign ( Fin , Paramstr ( 1 ) ) ; Assign ( Fout , Paramstr ( 2 ) ) ; Reset ( Fin , 1 ) ; Rewrite ( Fout , 1 ) ; Total :=0; Repeat BlockRead ( Fin , buf , Sizeof ( buf ) , NumRead ) ; BlockWrite ( Fout , Buf , NumRead, NumWritten ) ; inc ( Total , NumWritten ) ; Until ( NumRead=0) or ( NumWritten<>NumRead ) ; Write ( ' Copied ' , Total , ' bytes from f i l e ' , paramstr ( 1 ) ) ; Writeln ( ' to f i l e ' , paramstr ( 2 ) ) ; c l o s e ( f i n ) ; c l o s e ( fout ) ; end . 102 12.2. FUNCTIONS AND PROCEDURES Blockwrite Declaration: Procedure Blockwrite (Var F : File; Var Buffer; Var Count : Longint); Description: BlockWrite writes count records from buffer to the file F.A record is a block of bytes with size specified by the Rewrite (135) or Reset (134) statement. If the records couldn't be written to disk, a run-time error is generated. This behavior can be controlled by the {$i} switch. Errors: A run-time error is generated if, for some reason, the records couldn't be written to disk. See also: Blockread (102),Close (104), Rewrite (135), Assign (100) For the example, see Blockread (102). Break Declaration: Procedure Break; Description: Break jumps to the statement following the end of the current repetitive statement. The code between the Break call and the end of the repetitive statement is skipped. The condition of the repetitive statement is NOT evaluated. This can be used with For, varrepeat and While statements. Note that while this is a procedure, Break is a reserved word and hence cannot be redefined. Errors: None. See also: Continue (105), Exit (111) Program Example87 ; { Program to demonstrate the Break f u n c t i o n . } Var I : l o n g i n t ; begin I :=0; While I <10 Do begin Inc ( I ) ; I f I >5 Then Break ; Writeln ( i ) ; end ; I :=0; Repeat Inc ( I ) ; I f I >5 Then Break ; Writeln ( i ) ; Until I >=10; For I :=1 to 10 do begin 103 12.2. FUNCTIONS AND PROCEDURES I f I >5 Then Break ; Writeln ( i ) ; end ; end . Chdir Declaration: Procedure Chdir (const S : string); Description: Chdir changes the working directory of the process to S. Errors: If the directory S doesn't exist, a run-time error is generated. See also: Mkdir (127), Rmdir (136) Program Example7 ; { Program to demonstrate the ChDir f u n c t i o n . } begin { $I -} ChDir ( ParamStr ( 1 ) ) ; i f IOresult <>0 then Writeln ( ' Cannot change to d i r e c t o r y : ' , paramstr ( 1 ) ) ; end . Chr Declaration: Function Chr (X : byte) : Char; Description: Chr returns the character which has ASCII value X. Errors: None. See also: Ord (128), Str (144) Program Example8 ; { Program to demonstrate the Chr f u n c t i o n . } begin Write ( chr ( 1 0 ) , chr ( 1 3 ) ) ; { The same e f f e c t as Writeln ; } end . Close Declaration: Procedure Close (Var F : Anyfiletype); Description: Close flushes the bu er of the file F and closes F. After a call to Close, data can no longer be read from or written to F. To reopen a file closed with Close, it isn't necessary to assign the file again. A call to Reset (134) or Rewrite (135) is su cient. Errors: None. 104 12.2. FUNCTIONS AND PROCEDURES See also: Assign (100), Reset (134), Rewrite (135), Flush (115) Program Example9 ; { Program to demonstrate the Close f u n c t i o n . } Var F : text ; begin Assign ( f , ' Test . txt ' ) ; ReWrite ( F ) ; Writeln ( F, ' Some text w r i t t e n to Test . txt ' ) ; c l o s e ( f ) ; { Flushes contents of b u f f e r to disk , c l o s e s the f i l e . Omitting t h i s may cause data NOT to be w r i t t e n to disk .} end . Concat Declaration: Function Concat (S1,S2 [,S3, ... ,Sn]) : String; Description: Concat concatenates the strings S1,S2 etc. to one long string. The resulting string is truncated at a length of 255 bytes. The same operation can be performed with the + operation. Errors: None. See also: Copy (106), Delete (108), Insert (120), Pos (130), Length (122) Program Example10 ; { Program to demonstrate the Concat f u n c t i o n . } VarS : String ; begin S:=Concat ( ' This can be done ' , ' E a s i e r ' , ' with the + o p e r a t o r ! ' ) ; end . Continue Declaration: Procedure Continue; Description: Continue jumps to the end of the current repetitive statement. The code between the Continue call and the end of the repetitive statement is skipped. The condition of the repetitive statement is then checked again. This can be used with For, varrepeat and While statements. Note that while this is a procedure, Continue is a reserved word and hence cannot be redefined. Errors: None. See also: Break (103), Exit (111) 105 12.2. FUNCTIONS AND PROCEDURES Program Example86 ; { Program to demonstrate the Continue f u n c t i o n . } Var I : l o n g i n t ; begin I :=0; While I <10 Do begin Inc ( I ) ; I f I <5 Then Continue ; Writeln ( i ) ; end ; I :=0; Repeat Inc ( I ) ; I f I <5 Then Continue ; Writeln ( i ) ; Until I >=10; For I :=1 to 10 do begin I f I <5 Then Continue ; Writeln ( i ) ; end ; end . Copy Declaration: Function Copy (Const S : String;Index : Integer;Count : Byte) : String; Description: Copy returns a string which is a copy if the Count characters in S, starting at position Index. If Count is larger than the length of the string S, the result is truncated. If Index is larger than the length of the string S, then an empty string is returned. Errors: None. See also: Delete (108), Insert (120), Pos (130) Program Example11 ; { Program to demonstrate the Copy f u n c t i o n . } Var S , T : String ; begin T:= '1234567' ; S:=Copy ( T, 1 , 2 ) ; { S:= '12 ' } S:=Copy ( T, 4 , 2 ) ; { S:= '45 ' } S:=Copy ( T, 4 , 8 ) ; { S:= '4567 ' } end . 106 12.2. FUNCTIONS AND PROCEDURES Cos Declaration: Function Cos (X : Real) : Real; Description: Cos returns the cosine of X, where X is an angle, in radians. Errors: None. See also: Arctan (100), Sin (141) Program Example12 ; { Program to demonstrate the Cos f u n c t i o n . } Var R : Real ; begin R:=Cos( Pi ) ; { R:=-1 } R:=Cos( Pi / 2 ) ; { R:=0 } R:=Cos ( 0 ) ; { R:=1 } end . CSeg Declaration: Function CSeg : Word; Description: CSeg returns the Code segment register. In Free Pascal, it returns always a zero, since Free Pascal is a 32 bit compiler. Errors: None. See also: DSeg (109), Seg (139), Ofs (128), Ptr (131) Program Example13 ; { Program to demonstrate the CSeg f u n c t i o n . } var W : word ; begin W:=CSeg ; {W:=0, provided f o r c o m p a t i b i l i t y , FPC i s 32 b i t .} end . Dec Declaration: Procedure Dec (Var X : Any ordinal type[; Decrement : Longint]); Description: Dec decreases the value of X with Decrement. If Decrement isn't specified, then 1 is taken as a default. Errors: A range check can occur, or an underflow error, if you try to decrease X below its minimum value. See also: Inc (120) 107 12.2. FUNCTIONS AND PROCEDURES Program Example14 ; { Program to demonstrate the Dec f u n c t i o n . } VarI : Integer ; L : Longint ; W : Word ; B : Byte ; Si : S h o r t I n t ; begin I :=1; L :=2; W:=3; B:=4; Si :=5; Dec ( i ) ; { i :=0 } Dec ( L , 2 ) ; { L:=0 } Dec (W, 2 ) ; { W:=1 } Dec ( B, - 2 ) ; { B:=6 } Dec ( Si , 0 ) ; { Si :=5 } end . Delete Declaration: Procedure Delete (var S : string;Index : Integer;Count : Integer); Description: Delete removes Count characters from string S, starting at position Index. All characters after the delected characters are shifted Count positions to the left, and the length of the string is adjusted. Errors: None. See also: Copy (106),Pos (130),Insert (120) Program Example15 ; { Program to demonstrate the Delete f u n c t i o n . } VarS : String ; begin S:= ' This i s not easy ! ' ; Delete ( S , 9 , 4 ) ; { S:= ' This i s easy ! ' } end . Dispose Declaration: Procedure Dispose (P : pointer); Procedure Dispiose (P : Typed Pointer; Des : Procedure); 108 12.2. FUNCTIONS AND PROCEDURES Description: The first form Dispose releases the memory allocated with a call to New (127). The pointer P must be typed. The released memory is returned to the heap. The second form of Dispose accepts as a first parameter a pointer to an object type, and as a second parameter the name of a destructor of this object. The destructor will be called, and the memory allocated for the object will be freed. Errors: An error will occur if the pointer doesn't point to a location in the heap. See also: New (127), Getmem (117), Freemem (116) Program Example16 ; { Program to demonstrate the Dispose and New f u n c t i o n s . } Type SS = String [ 2 0 ] ; AnObj = Object I : i n t e g e r ; Constructor I n i t ; Destructor Done ; end ; VarP : SS; T : AnObj ; Constructor Anobj . I n i t ; begin Writeln ( ' I n i t i a l i z i n g an i n s t a n c e of AnObj ! ' ) ; end ; Destructor AnObj . Done ; begin Writeln ( ' Destroying an i n s t a n c e of AnObj ! ' ) ; end ; begin New ( P) ; P := ' Hello , World ! ' ; Dispose ( P) ; { P i s undefined from here on !} New(T, I n i t ) ; T . i :=0; Dispose ( T, Done ) ; end . DSeg Declaration: Function DSeg : Word; Description: DSeg returns the data segment register. In Free Pascal, it returns always a zero, since Free Pascal is a 32 bit compiler. 109 12.2. FUNCTIONS AND PROCEDURES Errors: None. See also: CSeg (107), Seg (139), Ofs (128), Ptr (131) Program Example17 ; { Program to demonstrate the DSeg f u n c t i o n . } VarW : Word; begin W:=DSeg; {W:=0, This f u n c t i o n i s provided f o r c o m p a t i b i l i t y , FPC i s a 32 b i t c o m i l e r .} end . Eof Declaration: Function Eof [(F : Any file type)] : Boolean; Description: Eof returns True if the file-pointer has reached the end of the file, or if the file is empty. In all other cases Eof returns False. If no file F is specified, standard input is assumed. Errors: None. See also: Eoln (110), Assign (100), Reset (134), Rewrite (135) Program Example18 ; { Program to demonstrate the Eof f u n c t i o n . } Var T1, T2 : text ; C : Char ; begin { Set f i l e to read from . Empty means from standard input .} a s s i g n ( t1 , paramstr ( 1 ) ) ; reset ( t1 ) ; { Set f i l e to w r i t e to . Empty means to standard output . } a s s i g n ( t2 , paramstr ( 2 ) ) ; rewrite ( t2 ) ; While not eof ( t1 ) do begin read ( t1 , C) ; write ( t2 , C) ; end ; Close ( t1 ) ; Close ( t2 ) ; end . Eoln Declaration: Function Eoln [(F : Text)] : Boolean; 110 12.2. FUNCTIONS AND PROCEDURES Description: Eof returns True if the file pointer has reached the end of a line, which is demar- cated by a line-feed character (ASCII value 10), or if the end of the file is reached. In all other cases Eof returns False. If no file F is specified, standard input is assumed. It can only be used on files of type Text. Errors: None. See also: Eof (110), Assign (100), Reset (134), Rewrite (135) Program Example19 ; { Program to demonstrate the Eoln f u n c t i o n . } begin { This program waits f o r keyboard input . } { I t w i l l p r i n t True when an empty l i n e i s put in , and f a l s e when you type a non-empty l i n e . I t w i l l only stop when you p r e s s enter .} Writeln ( eoln ) ; end . Erase Declaration: Procedure Erase (Var F : Any file type); Description: Erase removes an unopened file from disk. The file should be assigned with Assign, but not opened with Reset or Rewrite Errors: A run-time error will be generated if the specified file doesn't exist, or is opened by the program. See also: Assign (100) Program Example20 ; { Program to demonstrate the Erase f u n c t i o n . } Var F : Text ; begin { Create a f i l e with a l i n e of text in i t } Assign ( F, ' t e s t . txt ' ) ; Rewrite ( F ) ; Writeln ( F, ' Try and f i n d t h i s when I ' ' m f i n i s h e d ! ' ) ; c l o s e ( f ) ; { Now remove the f i l e } Erase ( f ) ; end . Exit Declaration: Procedure Exit ([Var X : return type )]; 111 12.2. FUNCTIONS AND PROCEDURES Description: Exit exits the current subroutine, and returns control to the calling routine. If invoked in the main program routine, exit stops the program. The optional argu- ment X allows to specify a return value, in the case Exit is invoked in a function. The function result will then be equal to X. Errors: None. See also: Halt (117) Program Example21 ; { Program to demonstrate the Exit f u n c t i o n . } Procedure DoAnExit ( Yes : Boolean ) ; { This procedure demonstrates the normal Exit } begin Writeln ( ' Hello from DoAnExit ! ' ) ; I f Yes then begin Writeln ( ' B a i l i n g out e a r l y . ' ) ; exit ; end ; Writeln ( ' Continuing to the end . ' ) ; end ; Function P o s i t i v e ( Which : I n t e g e r ) : Boolean ; { This f u n c t i o n demonstrates the e x t r a FPC f e a t u r e of Exit : You can s p e c i f y a r e t u r n value f o r the f u n c t i o n } begin i f Which >0 then exit ( True ) else exit ( False ) ; end ; begin { This c a l l w i l l go to the end } DoAnExit ( False ) ; { This c a l l w i l l b a i l out e a r l y } DoAnExit ( True ) ; i f P o s i t i v e (-1) then Writeln ( ' The compiler i s nuts , -1 i s not p o s i t i v e . ' ) else Writeln ( ' The compiler i s not so bad , -1 seems to be n e g a t i v e . ' ) ; end . Exp Declaration: Function Exp (Var X : Real) : Real; 112 12.2. FUNCTIONS AND PROCEDURES Description: Exp returns the exponent of X, i.e. the number e to the power X. Errors: None. See also: Ln (123), Power (131) Program Example22 ; { Program to demonstrate the Exp f u n c t i o n . } begin Writeln ( Exp ( 1 ) : 8 : 2 ) ; { Should p r i n t 2 . 7 2 } end . Filepos Declaration: Function Filepos (Var F : Any file type) : Longint; Description: Filepos returns the current record position of the file-pointer in file F. It cannot be invoked with a file of type Text. If you try to do this, a compiler error will be generated. Errors: None. See also: Filesize (114) Program Example23 ; { Program to demonstrate the F i l e P o s f u n c t i o n . } Var F : File of Longint ; L , FP : l o n g i n t ; begin { F i l l a f i l e with data : Each p o s i t i o n c o n t a i n s the p o s i t i o n ! } Assign ( F, ' t e s t . dat ' ) ; Rewrite ( F ) ; For L:=0 to 100 do begin FP:= FilePos ( F ) ; Write ( F, FP) ; end ; Close ( F ) ; Reset ( F ) ; { I f a l l goes w e l l , nothing i s d i s p l a y e d here . } While not ( Eof ( F ) ) do begin FP:= FilePos ( F ) ; Read ( F, L ) ; i f L<>FP then Writeln ( ' Something wrong : Got ' , l , ' on pos ' , FP) ; end ; Close ( F ) ; Erase ( f ) ; end . 113 12.2. FUNCTIONS AND PROCEDURES Filesize Declaration: Function Filesize (Var F : Any file type) : Longint; Description: Filesize returns the total number of records in file F. It cannot be invoked with a file of type Text. (under linux, this also means that it cannot be invoked on pipes.) If F is empty, 0 is returned. Errors: None. See also: Filepos (113) Program Example24 ; { Program to demonstrate the F i l e S i z e f u n c t i o n . } Var F : File Of byte ; L : File Of Longint ; begin Assign ( F, paramstr ( 1 ) ) ; Reset ( F ) ; Writeln ( ' F i l e s i z e in bytes : ' , F i l e S i z e ( F ) ) ; Close ( F ) ; Assign ( L , paramstr ( 1 ) ) ; Reset ( L ) ; Writeln ( ' F i l e s i z e in Longints : ' , F i l e S i z e ( L ) ) ; Close ( f ) ; end . Fillchar Declaration: Procedure Fillchar (Var X;Count : Longint;Value : char or byte);; Description: Fillchar fills the memory starting at X with Count bytes or characters with value equal to Value. Errors: No checking on the size of X is done. See also: Fillword (115), Move (127) Program Example25 ; { Program to demonstrate the F i l l C h a r f u n c t i o n . } Var S : String [ 1 0 ] ; I : Byte ; begin For i :=10 downto 0 do begin { F i l l S with i spaces } FillChar ( S , SizeOf ( S ) , ' ' ) ; { Set Length } SetLength ( S , I ) ; Writeln ( s , ' ' ) ; end ; end . 114 12.2. FUNCTIONS AND PROCEDURES Fillword Declaration: Procedure Fillword (Var X;Count : Longint;Value : Word);; Description: Fillword fills the memory starting at X with Count words with value equal to Value. Errors: No checking on the size of X is done. See also: Fillchar (114), Move (127) Program Example76 ; { Program to demonstrate the FillWord f u n c t i o n . } Var W : Array [ 1 . . 1 0 0 ] of Word ; begin { Quick i n i t i a l i z a t i o n of array W } FillWord (W, 1 0 0 , 0 ) ; end . Flush Declaration: Procedure Flush (Var F : Text); Description: Flush empties the internal bu er of an opened file F and writes the contents to disk. The file is not closed as a result of this call. Errors: If the disk is full, a run-time error will be generated. See also: Close (104) Program Example26 ; { Program to demonstrate the Flush f u n c t i o n . } Var F : Text ; begin { Assign F to standard output } Assign ( F, ' ' ) ; Rewrite ( F ) ; Writeln ( F, ' This l i n e i s w r i t t e n f i r s t , but appears l a t e r ! ' ) ; { At t h i s point the text i s in the i n t e r n a l p a s c a l b u f f e r , and not yet w r i t t e n to standard output } Writeln ( ' This l i n e appears f i r s t , but i s w r i t t e n l a t e r ! ' ) ; { A w r i t e l n to ' output ' always causes a f l u s h - so t h i s text i s w r i t t e n to screen } Flush ( f ) ; { At t h i s point , the text w r i t t e n to F i s w r i t t e n to screen . } Write ( F, ' F i n i s h i n g ' ) ; Close ( f ) ; { C l o s i n g a f i l e always causes a f l u s h f i r s t } Writeln ( ' o f f . ' ) ; end . 115 12.2. FUNCTIONS AND PROCEDURES Frac Declaration: Function Frac (X : Real) : Real; Description: Frac returns the non-integer part of X. Errors: None. See also: Round (136), Int (121) Program Example27 ; { Program to demonstrate the Frac f u n c t i o n . } Var R : Real ; begin Writeln ( Frac ( 1 2 3 . 4 5 6 ) : 0 : 3 ) ; { P r i n t s O.456 } Writeln ( Frac ( - 1 2 3 . 4 5 6 ) : 0 : 3 ) ; { P r i n t s -O.456 } end . Freemem Declaration: Procedure Freemem (Var P : pointer; Count : Longint); Description: Freemem releases the memory occupied by the pointer P, of size Count (in bytes), and returns it to the heap. P should point to the memory allocated to a dynamical variable. Errors: An error will occur when P doesn't point to the heap. See also: Getmem (117), New (127), Dispose (108) Program Example28 ; { Program to demonstrate the FreeMem and GetMem f u n c t i o n s . } Var P : Pointer ; MM : Longint ; begin { Get memory fo r P } MM:=MemAvail ; Writeln ( ' Memory a v a i l a b l e b e f o r e GetMem : ' , MemAvail ) ; GetMem ( P, 8 0 ) ; MM:=MM-Memavail ; Write ( ' Memory a v a i l a b l e a f t e r GetMem : ' , MemAvail ) ; Writeln ( ' or ' ,MM, ' bytes l e s s than b e f o r e the c a l l . ' ) ; { f i l l i t with spaces } FillChar ( P , 8 0 , ' ' ) ; { Free the memory again } FreeMem ( P, 8 0 ) ; Writeln ( ' Memory a v a i l a b l e a f t e r FreeMem : ' , MemAvail ) ; end . 116 12.2. FUNCTIONS AND PROCEDURES Getdir Declaration: Procedure Getdir (drivenr : byte;var dir : string); Description: Getdir returns in dir the current directory on the drive drivenr, where drivenr is 1 for the first floppy drive, 3 for the first hard disk etc. A value of 0 returns the directory on the current disk. On linux, drivenr is ignored, as there is only one directory tree. Errors: An error is returned under dos, if the drive requested isn't ready. See also: Chdir (104) Program Example29 ; { Program to demonstrate the GetDir f u n c t i o n . } Var S : String ; begin GetDir ( 0 , S ) ; Writeln ( ' Current d i r e c t o r y i s : ' , S ) ; end . Getmem Declaration: Procedure Getmem (var p : pointer;size : Longint); Description: Getmem reserves Size bytes memory on the heap, and returns a pointer to this memory in p. If no more memory is available, nil is returned. Errors: None. See also: Freemem (116), Dispose (108), New (127) For an example, see Freemem (116). Halt Declaration: Procedure Halt [(Errnum : byte)]; Description: Halt stops program execution and returns control to the calling program. The optional argument Errnum specifies an exit value. If omitted, zero is returned. Errors: None. See also: Exit (111) Program Example30 ; { Program to demonstrate the Halt f u n c t i o n . } begin Writeln ( ' Before Halt . ' ) ; Halt ( 1 ) ; { Stop with e x i t code 1 } Writeln ( ' After Halt doesn ' ' t get executed . ' ) ; end . 117 12.2. FUNCTIONS AND PROCEDURES HexStr Declaration: Function HexStr (Value : longint; cnt : byte) : String; Description: HexStr returns a string with the hexadecimal representation of Value. The string has at most cnt charaters. (i.e. only the cnt rightmost nibbles are taken into account) To have a complete representation of a Longint-type value, you need 8 nibbles, i.e. cnt=8. Errors: None. See also: Str (144), Val (146), BinStr (101) Program example81 ; { Program to demonstrate the HexStr f u n c t i o n } Const Value = 45678; Var I : l o n g i n t ; begin For I :=1 to 10 do Writeln ( HexStr ( Value , I ) ) ; end . Hi Declaration: Function Hi (X : Ordinal type) : Word or byte; Description: Hi returns the high byte or word from X, depending on the size of X. If the size of X is 4, then the high word is returned. If the size is 2 then the high byte is returned. Hi cannot be invoked on types of size 1, such as byte or char. Errors: None See also: Lo (123) Program Example31 ; { Program to demonstrate the Hi f u n c t i o n . } varL : Longint ; W : Word ; begin L:=1 Shl 1 6 ; { = $10000 } W:=1 Shl 8 ; { = $100 } Writeln ( Hi ( L ) ) ; { P r i n t s 1 } Writeln ( Hi (W) ) ; { P r i n t s 1 } end . 118 12.2. FUNCTIONS AND PROCEDURES High Declaration: Function High (Type identifier or variable reference) : Longint; Description: The return value of High depends on it's argument: 1.If the argument is an ordinal type, High returns the lowest value in the range of the given ordinal type. 2.If the argument is an array type or an array type variable then High returns the highest possible value of it's index. 3.If the argument is an open array identifier in a function or procedure, then High returns the highest index of the array, as if the array has a zero-based index. Errors: None. See also: Low (124), Ord (128), Pred (131), Succ (144) Program example80 ; { Example to demonstrate the High and Low f u n c t i o n s . } Type TEnum = ( North , East , South , West ) ; TRange = 1 4 . . 5 5 ; TArray = Array [ 2 . . 1 0 ] of Longint ; Function Average ( Row : Array of Longint ) : Real ; Var I : l o n g i n t ; Temp : Real ; begin Temp := Row [ 0 ] ; For I : = 1 to High ( Row) do Temp := Temp + Row[ i ] ; Average := Temp / ( High ( Row)+1); end ; Var A : TEnum; B : TRange ; C : TArray ; I : l o n g i n t ; begin Writeln ( ' TEnum goes from : ' , Ord(Low(TEnum) ) , ' to ' , Ord( high (TEnum) ) , ' . ' ) ; Writeln ( ' A goes from : ' , Ord(Low(A) ) , ' to ' , Ord( high (A) ) , ' . ' ) ; Writeln ( ' TRange goes from : ' , Ord(Low( TRange ) ) , ' to ' , Ord( high ( TRange ) ) , ' . ' ) ; Writeln ( ' B goes from : ' , Ord(Low(B) ) , ' to ' , Ord( high (B) ) , ' . ' ) ; Writeln ( ' TArray index goes from : ' , Ord(Low( TArray ) ) , ' to ' , Ord( high ( TArray ) ) , ' . ' ) ; Writeln ( ' C index goes from : ' , Low(C) , ' to ' , high (C) , ' . ' ) ; For I :=Low(C) to High (C) do C[ i ]:= I ; Writeln ( ' Average : ' , Average ( c ) ) ; end . 119 12.2. FUNCTIONS AND PROCEDURES Inc Declaration: Procedure Inc (Var X : Any ordinal type[; Increment : Longint]); Description: Inc increases the value of X with Increment. If Increment isn't specified, then 1 is taken as a default. Errors: If range checking is on, then A range check can occur, or an overflow error, if you try to increase X over its maximum value. See also: Dec (107) Program Example32 ; { Program to demonstrate the Inc f u n c t i o n . } Const C : C a r d i n a l = 1; L : Longint = 1; I : I n t e g e r = 1; W : Word = 1; B : Byte = 1; SI : S h o r t I n t = 1; CH : Char = ' A' ; begin Inc ( C ) ; { C:=2 } Inc ( L , 5 ) ; { L:=6 } Inc ( I , - 3 ) ; { I :=-2 } Inc (W, 3 ) ; { W:=4 } Inc ( B, 1 0 0 ) ; { B:=101 } Inc ( SI , - 3) ; { Si :=-2 } Inc ( CH, 1 ) ; { ch := ' B' } end . Insert Declaration: Procedure Insert (Const Source : String;var S : String;Index : Longint); Description: Insert inserts string Source in string S, at position Index, shifting all characters after Index to the right. The resulting string is truncated at 255 characters, if needed. (i.e. for shortstrings) Errors: None. See also: Delete (108), Copy (106), Pos (130) Program Example33 ; { Program to demonstrate the I n s e r t f u n c t i o n . } Var S : String ; begin S:= ' Free Pascal i s d i f f i c u l t to use ! ' ; Insert ( ' NOT ' , S , pos ( ' d i f f i c u l t ' , S ) ) ; 120 12.2. FUNCTIONS AND PROCEDURES writeln ( s ) ; end . Int Declaration: Function Int (X : Real) : Real; Description: Int returns the integer part of any Real X, as a Real. Errors: None. See also: Frac (116), Round (136) Program Example34 ; { Program to demonstrate the Int f u n c t i o n . } begin Writeln ( Int ( 1 2 3 . 4 5 6 ) : 0 : 1 ) ; { P r i n t s 123.0 } Writeln ( Int ( - 1 2 3 . 4 5 6 ) : 0 : 1 ) ; { P r i n t s -123.0 } end . IOresult Declaration: Function IOresult : Word; Description: IOresult contains the result of any input/output call, when the {$i-} compiler directive is active, disabling IO checking. When the flag is read, it is reset to zero. If IOresult is zero, the operation completed successfully. If non-zero, an error occurred. The following errors can occur: dos errors : 2 File not found. 3 Path not found. 4 Too many open files. 5 Access denied. 6 Invalid file handle. 12 Invalid file-access mode. 15 Invalid disk number. 16 Cannot remove current directory. 17 Cannot rename across volumes. I/O errors : 100 Error when reading from disk. 101 Error when writing to disk. 102 File not assigned. 103 File not open. 104 File not opened for input. 105 File not opened for output. 121 12.2. FUNCTIONS AND PROCEDURES 106 Invalid number. Fatal errors : 150 Disk is write protected. 151 Unknown device. 152 Drive not ready. 153 Unknown command. 154 CRC check failed. 155 Invalid drive specified.. 156 Seek error on disk. 157 Invalid media type. 158 Sector not found. 159 Printer out of paper. 160 Error when writing to device. 161 Error when reading from device. 162 Hardware failure. Errors: None. See also: All I/O functions. Program Example35 ; { Program to demonstrate the IOResult f u n c t i o n . } Var F : text ; begin Assign ( f , paramstr ( 1 ) ) ; { $i -} Reset ( f ) ; { $i +} I f IOresult <>0 then writeln ( ' F i l e ' , paramstr ( 1 ) , ' doesn ' ' t e x i s t ' ) else writeln ( ' F i l e ' , paramstr ( 1 ) , ' e x i s t s ' ) ; end . Length Declaration: Function Length (S : String) : Byte; Description: Length returns the length of the string S, which is limited to 255 for shortstrings. If the strings S is empty, 0 is returned. Note: The length of the string S is stored in S[0] for shortstrings only. Ansistrings have their length stored elsewhere, the Length fuction should always be used on ansistrings. Errors: None. See also: Pos (130) 122 12.2. FUNCTIONS AND PROCEDURES Program Example36 ; { Program to demonstrate the Length f u n c t i o n . } Var S : String ; I : I n t e g e r ; begin S:= ' ' ; for i :=1 to 10 do begin S:=S+' ' ; Writeln ( Length ( S ) : 2 , ' : ' , s ) ; end ; end . Ln Declaration: Function Ln (X : Real) : Real; Description: Ln returns the natural logarithm of the Real parameter X. X must be positive. Errors: An run-time error will occur when X is negative. See also: Exp (112), Power (131) Program Example37 ; { Program to demonstrate the Ln f u n c t i o n . } begin Writeln ( Ln ( 1 ) ) ; { P r i n t s 0 } Writeln ( Ln( Exp ( 1 ) ) ) ; { P r i n t s 1 } end . Lo Declaration: Function Lo (O : Word or Longint) : Byte or Word; Description: Lo returns the low byte of its argument if this is of type Integer or Word. It returns the low word of its argument if this is of type Longint or Cardinal. Errors: None. See also: Ord (128), Chr (104), Hi (118) Program Example38 ; { Program to demonstrate the Lo f u n c t i o n . } Var L : Longint ; W : Word ; begin 123 12.2. FUNCTIONS AND PROCEDURES L :=(1 Shl 1 6 ) + ( 1 Shl 4 ) ; { $10010 } Writeln ( Lo( L ) ) ; { P r i n t s 16 } W:=(1 Shl 8 ) + ( 1 Shl 4 ) ; { $110 } Writeln ( Lo(W) ) ; { P r i n t s 16 } end . LongJmp Declaration: Procedure LongJmp (Var env : Jmp Buf; Value : Longint); Description: LongJmp jumps to the adress in the env jmp buf, and resores the registers that were stored in it at the corresponding SetJmp (139) call. In e ect, program flow will continue at the SetJmp call, which will return value instead of 0. If you pas a value equal to zero, it will be converted to 1 before passing it on. The call will not return, so it must be used with extreme care. This can be used for error recovery, for instance when a segmentation fault occurred. Errors: None. See also: SetJmp (139) For an example, see SetJmp (139) Low Declaration: Function Low (Type identifier or variable reference) : Longint; Description: The return value of Low depends on it's argument: 1.If the argument is an ordinal type, Low returns the lowest value in the range of the given ordinal type. 2.If the argument is an array type or an array type variable then Low returns the lowest possible value of it's index. Errors: None. See also: High (119), Ord (128), Pred (131), Succ (144) for an example, see High (119). Lowercase Declaration: Function Lowercase (C : Char or String) : Char or String; Description: Lowercase returns the lowercase version of its argument C. If its argument is a string, then the complete string is converted to lowercase. The type of the returned value is the same as the type of the argument. Errors: None. See also: Upcase (146) 124 12.2. FUNCTIONS AND PROCEDURES Program Example73 ; { Program to demonstrate the Lowercase f u n c t i o n . } Var I : Longint ; begin For i := ord ( ' A' ) to ord ( ' Z' ) do write ( lowercase ( chr ( i ) ) ) ; Writeln ; Writeln ( Lowercase ( ' ABCDEFGHIJKLMNOPQRSTUVWXYZ' ) ) ; end . Mark Declaration: Procedure Mark (Var P : Pointer); Description: Mark copies the current heap-pointer to P. Errors: None. See also: Getmem (117), Freemem (116), New (127), Dispose (108), Maxavail (125) Program Example39 ; { Program to demonstrate the Mark and Release f u n c t i o n s . } Var P, PP, PPP,MM : Pointer ; begin Getmem ( P, 1 0 0 ) ; Mark (MM) ; Writeln ( ' Getmem 10 0 : Memory a v a i l a b l e : ' , MemAvail , ' ( marked ) ' ) ; GetMem ( PP, 1 0 0 0 ) ; Writeln ( ' Getmem 1000 : Memory a v a i l a b l e : ' , MemAvail ) ; GetMem ( PPP, 1 0 0 0 0 0 ) ; Writeln ( ' Getmem 10000 : Memory a v a i l a b l e : ' , MemAvail ) ; Release (MM) ; Writeln ( ' Released : Memory a v a i l a b l e : ' , MemAvail ) ; { At t h i s point , PP and PPP are i n v a l i d ! } end . Maxavail Declaration: Function Maxavail : Longint; Description: Maxavail returns the size, in bytes, of the biggest free memory block in the heap. Remark: The heap grows dynamically if more memory is needed than is available. Errors: None. See also: Release (134), Memavail (126),Freemem (116), Getmem (117) 125 12.2. FUNCTIONS AND PROCEDURES Program Example40 ; { Program to demonstrate the MaxAvail f u n c t i o n . } VarP : Pointer ; I : l o n g i n t ; begin { This w i l l a l l o c a t e memory u n t i l there i s no more memory} I :=0; While MaxAvail >=1000 do begin Inc ( I ) ; GetMem ( P, 1 0 0 0 ) ; end ; { Default 4MB heap i s a l l o c a t e d , so 4000 blocks should be a l l o c a t e d . When compiled with the -Ch10000 switch , the program w i l l be able to a l l o c a t e 10 block } Writeln ( ' A l l o c a t e d ' , i , ' blocks of 1000 bytes ' ) ; end . Memavail Declaration: Function Memavail : Longint; Description: Memavail returns the size, in bytes, of the free heap memory. Remark: The heap grows dynamically if more memory is needed than is available. Errors: None. See also: Maxavail (125),Freemem (116), Getmem (117) Program Example41 ; { Program to demonstrate the MemAvail f u n c t i o n . } VarP, PP : Pointer ; begin GetMem ( P, 1 0 0 ) ; GetMem ( PP, 1 0 0 0 0 ) ; FreeMem ( P, 1 0 0 ) ; { Due to the heap f r a g me n t a t i o n i n t r o d u c e d By the p r e v i o u s c a l l s , the maximum amount of memory i s n ' t equal to the maximum block s i z e a v a i l a b l e . } Writeln ( ' Total heap a v a i l a b l e ( Bytes ) : ' , MemAvail ) ; Writeln ( ' Largest block a v a i l a b l e ( Bytes ) : ' , MaxAvail ) ; end . 126 12.2. FUNCTIONS AND PROCEDURES Mkdir Declaration: Procedure Mkdir (const S : string); Description: Mkdir creates a new directory S. Errors: If a parent-directory of directory S doesn't exist, a run-time error is generated. See also: Chdir (104), Rmdir (136) For an example, see Rmdir (136). Move Declaration: Procedure Move (var Source,Dest;Count : Longint); Description: Move moves Count bytes from Source to Dest. Errors: If either Dest or Source is outside the accessible memory for the process, then a run-time error will be generated. With older versions of the compiler, a segmentation- fault will occur. See also: Fillword (115), Fillchar (114) Program Example42 ; { Program to demonstrate the Move f u n c t i o n . } Var S1 , S2 : String [ 3 0 ] ; begin S1:= ' Hello World ! ' ; S2:= ' Bye , bye ! ' ; Move ( S1 , S2 , Sizeof ( S1 ) ) ; Writeln ( S2 ) ; end . New Declaration: Procedure New (Var P : Pointer[, Constructor]); Description: New allocates a new instance of the type pointed to by P, and puts the address in P. If P is an object, then it is possible to specify the name of the constructor with which the instance will be created. Errors: If not enough memory is available, Nil will be returned. See also: Dispose (108), Freemem (116), Getmem (117), Memavail (126), Maxavail (125) For an example, see Dispose (108). 127 12.2. FUNCTIONS AND PROCEDURES Odd Declaration: Function Odd (X : Longint) : Boolean; Description: Odd returns True if X is odd, or False otherwise. Errors: None. See also: Abs (99), Ord (128) Program Example43 ; { Program to demonstrate the Odd f u n c t i o n . } begin I f Odd( 1 ) Then Writeln ( ' Ev er y th i n g OK with 1 ! ' ) ; I f Not Odd( 2 ) Then Writeln ( ' E v ery t h in g OK with 2 ! ' ) ; end . Ofs Declaration: Function Ofs Var X : Longint; Description: Ofs returns the o set of the address of a variable. This function is only supported for compatibility. In Free Pascal, it returns always the complete address of the variable, since Free Pascal is a 32 bit compiler. Errors: None. See also: DSeg (109), CSeg (107), Seg (139), Ptr (131) Program Example44 ; { Program to demonstrate the Ofs f u n c t i o n . } Var W : Pointer ; begin W:= Pointer ( Ofs (W) ) ; { W c o n t a i n s i t s own o f f s e t . } end . Ord Declaration: Function Ord (X : Any ordinal type) : Longint; Description: Ord returns the Ordinal value of a ordinal-type variable X. Errors: None. See also: Chr (104), Succ (144), Pred (131), High (119), Low (124) 128 12.2. FUNCTIONS AND PROCEDURES Program Example45 ; { Program to demonstrate the Ord , Pred , Succ f u n c t i o n s . } Type TEnum = ( Zero , One , Two, Three , Four ) ; VarX : Longint ; Y : TEnum; begin X:=125; Writeln ( Ord(X) ) ; { P r i n t s 125 } X:=Pred (X) ; Writeln ( Ord(X) ) ; { p r i n t s 124 } Y:= One ; Writeln ( Ord( y ) ) ; { P r i n t s 1 } Y:=Succ (Y) ; Writeln ( Ord(Y) ) ; { P r i n t s 2} end . Paramcount Declaration: Function Paramcount : Longint; Description: Paramcount returns the number of command-line arguments. If no arguments were given to the running program, 0 is returned. Errors: None. See also: Paramstr (129) Program Example46 ; { Program to demonstrate the ParamCount and ParamStr f u n c t i o n s . } VarI : Longint ; begin Writeln ( paramstr ( 0 ) , ' : Got ' , ParamCount , ' command-l i n e parameters : ' ) ; For i :=1 to ParamCount do Writeln ( ParamStr ( i ) ) ; end . Paramstr Declaration: Function Paramstr (L : Longint) : String; Description: Paramstr returns the L-th command-line argument. L must be between 0 and Paramcount, these values included. The zeroth argument is the name with which the program was started. In all cases, the command-line will be truncated to a length of 255, even though the operating system may support bigger command-lines. If you want to access the 129 12.2. FUNCTIONS AND PROCEDURES complete command-line, you must use the argv pointer to access the Real values of the command-line parameters. Errors: None. See also: Paramcount (129) For an example, see Paramcount (129). Pi Declaration: Function Pi : Real; Description: Pi returns the value of Pi (3.1415926535897932385). Errors: None. See also: Cos (107), Sin (141) Program Example47 ; { Program to demonstrate the Pi f u n c t i o n . } begin Writeln ( Pi ) ; {3.1415926} Writeln ( Sin ( Pi ) ) ; end . Pos Declaration: Function Pos (Const Substr : String;Const S : String) : Byte; Description: Pos returns the index of Substr in S, if S contains Substr. In case Substr isn't found, 0 is returned. The search is case-sensitive. Errors: None See also: Length (122), Copy (106), Delete (108), Insert (120) Program Example48 ; { Program to demonstrate the Pos f u n c t i o n . } VarS : String ; begin S:= ' The f i r s t space in t h i s sentence i s at p o s i t i o n : ' ; Writeln ( S , pos ( ' ' , S ) ) ; S:= ' The l a s t l e t t e r of the alphabet doesn ' ' t appear in t h i s sentence ' ; I f ( Pos ( ' Z' , S)=0) and ( Pos( ' z ' , S)=0) then Writeln ( S ) ; end . 130 12.2. FUNCTIONS AND PROCEDURES Power Declaration: Function Power (base,expon : Real) : Real; Description: Power returns the value of base to the power expon. Base and expon can be of type Longint, in which case the result will also be a Longint. The function actually returns Exp(expon*Ln(base)) Errors: None. See also: Exp (112), Ln (123) Program Example78 ; { Program to demonstrate the Power f u n c t i o n . } begin Writeln ( Power ( exp ( 1 . 0 ) , 1 . 0 ) : 8 : 2 ) ; { Should p r i n t 2 . 7 2 } end . Pred Declaration: Function Pred (X : Any ordinal type) : Same type; Description: Pred returns the element that precedes the element that was passed to it. If it is applied to the first value of the ordinal type, and the program was compiled with range checking on ({$R+}, then a run-time error will be generated. Errors: Run-time error 201 is generated when the result is out of range. See also: Ord (128), Pred (131), High (119), Low (124) for an example, see Ord (128) Ptr Declaration: Function Ptr (Sel,Off : Longint) : Pointer; Description: Ptr returns a pointer, pointing to the address specified by segment Sel and o set Off. Remarks: 1.In the 32-bit flat-memory model supported by Free Pascal, this function is obsolete. 2.The returned address is simply the o set. If you recompile the RTL with -dDoMapping defined, then the compiler returns the following : ptr := pointer($e0000000+sel shl 4+off) under dos, or ptr := pointer(sel shl 4+off) on other OSes. Errors: None. See also: Addr (99) 131 12.2. FUNCTIONS AND PROCEDURES Program Example59 ; { Program to demonstrate the Ptr f u n c t i o n . } Var P : String ; S : String ; begin S:= ' Hello , World ! ' ; P:= Ptr ( Seg( S ) , Longint ( Ofs ( S ) ) ) ; {P now p o i n t s to S !} Writeln ( P ); end . Random Declaration: Function Random [(L : Longint)] : Longint or Real; Description: Random returns a random number larger or equal to 0 and strictly less than L. If the argument L is omitted, a Real number between 0 and 1 is returned. (0 included, 1 excluded) Errors: None. See also: Randomize (132) Program Example49 ; { Program to demonstrate the Random and Randomize f u n c t i o n s . } Var I , Count , guess : Longint ; R : Real ; begin Randomize ; { This way we generate a new sequence every time the program i s run } Count :=0; For i :=1 to 1000 do I f Random>0.5 then inc ( Count ) ; Writeln ( ' Generated ' , Count , ' numbers > 0.5' ) ; Writeln ( ' out of 1000 generated numbers . ' ) ; count :=0; For i :=1 to 5 do begin write ( ' Guess a number between 1 and 5 : ' ) ; readln ( Guess ) ; I f Guess=Random(5)+1 then inc ( count ) ; end ; Writeln ( ' You guessed ' , Count , ' out of 5 c o r r e c t . ' ) ; end . Randomize Declaration: Procedure Randomize ; 132 12.2. FUNCTIONS AND PROCEDURES Description: Randomize initializes the random number generator of Free Pascal, by giving a value to Randseed, calculated with the system clock. Errors: None. See also: Random (132) For an example, see Random (132). Read Declaration: Procedure Read ([Var F : Any file type], V1 [, V2, ... , Vn]); Description: Read reads one or more values from a file F, and stores the result in V1, V2, etc.; If no file F is specified, then standard input is read. If F is of type Text, then the variables V1, V2 etc. must be of type Char, Integer, Real, String or PChar. If F is a typed file, then each of the variables must be of the type specified in the declaration of F. Untyped files are not allowed as an argument. Errors: If no data is available, a run-time error is generated. This behavior can be con- trolled with the {$i} compiler switch. See also: Readln (133), Blockread (102), Write (147), Blockwrite (103) Program Example50 ; { Program to demonstrate the Read ( Ln ) f u n c t i o n . } Var S : String ; C : Char ; F : File of char ; begin Assign ( F, ' ex50 . pp ' ) ; Reset ( F ) ; C:= ' A' ; Writeln ( ' The c h a r a c t e r s b e f o r e the f i r s t space in ex50 . pp are : ' ) ; While not Eof ( f ) and ( C<>' ' ) do Begin Read ( F, C) ; Write ( C) ; end ; Writeln ; Close ( F ) ; Writeln ( ' Type some words . An empty l i n e ends the program . ' ) ; repeat Readln ( S ) ; u n t i l S=' ' ; end . Readln Declaration: Procedure Readln [Var F : Text], V1 [, V2, ... , Vn]); 133 12.2. FUNCTIONS AND PROCEDURES Description: Read reads one or more values from a file F, and stores the result in V1, V2, etc. After that it goes to the next line in the file (defined by the LineFeed (#10) character). If no file F is specified, then standard input is read. The variables V1, V2 etc. must be of type Char, Integer, Real, String or PChar. Errors: If no data is available, a run-time error is generated. This behavior can be con- trolled with the {$i} compiler switch. See also: Read (133), Blockread (102), Write (147), Blockwrite (103) For an example, see Read (133). Release Declaration: Procedure Release (Var P : pointer); Description: Release sets the top of the Heap to the location pointed to by P. All memory at a location higher than P is marked empty. Errors: A run-time error will be generated if P points to memory outside the heap. See also: Mark (125), Memavail (126), Maxavail (125), Getmem (117), Freemem (116) New (127), Dispose (108) For an example, see Mark (125). Rename Declaration: Procedure Rename (Var F : Any Filetype; Const S : String); Description: Rename changes the name of the assigned file F to S. F must be assigned, but not opened. Errors: A run-time error will be generated if F isn't assigned, or doesn't exist. See also: Erase (111) Program Example77 ; { Program to demonstrate the Rename f u n c t i o n . } Var F : Text ; begin Assign ( F, paramstr ( 1 ) ) ; Rename ( F, paramstr ( 2 ) ) ; end . Reset Declaration: Procedure Reset (Var F : Any File Type[; L : Longint]); Description: Reset opens a file F for reading. F can be any file type. If F is an untyped or typed file, then it is opened for reading and writing. If F is an untyped file, the record size can be specified in the optional parameter L. Default a value of 128 is used. 134 12.2. FUNCTIONS AND PROCEDURES Errors: If the file cannot be opened for reading, then a run-time error is generated. This behavior can be changed by the {$i} compiler switch. See also: Rewrite (135), Assign (100), Close (104), Append (100) Program Example51 ; { Program to demonstrate the Reset f u n c t i o n . } Function F i l e E x i s t s ( Name : String ) : boolean ; Var F : File ; begin { $i -} Assign ( F, Name) ; Reset ( F ) ; { $I +} F i l e E x i s t s :=( IoResult =0) and ( Name<>' ' ) ; Close ( f ) ; end ; begin I f F i l e E x i s t s ( Paramstr ( 1 ) ) then Writeln ( ' F i l e found ' ) else Writeln ( ' F i l e NOT found ' ) ; end . Rewrite Declaration: Procedure Rewrite (Var F : Any File Type[; L : Longint]); Description: Rewrite opens a file F for writing. F can be any file type. If F is an untyped or typed file, then it is opened for reading and writing. If F is an untyped file, the record size can be specified in the optional parameter L. Default a value of 128 is used. if Rewrite finds a file with the same name as F, this file is truncated to length 0. If it doesn't find such a file, a new file is created. Errors: If the file cannot be opened for writing, then a run-time error is generated. This behavior can be changed by the {$i} compiler switch. See also: Reset (134), Assign (100), Close (104), Flush (115), Append (100) Program Example52 ; { Program to demonstrate the Rewrite f u n c t i o n . } Var F : File ; I : l o n g i n t ; begin Assign ( F, ' Test . dat ' ) ; { Create the f i l e . R e c o r d s i z e i s 4 } 135 12.2. FUNCTIONS AND PROCEDURES Rewrite ( F, Sizeof ( I ) ) ; For I :=1 to 10 do BlockWrite ( F, I , 1 ) ; c l o s e ( f ) ; { F c o n t a i n s now a b i n a ry r e p r e s e n t a t i o n of 10 l o n g i n t s going from 1 to 10 } end . Rmdir Declaration: Procedure Rmdir (const S : string); Description: Rmdir removes the directory S. Errors: If S doesn't exist, or isn't empty, a run-time error is generated. See also: Chdir (104), Mkdir (127) Program Example53 ; { Program to demonstrate the MkDir and RmDir f u n c t i o n s . } Const D : String [ 8 ] = ' TEST. DIR ' ; Var S : String ; begin Writeln ( ' Making d i r e c t o r y ' , D) ; Mkdir ( D) ; Writeln ( ' Changing d i r e c t o r y to ' , D) ; ChDir ( D) ; GetDir ( 0 , S ) ; Writeln ( ' Current D i r e c t o r y i s : ' , S ) ; WRiteln ( ' Going back ' ) ; ChDir ( ' . . ' ) ; Writeln ( ' Removing d i r e c t o r y ' , D) ; RmDir ( D) ; end . Round Declaration: Function Round (X : Real) : Longint; Description: Round rounds X to the closest integer, which may be bigger or smaller than X. Errors: None. See also: Frac (116), Int (121), Trunc (145) Program Example54 ; { Program to demonstrate the Round f u n c t i o n . } begin 136 12.2. FUNCTIONS AND PROCEDURES Writeln ( Round ( 1 2 3 . 4 5 6 ) ) ; { P r i n t s 124 } Writeln ( Round ( - 1 2 3 . 4 5 6 ) ) ; { P r i n t s -124 } Writeln ( Round ( 1 2 . 3 4 5 6 ) ) ; { P r i n t s 12 } Writeln ( Round ( - 1 2 . 3 4 5 6 ) ) ; { P r i n t s -12 } end . Runerror Declaration: Procedure Runerror (ErrorCode : Word); Description: Runerror stops the execution of the program, and generates a run-time error ErrorCode. Errors: None. See also: Exit (111), Halt (117) Program Example55 ; { Program to demonstrate the RunError f u n c t i o n . } begin { The program w i l l stop end emit a run-e r r o r 106 } RunError ( 1 0 6 ) ; end . Seek Declaration: Procedure Seek (Var F; Count : Longint); Description: Seek sets the file-pointer for file F to record Nr. Count. The first record in a file has Count=0. F can be any file type, except Text. If F is an untyped file, with no record size specified in Reset (134) or Rewrite (135), 128 is assumed. Errors: A run-time error is generated if Count points to a position outside the file, or the file isn't opened. See also: Eof (110), SeekEof (138), SeekEoln (138) Program Example56 ; { Program to demonstrate the Seek f u n c t i o n . } VarF : File ; I , j : l o n g i n t ; begin { Create a f i l e and f i l l i t with data } Assign ( F, ' t e s t . dat ' ) ; Rewrite ( F ) ; { Create f i l e } Close ( f ) ; FileMode :=2; ReSet ( F, Sizeof ( i ) ) ; { Opened read / w r i t e } 137 12.2. FUNCTIONS AND PROCEDURES For I :=0 to 10 do BlockWrite ( F, I , 1 ) ; { Go Back to the b e g i n i n g of the f i l e } Seek ( F , 0 ) ; For I :=0 to 10 do begin BlockRead ( F, J , 1 ) ; I f J<>I then Writeln ( ' Error : expected ' , i , ' , got ' , j ) ; end ; Close ( f ) ; end . SeekEof Declaration: Function SeekEof [(Var F : text)] : Boolean; Description: SeekEof returns True is the file-pointer is at the end of the file. It ignores all whitespace. Calling this function has the e ect that the file-position is advanced until the first non-whitespace character or the end-of-file marker is reached. If the end-of-file marker is reached, True is returned. Otherwise, False is returned. If the parameter F is omitted, standard Input is assumed. Errors: A run-time error is generated if the file F isn't opened. See also: Eof (110), SeekEoln (138), Seek (137) Program Example57 ; { Program to demonstrate the SeekEof f u n c t i o n . } Var C : Char ; begin { t h i s w i l l p r i n t a l l c h a r a c t e r s from standard input except Whitespace c h a r a c t e r s . } While Not SeekEof do begin Read ( C) ; Write ( C) ; end ; end . SeekEoln Declaration: Function SeekEoln [(Var F : text)] : Boolean; Description: SeekEoln returns True is the file-pointer is at the end of the current line. It ignores all whitespace. Calling this function has the e ect that the file-position is advanced until the first non-whitespace character or the end-of-line marker is reached. If the end-of-line marker is reached, True is returned. Otherwise, False is returned. The end-of-line marker is defined as #10, the LineFeed character. If the parameter F is omitted, standard Input is assumed. Errors: A run-time error is generated if the file F isn't opened. 138 12.2. FUNCTIONS AND PROCEDURES See also: Eof (110), SeekEof (138), Seek (137) Program Example58 ; { Program to demonstrate the SeekEoln f u n c t i o n . } VarC : Char; begin { This w i l l read the f i r s t l i n e of standard output and p r i n t a l l c h a r a c t e r s except whitespace . } While not SeekEoln do Begin Read ( c ) ; Write ( c ) ; end ; end . Seg Declaration: Function Seg Var X : Longint; Description: Seg returns the segment of the address of a variable. This function is only sup- ported for compatibility. In Free Pascal, it returns always 0, since Free Pascal is a 32 bit compiler, segments have no meaning. Errors: None. See also: DSeg (109), CSeg (107), Ofs (128), Ptr (131) Program Example60 ; { Program to demonstrate the Seg f u n c t i o n . } VarW : Word; begin W:=Seg(W) ; { W c o n t a i n s i t s own Segment } end . SetJmp Declaration: Function SetJmp (Var Env : Jmp Buf) : Longint; Description: SetJmp fills env with the necessary data for a jump back to the point where it was called. It returns zero if called in this way. If the function returns nonzero, then it means that a call to LongJmp (124) with env as an argument was made somewhere in the program. Errors: None. See also: LongJmp (124) 139 12.2. FUNCTIONS AND PROCEDURES program example79 ; { Program to demonstrate the setjmp , longjmp f u n c t i o n s } procedure dojmp ( var env : jmp buf ; value : l o n g i n t ) ; begin value :=2; Writeln ( ' Going to jump ! ' ) ; { This w i l l r e t u r n to the setjmp c a l l , and r e t u r n value i n s t e a d of 0 } longjmp ( env , value ) ; end ; var env : jmp buf ; begin i f setjmp ( env )=0 then begin writeln ( ' Passed f i r s t time . ' ) ; dojmp ( env , 2 ) ; end else writeln ( ' Passed second time . ' ) ; end . SetLength Declaration: Procedure SetLength(var S : String; Len : Longint); Description: SetLength sets the length of the string S to Len. S can be an ansistring or a short string. For ShortStrings, Len can maximally be 255. For AnsiStrings it can have any value. For AnsiString strings, SetLength must be used to set the length of the string. Errors: None. See also: Length (122) Program Example85 ; { Program to demonstrate the SetLength f u n c t i o n . } Var S : String ; begin FillChar ( S [ 1 ] , 1 0 0 , # 3 2 ) ; S e t l e n g t h ( S , 1 0 0 ) ; Writeln ( ' " ' , S , ' " ' ) ; end . SetTextBuf Declaration: Procedure SetTextBuf (Var f : Text; Var Buf[; Size : Word]); 140 12.2. FUNCTIONS AND PROCEDURES Description: SetTextBuf assigns an I/O bu er to a text file. The new bu er is located at Buf and is Size bytes long. If Size is omitted, then SizeOf(Buf) is assumed. The standard bu er of any text file is 128 bytes long. For heavy I/0 operations this may prove too slow. The SetTextBuf procedure allows you to set a bigger bu er for your application, thus reducing the number of system calls, and thus reducing the load on the system resources. The maximum size of the newly assigned bu er is 65355 bytes. Remark 1: Never assign a new bu er to an opened file. You can assign a new bu er immediately after a call to Rewrite (135), Reset (134) or Append, but not after you read from/wrote to the file. This may cause loss of data. If you still want to assign a new bu er after read/write operations have been performed, flush the file first. This will ensure that the current bu er is emptied. Remark 2: Take care that the bu er you assign is always valid. If you assign a local variable as a bu er, then after your program exits the local program block, the bu er will no longer be valid, and stack problems may occur. Errors: No checking on Size is done. See also: Assign (100), Reset (134), Rewrite (135), Append (100) Program Example61 ; { Program to demonstrate the SetTextBuf f u n c t i o n . } VarFin , Fout : Text ; Ch : Char ; Bufin , Bufout : Array [ 1 . . 1 0 0 0 0 ] of byte ; begin Assign ( Fin , paramstr ( 1 ) ) ; Reset ( Fin ) ; Assign ( Fout , paramstr ( 2 ) ) ; Rewrite ( Fout ) ; { This i s harmless b e f o r e IO has begun } { Try t h i s program again on a big f i l e , a f t e r commenting out the f o l l o w i n g 2 l i n e s and r e c o m p i l i n g i t . } SetTextBuf ( Fin , Bufin ) ; SetTextBuf ( Fout , Bufout ) ; While not eof ( Fin ) do begin Read ( Fin , ch ) ; write ( Fout , ch ) ; end ; Close ( Fin ) ; Close ( Fout ) ; end . Sin Declaration: Function Sin (X : Real) : Real; Description: Sin returns the sine of its argument X, where X is an angle in radians. Errors: None. 141 12.2. FUNCTIONS AND PROCEDURES See also: Cos (107), Pi (130), Exp (112), Ln (123) Program Example62 ; { Program to demonstrate the Sin f u n c t i o n . } begin Writeln ( Sin ( Pi ) : 0 : 1 ) ; { P r i n t s 0 . 0 } Writeln ( Sin ( Pi / 2 ) : 0 : 1 ) ; { P r i n t s 1 . 0 } end . SizeOf Declaration: Function SizeOf (X : Any Type) : Longint; Description: SizeOf returns the size, in bytes, of any variable or type-identifier. Remark: this isn't really a RTL function. Its result is calculated at compile-time, and hard-coded in your executable. Errors: None. See also: Addr (99) Program Example63 ; { Program to demonstrate the SizeOf f u n c t i o n . } VarI : Longint ; S : String [ 1 0 ] ; begin Writeln ( SizeOf ( I ) ) ; { P r i n t s 4 } Writeln ( SizeOf ( S ) ) ; { P r i n t s 11 } end . Sptr Declaration: Function Sptr : Pointer; Description: Sptr returns the current stack pointer. Errors: None. See also: SSeg (143) Program Example64 ; { Program to demonstrate the SPtr f u n c t i o n . } VarP : Longint ; begin P:= Sptr ; { P Contains now the c u r r e n t stack p o s i t i o n . } end . 142 12.2. FUNCTIONS AND PROCEDURES Sqr Declaration: Function Sqr (X : Real) : Real; Description: Sqr returns the square of its argument X. Errors: None. See also: Sqrt (143), Ln (123), Exp (112) Program Example65 ; { Program to demonstrate the Sqr f u n c t i o n . } Var i : I n t e g e r ; begin For i :=1 to 10 do writeln ( Sqr ( i ) : 3 ) ; end . Sqrt Declaration: Function Sqrt (X : Real) : Real; Description: Sqrt returns the square root of its argument X, which must be positive. Errors: If X is negative, then a run-time error is generated. See also: Sqr (143), Ln (123), Exp (112) Program Example66 ; { Program to demonstrate the Sqrt f u n c t i o n . } begin Writeln ( Sqrt ( 4 ) : 0 : 3 ) ; { P r i n t s 2 . 0 0 0 } Writeln ( Sqrt ( 2 ) : 0 : 3 ) ; { P r i n t s 1 . 4 1 4 } end . SSeg Declaration: Function SSeg : Longint; Description: SSeg returns the Stack Segment. This function is only supported for compatibility reasons, as Sptr returns the correct contents of the stackpointer. Errors: None. See also: Sptr (142) Program Example67 ; { Program to demonstrate the SSeg f u n c t i o n . } Var W : Longint ; begin W:=SSeg ; end . 143 12.2. FUNCTIONS AND PROCEDURES Str Declaration: Procedure Str (Var X[:NumPlaces[:Decimals]]; Var S : String); Description: Str returns a string which represents the value of X. X can be any numerical type. The optional NumPLaces and Decimals specifiers control the formatting of the string. Errors: None. See also: Val (146) Program Example68 ; { Program to demonstrate the Str f u n c t i o n . } Var S : String ; Function IntToStr ( I : Longint ) : String ; Var S : String ; begin Str ( I , S ) ; IntToStr :=S ; end ; begin S:= ' ' +IntToStr (-233)+ ' ' ; Writeln ( S ) ; end . Succ Declaration: Function Succ (X : Any ordinal type) : Same type; Description: Succ returns the element that succeeds the element that was passed to it. If it is applied to the last value of the ordinal type, and the program was compiled with range checking on ({$R+}), then a run-time error will be generated. Errors: Run-time error 201 is generated when the result is out of range. See also: Ord (128), Pred (131), High (119), Low (124) for an example, see Ord (128). Swap Declaration: Function Swap (X) : Type of X; Description: Swap swaps the high and low order bytes of X if X is of type Word or Integer, or swaps the high and low order words of X if X is of type Longint or Cardinal. The return type is the type of X Errors: None. See also: Lo (123), Hi (118) 144 12.2. FUNCTIONS AND PROCEDURES Program Example69 ; { Program to demonstrate the Swap f u n c t i o n . } Var W : Word ; L : Longint ; begin W:= $1234 ; W:=Swap(W) ; i f W<>$3412 then writeln ( ' Error when swapping word ! ' ) ; L:= $12345678 ; L:=Swap( L ) ; i f L<>$56781234 then writeln ( ' Error when swapping Longint ! ' ) ; end . Trunc Declaration: Function Trunc (X : Real) : Longint; Description: Trunc returns the integer part of X, which is always smaller than (or equal to) X in absolute value. Errors: None. See also: Frac (116), Int (121), Round (136) Program Example54 ; { Program to demonstrate the Trunc f u n c t i o n . } begin Writeln ( Trunc ( 1 2 3 . 4 5 6 ) ) ; { P r i n t s 123 } Writeln ( Trunc ( - 1 2 3 . 4 5 6 ) ) ; { P r i n t s -123 } Writeln ( Trunc ( 1 2 . 3 4 5 6 ) ) ; { P r i n t s 12 } Writeln ( Trunc ( - 1 2 . 3 4 5 6 ) ) ; { P r i n t s -12 } end . Truncate Declaration: Procedure Truncate (Var F : file); Description: Truncate truncates the (opened) file F at the current file position. Errors: Errors are reported by IOresult. See also: Append (100), Filepos (113), Seek (137) Program Example71 ; { Program to demonstrate the Truncate f u n c t i o n . } Var F : File of l o n g i n t ; 145 12.2. FUNCTIONS AND PROCEDURES I , L : Longint ; begin Assign ( F, ' t e s t . dat ' ) ; Rewrite ( F ) ; For I :=1 to 10 Do Write ( F, I ) ; Writeln ( ' F i l e s i z e b e f o r e Truncate : ' , F i l e S i z e ( F ) ) ; Close ( f ) ; Reset ( F ) ; Repeat Read ( F, I ) ; Until i =5; Truncate ( F ) ; Writeln ( ' F i l e s i z e a f t e r Truncate : ' , F i l e s i z e ( F ) ) ; Close ( f ) ; end . Upcase Declaration: Function Upcase (C : Char or string) : Char or String; Description: Upcase returns the uppercase version of its argument C. If its argument is a string, then the complete string is converted to uppercase. The type of the returned value is the same as the type of the argument. Errors: None. See also: Lowercase (124) Program Example72 ; { Program to demonstrate the Upcase f u n c t i o n . } Var I : Longint ; begin For i := ord ( ' a ' ) to ord ( ' z ' ) do write ( upcase ( chr ( i ) ) ) ; Writeln ; { This doesn ' t work in TP, but i t does in Free Pascal } Writeln ( Upcase ( ' a b c d e f g h i j k l m n o p q r s t u v w x y z ' ) ) ; end . Val Declaration: Procedure Val (const S : string;var V;var Code : word); Description: Val converts the value represented in the string S to a numerical value, and stores this value in the variable V, which can be of type Longint, Real and Byte. If the conversion isn't succesfull, then the parameter Code contains the index of the character in S which prevented the conversion. The string S isn't allowed to contain spaces. 146 12.2. FUNCTIONS AND PROCEDURES Errors: If the conversion doesn't succeed, the value of Code indicates the position where the conversion went wrong. See also: Str (144) Program Example74 ; { Program to demonstrate the Val f u n c t i o n . } Var I , Code : I n t e g e r ; begin Val ( ParamStr ( 1 ) , I , Code ) ; I f Code<>0 then Writeln ( ' Error at p o s i t i o n ' , code , ' : ' , Paramstr ( 1 ) [ Code ] ) else Writeln ( ' Value : ' , I ) ; end . Write Declaration: Procedure Write ([Var F : Any filetype;] V1 [; V2; ... , Vn)]; Description: Write writes the contents of the variables V1, V2 etc. to the file F. F can be a typed file, or a Text file. If F is a typed file, then the variables V1, V2 etc. must be of the same type as the type in the declaration of F. Untyped files are not allowed. If the parameter F is omitted, standard output is assumed. If F is of type Text, then the necessary conversions are done such that the output of the variables is in human-readable format. This conversion is done for all numerical types. Strings are printed exactly as they are in memory, as well as PChar types. The format of the numerical conversions can be influenced through the following modifiers: OutputVariable : NumChars [: Decimals ] This will print the value of OutputVariable with a minimum of NumChars characters, from which Decimals are reserved for the decimals. If the number cannot be represented with NumChars characters, NumChars will be increased, until the representation fits. If the representation requires less than NumChars characters then the output is filled up with spaces, to the left of the generated string, thus resulting in a right-aligned representation. If no formatting is specified, then the number is written using its natural length, with nothing in front of it if it's positive, and a minus sign if it's negative. Real numbers are, by default, written in scientific notation. Errors: If an error occurs, a run-time error is generated. This behavior can be controlled with the {$i} switch. See also: WriteLn (147), Read (133), Readln (133), Blockwrite (103) WriteLn Declaration: Procedure WriteLn [([Var F : Text;] [V1 [; V2; ... , Vn)]]; Description: WriteLn does the same as Write (147) for text files, and emits a Carriage Return - LineFeed character pair after that. If the parameter F is omitted, standard output is assumed. If no variables are specified, a Carriage Return - LineFeed character pair is emitted, resulting in a new line in the file F. Remark: Under linux, the Carriage Return character is omitted, as customary in Unix environments. 147 12.2. FUNCTIONS AND PROCEDURES Errors: If an error occurs, a run-time error is generated. This behavior can be controlled with the {$i} switch. See also: Write (147), Read (133), Readln (133), Blockwrite (103) Program Example75 ; { Program to demonstrate the Write ( ln ) f u n c t i o n . } VarF : File of Longint ; L : Longint ; begin Write ( ' This i s on the f i r s t l i n e ! ' ) ; { No CR/LF p a i r ! } Writeln ( ' And t h i s too . . . ' ) ; Writeln ( ' But t h i s i s a l r e a d y on the second l i n e . . . ' ) ; Assign ( f , ' t e s t . dat ' ) ; Rewrite ( f ) ; For L:=1 to 10 do write ( F, L ) ; { No w r i t e l n allowed here ! } Close ( f ) ; end . 148 Index Abs, 99 High, 119 Addr, 99 Append, 100 Inc, 120 Arctan, 100 Insert, 120 Assign, 100 Int, 121 Assigned, 101 IOresult, 121 BinStr, 101 Length, 122 Blockread, 102 Ln, 123 Blockwrite, 103 Lo, 123 Break, 103 LongJmp, 124 Low, 124 Chdir, 104 Lowercase, 124 Chr, 104 Close, 104 Mark, 125 Concat, 105 Maxavail, 125 Continue, 105 Memavail, 126 Copy, 106 Mkdir, 127 Cos, 107 Move, 127 CSeg, 107 New, 127 Dec, 107 Odd, 128 Delete, 108 Ofs, 128 Dispose, 108 Ord, 128 DSeg, 109 Paramcount, 129 Eof, 110 Paramstr, 129 Eoln, 110 Pi, 130 Erase, 111 Pos, 130 Exit, 111 Power, 131 Exp, 112 Pred, 131 Ptr, 131 Filepos, 113 Filesize, 114 Random, 132 Fillchar, 114 Randomize, 132 Fillword, 115 Read, 133 Flush, 115 Readln, 133 Frac, 116 Release, 134 Freemem, 116 Rename, 134 Reset, 134 Getdir, 117 Rewrite, 135 Getmem, 117 Rmdir, 136 Halt, 117 Round, 136 HexStr, 118 Runerror, 137 Hi, 118 Seek, 137 149 INDEX SeekEof, 138 SeekEoln, 138 Seg, 139 SetJmp, 139 SetLength, 140 SetTextBuf, 140 Sin, 141 SizeOf, 142 Sptr, 142 Sqr, 143 Sqrt, 143 SSeg, 143 Str, 144 Succ, 144 Swap, 144 Trunc, 145 Truncate, 145 Upcase, 146 Val, 146 Write, 147 WriteLn, 147 150