Class::Accessor - Automated accessor generation
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);
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".
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.
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.
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.
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.
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.)
__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.
__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.
__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
orthogonality 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 $foo->foo; # BOOM! Can't read from this accessor.
If you prefer a Moose-like interface to create accessors, you can use
"has" by importing this module like this:
use Class::Accessor "antlers";
or
use Class::Accessor "moose-like";
Then you can declare accessors like this:
has alpha => ( is => "rw", isa => "Str" );
has beta => ( is => "ro", isa => "Str" );
has gamma => ( is => "wo", isa => "Str" );
Currently only the "is" attribute is supported. And our "is"
also supports the "wo" value to make a write-only accessor.
If you are using the Moose-like interface then you should use the
"extends" rather than tweaking your @ISA directly. Basically,
replace
@ISA = qw/Foo Bar/;
with
extends(qw/Foo Bar/);
An accessor generated by Class::Accessor looks something like this:
# Your foo may vary.
sub foo {
my($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.
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().
$obj->set($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.
$value = $obj->get($key);
@values = $obj->get(@keys);
get() defines how data is retrieved from your objects.
override this method to change how it is retrieved.
$accessor = __PACKAGE__->make_accessor($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().
$read_only_accessor = __PACKAGE__->make_ro_accessor($field);
Generates a subroutine reference which acts as a read-only accessor for the
given $field. It only calls
get().
Override
get() to change the behavior of your accessors.
$write_only_accessor = __PACKAGE__->make_wo_accessor($field);
Generates a subroutine reference 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.
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.
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.
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, @_);
}
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.
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".
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().
Copyright 2017 Marty Pauley <
[email protected]>
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.
Michael G Schwern <
[email protected]>
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 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.