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Class::Accessor(3pm) - phpMan
Class::Accessor(3pm) User Contributed Perl Documentation Class::Accessor(3pm)
NAME
Class::Accessor - Automated accessor generation
SYNOPSIS
package Foo;
use base qw(Class::Accessor);
Foo->follow_best_practice;
Foo->mk_accessors(qw(name role salary));
# or if you prefer a Moose-like interface...
package Foo;
use Class::Accessor "antlers";
has name => ( is => "rw", isa => "Str" );
has role => ( is => "rw", isa => "Str" );
has salary => ( is => "rw", isa => "Num" );
# Meanwhile, in a nearby piece of code!
# Class::Accessor provides new().
my $mp = Foo->new({ name => "Marty", role => "JAPH" });
my $job = $mp->role; # gets $mp->{role}
$mp->salary(400000); # sets $mp->{salary} = 400000 # I wish
# like my @info = @{$mp}{qw(name role)}
my @info = $mp->get(qw(name role));
# $mp->{salary} = 400000
$mp->set('salary', 400000);
DESCRIPTION
This module automagically generates accessors/mutators for your class.
Most of the time, writing accessors is an exercise in cutting and pasting. You usually
wind up with a series of methods like this:
sub name {
my $self = shift;
if(@_) {
$self->{name} = $_[0];
}
return $self->{name};
}
sub salary {
my $self = shift;
if(@_) {
$self->{salary} = $_[0];
}
return $self->{salary};
}
# etc...
One for each piece of data in your object. While some will be unique, doing value checks
and special storage tricks, most will simply be exercises in repetition. Not only is it
Bad Style to have a bunch of repetitious code, but it's also simply not lazy, which is the
real tragedy.
If you make your module a subclass of Class::Accessor and declare your accessor fields
with mk_accessors() then you'll find yourself with a set of automatically generated
accessors which can even be customized!
The basic set up is very simple:
package Foo;
use base qw(Class::Accessor);
Foo->mk_accessors( qw(far bar car) );
Done. Foo now has simple far(), bar() and car() accessors defined.
Alternatively, if you want to follow Damian's best practice guidelines you can use:
package Foo;
use base qw(Class::Accessor);
Foo->follow_best_practice;
Foo->mk_accessors( qw(far bar car) );
Note: you must call "follow_best_practice" before calling "mk_accessors".
Moose-like
By popular demand we now have a simple Moose-like interface. You can now do:
package Foo;
use Class::Accessor "antlers";
has far => ( is => "rw" );
has bar => ( is => "rw" );
has car => ( is => "rw" );
Currently only the "is" attribute is supported.
CONSTRUCTOR
Class::Accessor provides a basic constructor, "new". It generates a hash-based object and
can be called as either a class method or an object method.
new
my $obj = Foo->new;
my $obj = $other_obj->new;
my $obj = Foo->new(\%fields);
my $obj = $other_obj->new(\%fields);
It takes an optional %fields hash which is used to initialize the object (handy if you use
read-only accessors). The fields of the hash correspond to the names of your accessors,
so...
package Foo;
use base qw(Class::Accessor);
Foo->mk_accessors('foo');
my $obj = Foo->new({ foo => 42 });
print $obj->foo; # 42
however %fields can contain anything, new() will shove them all into your object.
MAKING ACCESSORS
follow_best_practice
In Damian's Perl Best Practices book he recommends separate get and set methods with the
prefix set_ and get_ to make it explicit what you intend to do. If you want to create
those accessor methods instead of the default ones, call:
__PACKAGE__->follow_best_practice
before you call any of the accessor-making methods.
accessor_name_for / mutator_name_for
You may have your own crazy ideas for the names of the accessors, so you can make those
happen by overriding "accessor_name_for" and "mutator_name_for" in your subclass. (I
copied that idea from Class::DBI.)
mk_accessors
__PACKAGE__->mk_accessors(@fields);
This creates accessor/mutator methods for each named field given in @fields. Foreach
field in @fields it will generate two accessors. One called "field()" and the other
called "_field_accessor()". For example:
# Generates foo(), _foo_accessor(), bar() and _bar_accessor().
__PACKAGE__->mk_accessors(qw(foo bar));
See "Overriding autogenerated accessors" in CAVEATS AND TRICKS for details.
mk_ro_accessors
__PACKAGE__->mk_ro_accessors(@read_only_fields);
Same as mk_accessors() except it will generate read-only accessors (ie. true accessors).
If you attempt to set a value with these accessors it will throw an exception. It only
uses get() and not set().
package Foo;
use base qw(Class::Accessor);
Foo->mk_ro_accessors(qw(foo bar));
# Let's assume we have an object $foo of class Foo...
print $foo->foo; # ok, prints whatever the value of $foo->{foo} is
$foo->foo(42); # BOOM! Naughty you.
mk_wo_accessors
__PACKAGE__->mk_wo_accessors(@write_only_fields);
Same as mk_accessors() except it will generate write-only accessors (ie. mutators). If
you attempt to read a value with these accessors it will throw an exception. It only uses
set() and not get().
NOTE I'm not entirely sure why this is useful, but I'm sure someone will need it. If
you've found a use, let me know. Right now it's here for orthoginality and because it's
easy to implement.
package Foo;
use base qw(Class::Accessor);
Foo->mk_wo_accessors(qw(foo bar));
# Let's assume we have an object $foo of class Foo...
$foo->foo(42); # OK. Sets $self->{foo} = 42
print $foo->foo; # BOOM! Can't read from this accessor.
Moose!
If you prefer a Moose-like interface to create accessors, you can use "has" by importing
this module like this:
use Class::Accessor "antlers";
or
use Class::Accessor "moose-like";
Then you can declare accessors like this:
has alpha => ( is => "rw", isa => "Str" );
has beta => ( is => "ro", isa => "Str" );
has gamma => ( is => "wo", isa => "Str" );
Currently only the "is" attribute is supported. And our "is" also supports the "wo" value
to make a write-only accessor.
If you are using the Moose-like interface then you should use the "extends" rather than
tweaking your @ISA directly. Basically, replace
@ISA = qw/Foo Bar/;
with
extends(qw/Foo Bar/);
DETAILS
An accessor generated by Class::Accessor looks something like this:
# Your foo may vary.
sub foo {
my($self) = shift;
if(@_) { # set
return $self->set('foo', @_);
}
else {
return $self->get('foo');
}
}
Very simple. All it does is determine if you're wanting to set a value or get a value and
calls the appropriate method. Class::Accessor provides default get() and set() methods
which your class can override. They're detailed later.
Modifying the behavior of the accessor
Rather than actually modifying the accessor itself, it is much more sensible to simply
override the two key methods which the accessor calls. Namely set() and get().
If you -really- want to, you can override make_accessor().
set
$obj->set($key, $value);
$obj->set($key, @values);
set() defines how generally one stores data in the object.
override this method to change how data is stored by your accessors.
get
$value = $obj->get($key);
@values = $obj->get(@keys);
get() defines how data is retreived from your objects.
override this method to change how it is retreived.
make_accessor
$accessor = __PACKAGE__->make_accessor($field);
Generates a subroutine reference which acts as an accessor for the given $field. It calls
get() and set().
If you wish to change the behavior of your accessors, try overriding get() and set()
before you start mucking with make_accessor().
make_ro_accessor
$read_only_accessor = __PACKAGE__->make_ro_accessor($field);
Generates a subroutine refrence which acts as a read-only accessor for the given $field.
It only calls get().
Override get() to change the behavior of your accessors.
make_wo_accessor
$read_only_accessor = __PACKAGE__->make_wo_accessor($field);
Generates a subroutine refrence which acts as a write-only accessor (mutator) for the
given $field. It only calls set().
Override set() to change the behavior of your accessors.
EXCEPTIONS
If something goes wrong Class::Accessor will warn or die by calling Carp::carp or
Carp::croak. If you don't like this you can override _carp() and _croak() in your
subclass and do whatever else you want.
EFFICIENCY
Class::Accessor does not employ an autoloader, thus it is much faster than you'd think.
Its generated methods incur no special penalty over ones you'd write yourself.
accessors:
Rate Basic Fast Faster Direct
Basic 367589/s -- -51% -55% -89%
Fast 747964/s 103% -- -9% -77%
Faster 819199/s 123% 10% -- -75%
Direct 3245887/s 783% 334% 296% --
mutators:
Rate Acc Fast Faster Direct
Acc 265564/s -- -54% -63% -91%
Fast 573439/s 116% -- -21% -80%
Faster 724710/s 173% 26% -- -75%
Direct 2860979/s 977% 399% 295% --
Class::Accessor::Fast is faster than methods written by an average programmer (where
"average" is based on Schwern's example code).
Class::Accessor is slower than average, but more flexible.
Class::Accessor::Faster is even faster than Class::Accessor::Fast. It uses an array
internally, not a hash. This could be a good or bad feature depending on your point of
view.
Direct hash access is, of course, much faster than all of these, but it provides no
encapsulation.
Of course, it's not as simple as saying "Class::Accessor is slower than average". These
are benchmarks for a simple accessor. If your accessors do any sort of complicated work
(such as talking to a database or writing to a file) the time spent doing that work will
quickly swamp the time spend just calling the accessor. In that case, Class::Accessor and
the ones you write will be roughly the same speed.
EXAMPLES
Here's an example of generating an accessor for every public field of your class.
package Altoids;
use base qw(Class::Accessor Class::Fields);
use fields qw(curiously strong mints);
Altoids->mk_accessors( Altoids->show_fields('Public') );
sub new {
my $proto = shift;
my $class = ref $proto || $proto;
return fields::new($class);
}
my Altoids $tin = Altoids->new;
$tin->curiously('Curiouser and curiouser');
print $tin->{curiously}; # prints 'Curiouser and curiouser'
# Subclassing works, too.
package Mint::Snuff;
use base qw(Altoids);
my Mint::Snuff $pouch = Mint::Snuff->new;
$pouch->strong('Blow your head off!');
print $pouch->{strong}; # prints 'Blow your head off!'
Here's a simple example of altering the behavior of your accessors.
package Foo;
use base qw(Class::Accessor);
Foo->mk_accessors(qw(this that up down));
sub get {
my $self = shift;
# Note every time someone gets some data.
print STDERR "Getting @_\n";
$self->SUPER::get(@_);
}
sub set {
my ($self, $key) = splice(@_, 0, 2);
# Note every time someone sets some data.
print STDERR "Setting $key to @_\n";
$self->SUPER::set($key, @_);
}
CAVEATS AND TRICKS
Class::Accessor has to do some internal wackiness to get its job done quickly and
efficiently. Because of this, there's a few tricks and traps one must know about.
Hey, nothing's perfect.
Don't make a field called DESTROY
This is bad. Since DESTROY is a magical method it would be bad for us to define an
accessor using that name. Class::Accessor will carp if you try to use it with a field
named "DESTROY".
Overriding autogenerated accessors
You may want to override the autogenerated accessor with your own, yet have your custom
accessor call the default one. For instance, maybe you want to have an accessor which
checks its input. Normally, one would expect this to work:
package Foo;
use base qw(Class::Accessor);
Foo->mk_accessors(qw(email this that whatever));
# Only accept addresses which look valid.
sub email {
my($self) = shift;
my($email) = @_;
if( @_ ) { # Setting
require Email::Valid;
unless( Email::Valid->address($email) ) {
carp("$email doesn't look like a valid address.");
return;
}
}
return $self->SUPER::email(@_);
}
There's a subtle problem in the last example, and it's in this line:
return $self->SUPER::email(@_);
If we look at how Foo was defined, it called mk_accessors() which stuck email() right into
Foo's namespace. There *is* no SUPER::email() to delegate to! Two ways around this...
first is to make a "pure" base class for Foo. This pure class will generate the accessors
and provide the necessary super class for Foo to use:
package Pure::Organic::Foo;
use base qw(Class::Accessor);
Pure::Organic::Foo->mk_accessors(qw(email this that whatever));
package Foo;
use base qw(Pure::Organic::Foo);
And now Foo::email() can override the generated Pure::Organic::Foo::email() and use it as
SUPER::email().
This is probably the most obvious solution to everyone but me. Instead, what first made
sense to me was for mk_accessors() to define an alias of email(), _email_accessor().
Using this solution, Foo::email() would be written with:
return $self->_email_accessor(@_);
instead of the expected SUPER::email().
AUTHORS
Copyright 2009 Marty Pauley <marty+perl AT kasei.com>
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself. That means either (a) the GNU General Public License or (b) the
Artistic License.
ORIGINAL AUTHOR
Michael G Schwern <schwern AT pobox.com>
THANKS
Liz and RUZ for performance tweaks.
Tels, for his big feature request/bug report.
Various presenters at YAPC::Asia 2009 for criticising the non-Moose interface.
SEE ALSO
See Class::Accessor::Fast and Class::Accessor::Faster if speed is more important than
flexibility.
These are some modules which do similar things in different ways Class::Struct,
Class::Methodmaker, Class::Generate, Class::Class, Class::Contract, Moose, Mouse
See Class::DBI for an example of this module in use.
perl v5.10.0 2009-09-15 Class::Accessor(3pm)
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