IRIX Base Documentation 2001 May/SGI IRIX Base Documentation 2001 May.iso/usr/share/catman/p_man/cat3/perl5/overload.z/overload

Text File | 1998-10-30 | 23.5 KB | 661 lines

oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



NNNNAAAAMMMMEEEE
     overload - Package for overloading perl operations

SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
         package SomeThing;

         use overload
             '+' => \&myadd,
             '-' => \&mysub;
             # etc
         ...

         package main;
         $a = new SomeThing 57;
         $b=5+$a;
         ...
         if (overload::Overloaded $b) {...}
         ...
         $strval = overload::StrVal $b;


CCCCAAAAVVVVEEEEAAAATTTT SSSSCCCCRRRRIIIIPPPPTTTTOOOORRRR
     Overloading of operators is a subject not to be taken lightly.  Neither
     its precise implementation, syntax, nor semantics are 100% endorsed by
     Larry Wall.  So any of these may be changed at some point in the future.

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     DDDDeeeeccccllllaaaarrrraaaattttiiiioooonnnn ooooffff oooovvvveeeerrrrllllooooaaaaddddeeeedddd ffffuuuunnnnccccttttiiiioooonnnnssss

     The compilation directive

         package Number;
         use overload
             "+" => \&add,
             "*=" => "muas";

     declares function _N_u_m_b_e_r::_a_d_d() for addition, and method _m_u_a_s() in the
     "class" Number (or one of its base classes) for the assignment form *= of
     multiplication.

     Arguments of this directive come in (key, value) pairs.  Legal values are
     values legal inside a &{ ... } call, so the name of a subroutine, a
     reference to a subroutine, or an anonymous subroutine will all work.
     Note that values specified as strings are interpreted as methods, not
     subroutines.  Legal keys are listed below.

     The subroutine add will be called to execute $a+$b if $a is a reference
     to an object blessed into the package Number, or if $a is not an object
     from a package with defined mathemagic addition, but $b is a reference to
     a Number.  It can also be called in other situations, like $a+=7, or
     $a++.  See the section on _M_A_G_I_C _A_U_T_O_G_E_N_E_R_A_T_I_O_N.  (Mathemagical methods
     refer to methods triggered by an overloaded mathematical operator.)



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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



     Since overloading respects inheritance via the @ISA hierarchy, the above
     declaration would also trigger overloading of + and *= in all the
     packages which inherit from Number.

     CCCCaaaalllllllliiiinnnngggg CCCCoooonnnnvvvveeeennnnttttiiiioooonnnnssss ffffoooorrrr BBBBiiiinnnnaaaarrrryyyy OOOOppppeeeerrrraaaattttiiiioooonnnnssss

     The functions specified in the use overload ... directive are called with
     three (in one particular case with four, see the section on _L_a_s_t _R_e_s_o_r_t)
     arguments.  If the corresponding operation is binary, then the first two
     arguments are the two arguments of the operation.  However, due to
     general object calling conventions, the first argument should always be
     an object in the package, so in the situation of 7+$a, the order of the
     arguments is interchanged.  It probably does not matter when implementing
     the addition method, but whether the arguments are reversed is vital to
     the subtraction method.  The method can query this information by
     examining the third argument, which can take three different values:

     FALSE  the order of arguments is as in the current operation.

     TRUE   the arguments are reversed.

     undef  the current operation is an assignment variant (as in $a+=7), but
            the usual function is called instead.  This additional information
            can be used to generate some optimizations.

     CCCCaaaalllllllliiiinnnngggg CCCCoooonnnnvvvveeeennnnttttiiiioooonnnnssss ffffoooorrrr UUUUnnnnaaaarrrryyyy OOOOppppeeeerrrraaaattttiiiioooonnnnssss

     Unary operation are considered binary operations with the second argument
     being undef.  Thus the functions that overloads {"++"} is called with
     arguments ($a,undef,'') when $a++ is executed.

     OOOOvvvveeeerrrrllllooooaaaaddddaaaabbbblllleeee OOOOppppeeeerrrraaaattttiiiioooonnnnssss

     The following symbols can be specified in use overload:

     +o _A_r_i_t_h_m_e_t_i_c _o_p_e_r_a_t_i_o_n_s

              "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
              "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",

          For these operations a substituted non-assignment variant can be
          called if the assignment variant is not available.  Methods for
          operations "+", "-", "+=", and "-=" can be called to automatically
          generate increment and decrement methods.  The operation "-" can be
          used to autogenerate missing methods for unary minus or abs.

     +o _C_o_m_p_a_r_i_s_o_n _o_p_e_r_a_t_i_o_n_s

              "<",  "<=", ">",  ">=", "==", "!=", "<=>",
              "lt", "le", "gt", "ge", "eq", "ne", "cmp",

          If the corresponding "spaceship" variant is available, it can be



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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



          used to substitute for the missing operation.  During sorting
          arrays, cmp is used to compare values subject to use overload.

     +o _B_i_t _o_p_e_r_a_t_i_o_n_s

              "&", "^", "|", "neg", "!", "~",

          "neg" stands for unary minus.  If the method for neg is not
          specified, it can be autogenerated using the method for subtraction.
          If the method for "!" is not specified, it can be autogenerated
          using the methods for "bool", or "\"\"", or "0+".

     +o _I_n_c_r_e_m_e_n_t _a_n_d _d_e_c_r_e_m_e_n_t

              "++", "--",

          If undefined, addition and subtraction methods can be used instead.
          These operations are called both in prefix and postfix form.

     +o _T_r_a_n_s_c_e_n_d_e_n_t_a_l _f_u_n_c_t_i_o_n_s

              "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",

          If abs is unavailable, it can be autogenerated using methods for "<"
          or "<=>" combined with either unary minus or subtraction.

     +o _B_o_o_l_e_a_n, _s_t_r_i_n_g _a_n_d _n_u_m_e_r_i_c _c_o_n_v_e_r_s_i_o_n

              "bool", "\"\"", "0+",

          If one or two of these operations are unavailable, the remaining
          ones can be used instead.  bool is used in the flow control
          operators (like while) and for the ternary "?:" operation.  These
          functions can return any arbitrary Perl value.  If the corresponding
          operation for this value is overloaded too, that operation will be
          called again with this value.

     +o _S_p_e_c_i_a_l

              "nomethod", "fallback", "=",

          see the section on _S_P_E_C_I_A_L _S_Y_M_B_O_L_S _F_O_R _u_s_e _o_v_e_r_l_o_a_d.

     See the section on _F_a_l_l_b_a_c_k for an explanation of when a missing method
     can be autogenerated.

     IIIInnnnhhhheeeerrrriiiittttaaaannnncccceeee aaaannnndddd oooovvvveeeerrrrllllooooaaaaddddiiiinnnngggg

     Inheritance interacts with overloading in two ways.






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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



     Strings as values of use overload directive
          If value in

            use overload key => value;

          is a string, it is interpreted as a method name.

     Overloading of an operation is inherited by derived classes
          Any class derived from an overloaded class is also overloaded.  The
          set of overloaded methods is the union of overloaded methods of all
          the ancestors. If some method is overloaded in several ancestor,
          then which description will be used is decided by the usual
          inheritance rules:

          If A inherits from B and C (in this order), B overloads + with
          \&D::plus_sub, and C overloads + by "plus_meth", then the subroutine
          D::plus_sub will be called to implement operation + for an object in
          package A.

     Note that since the value of the fallback key is not a subroutine, its
     inheritance is not governed by the above rules.  In the current
     implementation, the value of fallback in the first overloaded ancestor is
     used, but this is accidental and subject to change.

SSSSPPPPEEEECCCCIIIIAAAALLLL SSSSYYYYMMMMBBBBOOOOLLLLSSSS FFFFOOOORRRR uuuusssseeee oooovvvveeeerrrrllllooooaaaadddd
     Three keys are recognized by Perl that are not covered by the above
     description.

     LLLLaaaasssstttt RRRReeeessssoooorrrrtttt

     "nomethod" should be followed by a reference to a function of four
     parameters.  If defined, it is called when the overloading mechanism
     cannot find a method for some operation.  The first three arguments of
     this function coincide with the arguments for the corresponding method if
     it were found, the fourth argument is the symbol corresponding to the
     missing method.  If several methods are tried, the last one is used.
     Say, 1-$a can be equivalent to

             &nomethodMethod($a,1,1,"-")

     if the pair "nomethod" => "nomethodMethod" was specified in the use
     overload directive.

     If some operation cannot be resolved, and there is no function assigned
     to "nomethod", then an exception will be raised via _d_i_e()-- unless
     "fallback" was specified as a key in use overload directive.

     FFFFaaaallllllllbbbbaaaacccckkkk

     The key "fallback" governs what to do if a method for a particular
     operation is not found.  Three different cases are possible depending on
     the value of "fallback":



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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



     +o undef         Perl tries to use a substituted method (see the section
                     on _M_A_G_I_C _A_U_T_O_G_E_N_E_R_A_T_I_O_N).  If this fails, it then tries
                     to calls "nomethod" value; if missing, an exception will
                     be raised.

     +o TRUE          The same as for the undef value, but no exception is
                     raised.  Instead, it silently reverts to what it would
                     have done were there no use overload present.

     +o defined, but FALSE
                     No autogeneration is tried.  Perl tries to call
                     "nomethod" value, and if this is missing, raises an
                     exception.

     NNNNooootttteeee.... "fallback" inheritance via @ISA is not carved in stone yet, see the
     section on _I_n_h_e_r_i_t_a_n_c_e _a_n_d _o_v_e_r_l_o_a_d_i_n_g.

     CCCCooooppppyyyy CCCCoooonnnnssssttttrrrruuuuccccttttoooorrrr

     The value for "=" is a reference to a function with three arguments,
     i.e., it looks like the other values in use overload. However, it does
     not overload the Perl assignment operator. This would go against Camel
     hair.

     This operation is called in the situations when a mutator is applied to a
     reference that shares its object with some other reference, such as

             $a=$b;
             $a++;

     To make this change $a and not change $b, a copy of $$a is made, and $a
     is assigned a reference to this new object.  This operation is done
     during execution of the $a++, and not during the assignment, (so before
     the increment $$a coincides with $$b).  This is only done if ++ is
     expressed via a method for '++' or '+='.  Note that if this operation is
     expressed via '+' a nonmutator, i.e., as in

             $a=$b;
             $a=$a+1;

     then $a does not reference a new copy of $$a, since $$a does not appear
     as lvalue when the above code is executed.

     If the copy constructor is required during the execution of some mutator,
     but a method for '=' was not specified, it can be autogenerated as a
     string copy if the object is a plain scalar.

     EEEExxxxaaaammmmpppplllleeee
          The actually executed code for






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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



                  $a=$b;
                  Something else which does not modify $a or $b....
                  ++$a;

          may be

                  $a=$b;
                  Something else which does not modify $a or $b....
                  $a = $a->clone(undef,"");
                  $a->incr(undef,"");

          if $b was mathemagical, and '++' was overloaded with \&incr, '=' was
          overloaded with \&clone.

MMMMAAAAGGGGIIIICCCC AAAAUUUUTTTTOOOOGGGGEEEENNNNEEEERRRRAAAATTTTIIIIOOOONNNN
     If a method for an operation is not found, and the value for  "fallback"
     is TRUE or undefined, Perl tries to autogenerate a substitute method for
     the missing operation based on the defined operations.  Autogenerated
     method substitutions are possible for the following operations:

     _A_s_s_i_g_n_m_e_n_t _f_o_r_m_s _o_f _a_r_i_t_h_m_e_t_i_c _o_p_e_r_a_t_i_o_n_s
                     $a+=$b can use the method for "+" if the method for "+="
                     is not defined.

     _C_o_n_v_e_r_s_i_o_n _o_p_e_r_a_t_i_o_n_s
                     String, numeric, and boolean conversion are calculated in
                     terms of one another if not all of them are defined.

     _I_n_c_r_e_m_e_n_t _a_n_d _d_e_c_r_e_m_e_n_t
                     The ++$a operation can be expressed in terms of $a+=1 or
                     $a+1, and $a-- in terms of $a-=1 and $a-1.

     abs($a)         can be expressed in terms of $a<0 and -$a (or 0-$a).

     _U_n_a_r_y _m_i_n_u_s     can be expressed in terms of subtraction.

     _N_e_g_a_t_i_o_n        ! and not can be expressed in terms of boolean
                     conversion, or string or numerical conversion.

     _C_o_n_c_a_t_e_n_a_t_i_o_n   can be expressed in terms of string conversion.

     _C_o_m_p_a_r_i_s_o_n _o_p_e_r_a_t_i_o_n_s
                     can be expressed in terms of its "spaceship" counterpart:
                     either <=> or cmp:

                         <, >, <=, >=, ==, !=        in terms of <=>
                         lt, gt, le, ge, eq, ne      in terms of cmp


     _C_o_p_y _o_p_e_r_a_t_o_r   can be expressed in terms of an assignment to the
                     dereferenced value, if this value is a scalar and not a
                     reference.



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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



WWWWAAAARRRRNNNNIIIINNNNGGGG
     The restriction for the comparison operation is that even if, for
     example, `cmp' should return a blessed reference, the autogenerated `lt'
     function will produce only a standard logical value based on the
     numerical value of the result of `cmp'.  In particular, a working numeric
     conversion is needed in this case (possibly expressed in terms of other
     conversions).

     Similarly, .=  and x= operators lose their mathemagical properties if the
     string conversion substitution is applied.

     When you _c_h_o_p() a mathemagical object it is promoted to a string and its
     mathemagical properties are lost.  The same can happen with other
     operations as well.

RRRRuuuunnnn----ttttiiiimmmmeeee OOOOvvvveeeerrrrllllooooaaaaddddiiiinnnngggg
     Since all use directives are executed at compile-time, the only way to
     change overloading during run-time is to

         eval 'use overload "+" => \&addmethod';

     You can also use

         eval 'no overload "+", "--", "<="';

     though the use of these constructs during run-time is questionable.

PPPPuuuubbbblllliiiicccc ffffuuuunnnnccccttttiiiioooonnnnssss
     Package overload.pm provides the following public functions:

     overload::StrVal(arg)
          Gives string value of arg as in absence of stringify overloading.

     overload::Overloaded(arg)
          Returns true if arg is subject to overloading of some operations.

     overload::Method(obj,op)
          Returns undef or a reference to the method that implements op.

IIIIMMMMPPPPLLLLEEEEMMMMEEEENNNNTTTTAAAATTTTIIIIOOOONNNN
     What follows is subject to change RSN.

     The table of methods for all operations is cached in magic for the symbol
     table hash for the package.  The cache is invalidated during processing
     of use overload, no overload, new function definitions, and changes in
     @ISA. However, this invalidation remains unprocessed until the next
     blessing into the package. Hence if you want to change overloading
     structure dynamically, you'll need an additional (fake) blessing to
     update the table.






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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



     (Every SVish thing has a magic queue, and magic is an entry in that
     queue.  This is how a single variable may participate in multiple forms
     of magic simultaneously.  For instance, environment variables regularly
     have two forms at once: their %ENV magic and their taint magic. However,
     the magic which implements overloading is applied to the stashes, which
     are rarely used directly, thus should not slow down Perl.)

     If an object belongs to a package using overload, it carries a special
     flag.  Thus the only speed penalty during arithmetic operations without
     overloading is the checking of this flag.

     In fact, if use overload is not present, there is almost no overhead for
     overloadable operations, so most programs should not suffer measurable
     performance penalties.  A considerable effort was made to minimize the
     overhead when overload is used in some package, but the arguments in
     question do not belong to packages using overload.  When in doubt, test
     your speed with use overload and without it.  So far there have been no
     reports of substantial speed degradation if Perl is compiled with
     optimization turned on.

     There is no size penalty for data if overload is not used. The only size
     penalty if overload is used in some package is that _a_l_l the packages
     acquire a magic during the next blessing into the package. This magic is
     three-words-long for packages without overloading, and carries the cache
     tabel if the package is overloaded.

     Copying ($a=$b) is shallow; however, a one-level-deep copying is carried
     out before any operation that can imply an assignment to the object $a
     (or $b) refers to, like $a++.  You can override this behavior by defining
     your own copy constructor (see the section on _C_o_p_y _C_o_n_s_t_r_u_c_t_o_r).

     It is expected that arguments to methods that are not explicitly supposed
     to be changed are constant (but this is not enforced).

AAAAUUUUTTTTHHHHOOOORRRR
     Ilya Zakharevich <_i_l_y_a@_m_a_t_h._m_p_s._o_h_i_o-_s_t_a_t_e._e_d_u>.

DDDDIIIIAAAAGGGGNNNNOOOOSSSSTTTTIIIICCCCSSSS
     When Perl is run with the ----DDDDoooo switch or its equivalent, overloading
     induces diagnostic messages.

     Using the m command of Perl debugger (see the _p_e_r_l_d_e_b_u_g manpage) one can
     deduce which operations are overloaded (and which ancestor triggers this
     overloading). Say, if eq is overloaded, then the method (eq is shown by
     debugger. The method () corresponds to the fallback key (in fact a
     presence of this method shows that this package has overloading enabled,
     and it is what is used by the Overloaded function).

BBBBUUUUGGGGSSSS
     Because it is used for overloading, the per-package hash %OVERLOAD now
     has a special meaning in Perl. The symbol table is filled with names
     looking like line-noise.



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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))



     For the purpose of inheritance every overloaded package behaves as if
     fallback is present (possibly undefined). This may create interesting
     effects if some package is not overloaded, but inherits from two
     overloaded packages.

     This document is confusing.

















































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oooovvvveeeerrrrllllooooaaaadddd((((3333))))                                                        oooovvvveeeerrrrllllooooaaaadddd((((3333))))























































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