Class::Struct(3) Perl Programmers Reference GuideClass::Struct(3)NAMEClass::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, ... };
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
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 imple
ments 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 declar
ing a "sub" of the same name in the package. (See Example
2.)
Each element's type can be scalar, array, hash, or class.
The "struct()" function
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 cur
rent 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".
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).
A function named "new" must not be explicitly defined in a
class created by "struct".
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.
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 ).
There is no significant speed gain between compile time
and run time class creation, there is just a new, more
standard order of events.
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 "'*'".
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 by default is initialized
to "undef" (but see the Initializing with new entry
elsewhere in this document).
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.
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 "'*@'").
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 ele
ment. 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 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 "'*%'").
With one or two arguments, the first argument is a key
specifying one element of the hash; the second argu
ment, 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" con
structor 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.
Initializing with "new"
"struct" always creates a constructor called "new". That
constructor may take a list of initializers for the vari
ous elements of the new struct.
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.
The initializer for a class element is also a hash refer
ence, and the contents of that hash are passed to the ele
ment's own constructor.
See Example 3 below for an example of initialization.
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, # 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);
$t->ru_stime->tv_usecs(0);
Example 2
An accessor function can be redefined in order to pro
vide additional checking of values, etc. Here, we
want the "count" element always to be nonnegative, so
we redefine the "count" accessor accordingly.
package MyObj;
use Class::Struct;
# declare the struct
struct ( 'MyObj', { count => '$', 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'
print "\$x->count(-5) = ", $x->count(-5), "\n";
# dies due to negative argument!
Example 3
The constructor of a generated class can be passed a
list of element=>value pairs, with which to initialize
the struct. If no initializer is specified for a par
ticular element, its default initialization is per
formed instead. Initializers for non-existent elements
are silently ignored.
Note that the initializer for a nested struct is spec
ified as an anonymous hash of initializers, which is
passed on to the nested struct's constructor.
use Class::Struct;
struct Breed =>
{
name => '$',
cross => '$',
};
struct Cat =>
[
name => '$',
kittens => '@',
markings => '%',
breed => 'Breed',
];
my $cat = Cat->new( name => 'Socks',
kittens => ['Monica', 'Kenneth'],
markings => { socks=>1, blaze=>"white" },
breed => { name=>'short-hair', cross=>1 },
);
print "Once a cat called ", $cat->name, "\n";
print "(which was a ", $cat->breed->name, ")\n";
print "had two kittens: ", join(' and ', @{$cat->kittens}), "\n";
Author and Modification History
Modified by Casey Tweten, 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.
2001-03-03 perl v5.6.1 Class::Struct(3)
