Class::Struct(3pm)

📦

version 5.42.3 · 10 versions

okay so bestie, we're looking at Class::Struct(3pm) from the Perl Programmers Reference Guide, and it's literally the page about declaring struct-like datatypes as Perl classes. iconic. 💅

✨ NAME ✨

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

🎀 SYNOPSIS 🎀

like, here's the whole vibe in code, bestie — copy it exactly:

           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. no cap, that's the whole point. 🫶

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.) lowkey important, bestie.

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

🦋 The `struct()` function 🦋

The struct function has three forms of parameter-list. it's giving options, but make it struct:

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. iykyk. ✨

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. vibe check: pick your fave. 🌈

bestie, listen: the class created by struct must not be a subclass of another class other than UNIVERSAL. that's a hard rule, fr fr.

It can, however, be used as a superclass for other classes. To facilitate this, the generated constructor method uses a two-argument blessing. Furthermore, if the class is hash-based, the key of each element is prefixed with the class name (see Perl Cookbook, Recipe 13.12). iconic behavior. 👑

also: a function named new must not be explicitly defined in a class created by struct. don't do it, bestie. 💀

The ELEMENT_LIST 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 (-w) is set. ngl, that warning is your friend. 🙈

🌈 Class Creation at Compile Time 🌈

Class::Struct can create your class at compile time. The main reason for doing this is obvious, so your class acts like every other class in Perl. Creating your class at compile time will make the order of events similar to using any other class (or Perl module). main character energy: your class shows up on time. ⭐

There is no significant speed gain between compile time and run time class creation, there is just a new, more standard order of events. so don't come for me about speed, okay? 💅

👑 Element Types and Accessor Methods 👑

The four element types — scalar, array, hash, and class — are represented by strings — '$', '@', '%', and a class name — optionally preceded by a '*'. obsessed with the little star. 🌟

The accessor method provided by struct for an element depends on the declared type of the element. let's break it down, bestie. 🎀

💖 Scalar (`'$'` or `'*$'`) 💖

The element is a scalar, and by default is initialized to undef (but see "Initializing with new"). literally undef until you say otherwise. 🦋

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

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. cute little distinction, no cap. ✨

🍓 Array (`'@'` or `'*@'`) 🍓

The element is an array, initialized by default to ().

With no argument, the accessor returns a reference to the element's whole array (whether or not the element was specified as '@' or '*@'). whole array ref, slay. 🫶

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. details matter, bestie. 💅

As a special case, when the accessor is called with an array reference as the sole argument, this causes an assignment of the whole array element. The object reference is returned. plot twist, but make it useful. 😭🔥

🌸 Hash (`'%'` or `'*%'`) 🌸

The element is a hash, initialized by default to ().

With no argument, the accessor returns a reference to the element's whole hash (whether or not the element was specified as '%' or '*%'). hash ref, iconic. 🎀

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. okay key, literally. 🔑

As a special case, when the accessor is called with a hash reference as the sole argument, this causes an assignment of the whole hash element. The object reference is returned. same energy, different container. 💜

🧚 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 not initialized by default. no default object, bestie — you gotta bring one. 🦋

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

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. star means ref, periodt. ⭐

🎀 Initializing with `new` 🎀

struct always creates a constructor called new. That constructor may take a list of initializers for the various elements of the new struct. yes bestie, it ate. 💅

Each initializer is a pair of values: element name => value. The initializer value for a scalar element is just a scalar value. The initializer for an array element is an array reference. The initializer for a hash is a hash reference. easy. 🌈

The initializer for a class element is an object of the corresponding class, or of one of its subclasses, or a reference to a hash containing named arguments to be passed to the element's constructor. classy. 👑

See Example 3 below for an example of initialization.

✨ EXAMPLES ✨

🌸 Example 1 🌸

Okay bestie, this is the nesting moment — giving a struct element a class type that is also a struct is literally how structs get nested. Here, Timeval represents a time (seconds and microseconds), and Rusage has two elements, each of them of type Timeval. Periodt.

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);

So $t->ru_utime and $t->ru_stime are straight up objects of type Timeval, and you're setting $t->ru_utime to 100.0 sec and $t->ru_stime to 5.0 sec. Slay 💅

🦋 Example 2 🦋

The way an accessor function can be redefined to give you extra checking of values, etc.? Obsessed. Here we want the count element to always be nonnegative, so we redefine the count accessor accordingly. No cap, that's it.

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!

Note the last one: it literally dies due to a negative argument. The audacity 😭 but like, that's the whole point of the override, bestie.

🎀 Example 3 🎀

Okay so the constructor of a generated class can be passed a list of element=>value pairs, with which to initialize the struct. Big main character energy. If no initializer is specified for a particular element, its default initialization is performed instead. And initializers for non-existent elements are silently ignored — the disrespect, but sure 💀

Heads up! 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";

See that or: line? That's the two ways of doing breed — object ref, or hash ref of initializers. Both iconic, both valid 🐱

👑 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.

And that's the whole vibe check, bestie! ✨ This little gem is living in perl v5.42.3, dated 2026-11-03, and honestly? She ate. Go build some structs, you absolute icon 🫶💖

📦 where this page comes from

alpine 3.24: perl-doc 5.42.3-r0 · Artistic-1.0-Perl OR GPL-1.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms