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

Text File | 1998-10-30 | 12.3 KB | 397 lines

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     Class::Struct - declare struct-like datatypes as Perl classes

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         use Class::Struct;
                 # declare struct, based on array:
         struct( CLASS_NAME => [ ELEMENT_NAME => ELEMENT_TYPE, ... ]);
                 # declare struct, based on hash:
         struct( CLASS_NAME => { ELEMENT_NAME => ELEMENT_TYPE, ... });

         package CLASS_NAME;
         use Class::Struct;
                 # declare struct, based on array, implicit class name:
         struct( ELEMENT_NAME => ELEMENT_TYPE, ... );

         package Myobj;
         use Class::Struct;
                 # declare struct with four types of elements:
         struct( s => '$', a => '@', h => '%', c => 'My_Other_Class' );

         $obj = new Myobj;               # constructor

                                         # scalar type accessor:
         $element_value = $obj->s;           # element value
         $obj->s('new value');               # assign to element

                                         # array type accessor:
         $ary_ref = $obj->a;                 # reference to whole array
         $ary_element_value = $obj->a(2);    # array element value
         $obj->a(2, 'new value');            # assign to array element

                                         # hash type accessor:
         $hash_ref = $obj->h;                # reference to whole hash
         $hash_element_value = $obj->h('x'); # hash element value
         $obj->h('x', 'new value');        # assign to hash element

                                         # class type accessor:
         $element_value = $obj->c;           # object reference
         $obj->c->method(...);               # call method of object
         $obj->c(new My_Other_Class);        # assign a new object


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     Class::Struct exports a single function, struct.  Given a list of element
     names and types, and optionally a class name, struct creates a Perl 5
     class that implements a "struct-like" data structure.

     The new class is given a constructor method, new, for creating struct
     objects.






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     Each element in the struct data has an accessor method, which is used to
     assign to the element and to fetch its value.  The default accessor can
     be overridden by declaring a sub of the same name in the package.  (See
     Example 2.)

     Each element's type can be scalar, array, hash, or class.

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     The struct function has three forms of parameter-list.

         struct( CLASS_NAME => [ ELEMENT_LIST ]);
         struct( CLASS_NAME => { ELEMENT_LIST });
         struct( ELEMENT_LIST );

     The first and second forms explicitly identify the name of the class
     being created.  The third form assumes the current package name as the
     class name.

     An object of a class created by the first and third forms is based on an
     array, whereas an object of a class created by the second form is based
     on a hash. The array-based forms will be somewhat faster and smaller; the
     hash-based forms are more flexible.

     The class created by struct must not be a subclass of another class other
     than UNIVERSAL.

     A function named new must not be explicitly defined in a class created by
     struct.

     The _E_L_E_M_E_N_T__L_I_S_T has the form

         NAME => TYPE, ...

     Each name-type pair declares one element of the struct. Each element name
     will be defined as an accessor method unless a method by that name is
     explicitly defined; in the latter case, a warning is issued if the
     warning flag (----wwww) is set.

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     The four element types -- scalar, array, hash, and class -- are
     represented by strings -- '$', '@', '%', and a class name -- optionally
     preceded by a '*'.

     The accessor method provided by struct for an element depends on the
     declared type of the element.

     Scalar ('$' or '*$')
          The element is a scalar, and is initialized to undef.

          The accessor's argument, if any, is assigned to the element.



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          If the element type is '$', the value of the element (after
          assignment) is returned. If the element type is '*$', a reference to
          the element is returned.

     Array ('@' or '*@')
          The element is an array, initialized to ().

          With no argument, the accessor returns a reference to the element's
          whole array.

          With one or two arguments, the first argument is an index specifying
          one element of the array; the second argument, if present, is
          assigned to the array element.  If the element type is '@', the
          accessor returns the array element value.  If the element type is
          '*@', a reference to the array element is returned.

     Hash ('%' or '*%')
          The element is a hash, initialized to ().

          With no argument, the accessor returns a reference to the element's
          whole hash.

          With one or two arguments, the first argument is a key specifying
          one element of the hash; the second argument, if present, is
          assigned to the hash element.  If the element type is '%', the
          accessor returns the hash element value.  If the element type is
          '*%', a reference to the hash element is returned.

     Class ('Class_Name' or '*Class_Name')
          The element's value must be a reference blessed to the named class
          or to one of its subclasses. The element is initialized to the
          result of calling the new constructor of the named class.

          The accessor's argument, if any, is assigned to the element. The
          accessor will croak if this is not an appropriate object reference.

          If the element type does not start with a '*', the accessor returns
          the element value (after assignment). If the element type starts
          with a '*', a reference to the element itself is returned.

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     Example 1
          Giving a struct element a class type that is also a struct is how
          structs are nested.  Here, timeval represents a time (seconds and
          microseconds), and rusage has two elements, each of which is of type
          timeval.

              use Class::Struct;







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              struct( rusage => {
                  ru_utime => timeval,  # seconds
                  ru_stime => timeval,  # microseconds
              });

              struct( timeval => [
                  tv_secs  => '$',
                  tv_usecs => '$',
              ]);

                  # create an object:
              my $t = new rusage;
                  # $t->ru_utime and $t->ru_stime are objects of type timeval.

                  # set $t->ru_utime to 100.0 sec and $t->ru_stime to 5.0 sec.
              $t->ru_utime->tv_secs(100);
              $t->ru_utime->tv_usecs(0);
              $t->ru_stime->tv_secs(5);
              $t->ru_stime->tv_usecs(0);


     Example 2
          An accessor function can be redefined in order to provide additional
          checking of values, etc.  Here, we want the count element always to
          be nonnegative, so we redefine the count accessor accordingly.

              package MyObj;
              use Class::Struct;

                          # declare the struct
              struct ( 'MyObj', { count => '$', stuff => '%' } );

                          # override the default accessor method for 'count'
              sub count {
                  my $self = shift;
                  if ( @_ ) {
                      die 'count must be nonnegative' if $_[0] < 0;
                      $self->{'count'} = shift;
                      warn "Too many args to count" if @_;
                  }
                  return $self->{'count'};
              }

              package main;
              $x = new MyObj;
              print "\$x->count(5) = ", $x->count(5), "\n";
                                      # prints '$x->count(5) = 5'

              print "\$x->count = ", $x->count, "\n";
                                      # prints '$x->count = 5'





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              print "\$x->count(-5) = ", $x->count(-5), "\n";
                                      # dies due to negative argument!


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     Renamed to Class::Struct and modified by Jim Miner, 1997-04-02.

         members() function removed.
         Documentation corrected and extended.
         Use of struct() in a subclass prohibited.
         User definition of accessor allowed.
         Treatment of '*' in element types corrected.
         Treatment of classes as element types corrected.
         Class name to struct() made optional.
         Diagnostic checks added.

     Originally Class::Template by Dean Roehrich.

         # Template.pm   --- struct/member template builder
         #   12mar95
         #   Dean Roehrich
         #
         # changes/bugs fixed since 28nov94 version:
         #  - podified
         # changes/bugs fixed since 21nov94 version:
         #  - Fixed examples.
         # changes/bugs fixed since 02sep94 version:
         #  - Moved to Class::Template.
         # changes/bugs fixed since 20feb94 version:
         #  - Updated to be a more proper module.
         #  - Added "use strict".
         #  - Bug in build_methods, was using @var when @$var needed.
         #  - Now using my() rather than local().
         #
         # Uses perl5 classes to create nested data types.
         # This is offered as one implementation of Tom Christiansen's "structs.pl"
         # idea.


















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