NAME

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

SYNOPSIS

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, ... );
# Declare struct at compile time
use Class::Struct CLASS_NAME => [ELEMENT_NAME => ELEMENT_TYPE, ...];
use Class::Struct CLASS_NAME => {ELEMENT_NAME => ELEMENT_TYPE, ...};
# declare struct at compile time, based on array, implicit
# class name:
package CLASS_NAME;
use Class::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' );
my $obj = Myobj->new;               # constructor
                                # scalar type accessor:
my $element_value = $obj->s;           # element value
$obj->s('new value');               # assign to element
                                # array type accessor:
my $ary_ref = $obj->a;                 # reference to whole array
my $ary_element_value = $obj->a(2);    # array element value
$obj->a(2, 'new value');            # assign to array element
                                # hash type accessor:
my $hash_ref = $obj->h;                # reference to whole hash
my $hash_element_value = $obj->h('x'); # hash element value
$obj->h('x', 'new value');          # assign to hash element
                                # class type accessor:
my $element_value = $obj->c;           # object reference
$obj->c->method(...);               # call method of object
$obj->c(new My_Other_Class);        # assign a new object

DESCRIPTION

"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.

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.

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

NAME => TYPE, ...

EXAMPLES

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;
struct( Rusage => {
    ru_utime => 'Timeval',  # user time used
    ru_stime => 'Timeval',  # system time used
});
struct( Timeval => [
    tv_secs  => '$',        # seconds
    tv_usecs => '$',        # microseconds
]);
# create an object:
my $t = Rusage->new(ru_utime=>Timeval->new(),
    ru_stime=>Timeval->new());
# $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->{'MyObj::count'} = shift;
        warn "Too many args to count" if @_;
    }
    return $self->{'MyObj::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'
print "\$x->count(-5) = ", $x->count(-5), "\n";
                        # dies due to negative argument!

Example 3

The constructor of a generated class can be passed a list of element=>value pairs, with which to initialize the struct. If no initializer is specified for a particular element, its default initialization is performed instead. Initializers for non-existent elements are silently ignored.

Note that the initializer for a nested class may be specified as an object of that class, or as a reference to a hash of initializers that are passed on to the nested struct's constructor.

use Class::Struct;
struct Breed =>
{
    name  => '$',
    cross => '$',
};
struct Cat =>
[
    name     => '$',
    kittens  => '@',
    markings => '%',
    breed    => 'Breed',
];
my $cat = Cat->new( name => 'Socks',
           kittens  => ['Monica', 'Kenneth'],
           markings => { socks=>1, blaze=>"white" },
           breed    => Breed->new(name=>'short-hair', cross=>1),
      or:  breed    => {name=>'short-hair', cross=>1},
                  );
print "Once a cat called ", $cat->name, "\n";
print "(which was a ", $cat->breed->name, ")\n";
print "had 2 kittens: ", join(' and ', @{$cat->kittens}), "\n";

Author and Modification History

Modified by Damian Conway, 2001-09-10, v0.62.

Modified implicit construction of nested objects.
Now will also take an object ref instead of requiring a hash ref.
Also default initializes nested object attributes to undef, rather
than calling object constructor without args
Original over-helpfulness was fraught with problems:
    * the class's constructor might not be called 'new'
    * the class might not have a hash-like-arguments constructor
    * the class might not have a no-argument constructor
    * "recursive" data structures didn't work well:
              package Person;
              struct { mother => 'Person', father => 'Person'};

Modified by Casey West, 2000-11-08, v0.59.

Added the ability for compile time class creation.

Modified by Damian Conway, 1999-03-05, v0.58.

Added handling of hash-like arg list to class ctor.
Changed to two-argument blessing in ctor to support
derivation from created classes.
Added classname prefixes to keys in hash-based classes
(refer to "Perl Cookbook", Recipe 13.12 for rationale).
Corrected behaviour of accessors for '*@' and '*%' struct
elements.  Package now implements documented behaviour when
returning a reference to an entire hash or array element.
Previously these were returned as a reference to a reference
to the element.

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.