Class::Accessor(3pm)

1Class::Accessor(3)    User Contributed Perl Documentation   Class::Accessor(3)
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NAME

6         Class::Accessor - Automated accessor generation
7

SYNOPSIS

9         package Employee;
10         use base qw(Class::Accessor);
11         Employee->mk_accessors(qw(name role salary));
12
13         # Meanwhile, in a nearby piece of code!
14         # Class::Accessor provides new().
15         my $mp = Foo->new({ name => "Marty", role => "JAPH" });
16
17         my $job = $mp->role;  # gets $mp->{role}
18         $mp->salary(400000);  # sets $mp->{salary} = 400000 (I wish)
19
20         # like my @info = @{$mp}{qw(name role)}
21         my @info = $mp->get(qw(name role));
22
23         # $mp->{salary} = 400000
24         $mp->set('salary', 400000);
25

DESCRIPTION

27       This module automagically generates accessors/mutators for your class.
28
29       Most of the time, writing accessors is an exercise in cutting and
30       pasting.  You usually wind up with a series of methods like this:
31
32           sub name {
33               my $self = shift;
34               if(@_) {
35                   $self->{name} = $_[0];
36               }
37               return $self->{name};
38           }
39
40           sub salary {
41               my $self = shift;
42               if(@_) {
43                   $self->{salary} = $_[0];
44               }
45               return $self->{salary};
46           }
47
48         # etc...
49
50       One for each piece of data in your object.  While some will be unique,
51       doing value checks and special storage tricks, most will simply be
52       exercises in repetition.  Not only is it Bad Style to have a bunch of
53       repetitious code, but its also simply not lazy, which is the real
54       tragedy.
55
56       If you make your module a subclass of Class::Accessor and declare your
57       accessor fields with mk_accessors() then you'll find yourself with a
58       set of automatically generated accessors which can even be customized!
59
60       The basic set up is very simple:
61
62           package My::Class;
63           use base qw(Class::Accessor);
64           My::Class->mk_accessors( qw(foo bar car) );
65
66       Done.  My::Class now has simple foo(), bar() and car() accessors
67       defined.
68
69   What Makes This Different?
70       What makes this module special compared to all the other method
71       generating modules ("SEE ALSO")?  By overriding the get() and set()
72       methods you can alter the behavior of the accessors class-wide.  Also,
73       the accessors are implemented as closures which should cost a bit less
74       memory than most other solutions which generate a new method for each
75       accessor.
76

METHODS

78   new
79           my $obj = Class->new;
80           my $obj = $other_obj->new;
81
82           my $obj = Class->new(\%fields);
83           my $obj = $other_obj->new(\%fields);
84
85       Class::Accessor provides a basic constructor.  It generates a hash-
86       based object and can be called as either a class method or an object
87       method.
88
89       It takes an optional %fields hash which is used to initialize the
90       object (handy if you use read-only accessors).  The fields of the hash
91       correspond to the names of your accessors, so...
92
93           package Foo;
94           use base qw(Class::Accessor);
95           Foo->mk_accessors('foo');
96
97           my $obj = Class->new({ foo => 42 });
98           print $obj->foo;    # 42
99
100       however %fields can contain anything, new() will shove them all into
101       your object.  Don't like it?  Override it.
102
103   mk_accessors
104           Class->mk_accessors(@fields);
105
106       This creates accessor/mutator methods for each named field given in
107       @fields.  Foreach field in @fields it will generate two accessors.  One
108       called "field()" and the other called "_field_accessor()".  For
109       example:
110
111           # Generates foo(), _foo_accessor(), bar() and _bar_accessor().
112           Class->mk_accessors(qw(foo bar));
113
114       See "Overriding autogenerated accessors" in CAVEATS AND TRICKS for
115       details.
116
117   mk_ro_accessors
118         Class->mk_ro_accessors(@read_only_fields);
119
120       Same as mk_accessors() except it will generate read-only accessors (ie.
121       true accessors).  If you attempt to set a value with these accessors it
122       will throw an exception.  It only uses get() and not set().
123
124           package Foo;
125           use base qw(Class::Accessor);
126           Class->mk_ro_accessors(qw(foo bar));
127
128           # Let's assume we have an object $foo of class Foo...
129           print $foo->foo;  # ok, prints whatever the value of $foo->{foo} is
130           $foo->foo(42);    # BOOM!  Naughty you.
131
132   mk_wo_accessors
133         Class->mk_wo_accessors(@write_only_fields);
134
135       Same as mk_accessors() except it will generate write-only accessors
136       (ie. mutators).  If you attempt to read a value with these accessors it
137       will throw an exception.  It only uses set() and not get().
138
139       NOTE I'm not entirely sure why this is useful, but I'm sure someone
140       will need it.  If you've found a use, let me know.  Right now its here
141       for orthoginality and because its easy to implement.
142
143           package Foo;
144           use base qw(Class::Accessor);
145           Class->mk_wo_accessors(qw(foo bar));
146
147           # Let's assume we have an object $foo of class Foo...
148           $foo->foo(42);      # OK.  Sets $self->{foo} = 42
149           print $foo->foo;    # BOOM!  Can't read from this accessor.
150

DETAILS

152       An accessor generated by Class::Accessor looks something like this:
153
154           # Your foo may vary.
155           sub foo {
156               my($self) = shift;
157               if(@_) {    # set
158                   return $self->set('foo', @_);
159               }
160               else {
161                   return $self->get('foo');
162               }
163           }
164
165       Very simple.  All it does is determine if you're wanting to set a value
166       or get a value and calls the appropriate method.  Class::Accessor
167       provides default get() and set() methods which your class can override.
168       They're detailed later.
169
170   follow_best_practice
171       In Damian's Perl Best Practices book he recommends separate get and set
172       methods with the prefix set_ and get_ to make it explicit what you
173       intend to do.  If you want to create those accessor methods instead of
174       the default ones, call:
175
176           __PACKAGE__->follow_best_practice
177
178   accessor_name_for / mutator_name_for
179       You may have your own crazy ideas for the names of the accessors, so
180       you can make those happen by overriding "accessor_name_for" and
181       "mutator_name_for" in your subclass.  (I copied that idea from
182       Class::DBI.)
183
184   Modifying the behavior of the accessor
185       Rather than actually modifying the accessor itself, it is much more
186       sensible to simply override the two key methods which the accessor
187       calls.  Namely set() and get().
188
189       If you -really- want to, you can override make_accessor().
190
191   set
192           $obj->set($key, $value);
193           $obj->set($key, @values);
194
195       set() defines how generally one stores data in the object.
196
197       override this method to change how data is stored by your accessors.
198
199   get
200           $value  = $obj->get($key);
201           @values = $obj->get(@keys);
202
203       get() defines how data is retreived from your objects.
204
205       override this method to change how it is retreived.
206
207   make_accessor
208           $accessor = Class->make_accessor($field);
209
210       Generates a subroutine reference which acts as an accessor for the
211       given $field.  It calls get() and set().
212
213       If you wish to change the behavior of your accessors, try overriding
214       get() and set() before you start mucking with make_accessor().
215
216   make_ro_accessor
217           $read_only_accessor = Class->make_ro_accessor($field);
218
219       Generates a subroutine refrence which acts as a read-only accessor for
220       the given $field.  It only calls get().
221
222       Override get() to change the behavior of your accessors.
223
224   make_wo_accessor
225           $read_only_accessor = Class->make_wo_accessor($field);
226
227       Generates a subroutine refrence which acts as a write-only accessor
228       (mutator) for the given $field.  It only calls set().
229
230       Override set() to change the behavior of your accessors.
231

EXCEPTIONS

233       If something goes wrong Class::Accessor will warn or die by calling
234       Carp::carp or Carp::croak.  If you don't like this you can override
235       _carp() and _croak() in your subclass and do whatever else you want.
236

EFFICIENCY

238       Class::Accessor does not employ an autoloader, thus it is much faster
239       than you'd think.  Its generated methods incur no special penalty over
240       ones you'd write yourself.
241
242         accessors:
243                      Rate   Basic Average    Fast  Faster  Direct
244         Basic    189150/s      --    -42%    -51%    -55%    -89%
245         Average  327679/s     73%      --    -16%    -22%    -82%
246         Fast     389212/s    106%     19%      --     -8%    -78%
247         Faster   421646/s    123%     29%      8%      --    -76%
248         Direct  1771243/s    836%    441%    355%    320%      --
249
250         mutators:
251                      Rate   Basic Average    Fast  Faster  Direct
252         Basic    173769/s      --    -34%    -53%    -59%    -90%
253         Average  263046/s     51%      --    -29%    -38%    -85%
254         Fast     371158/s    114%     41%      --    -13%    -78%
255         Faster   425821/s    145%     62%     15%      --    -75%
256         Direct  1699081/s    878%    546%    358%    299%      --
257
258       Class::Accessor::Fast is faster than methods written by an average
259       programmer (where "average" is based on Schwern's example code).
260
261       Class::Accessor is slower than average, but more flexible.
262
263       Class::Accessor::Faster is even faster than Class::Accessor::Fast.  It
264       uses an array internally, not a hash.  This could be a good or bad
265       feature depending on your point of view.
266
267       Direct hash access is, of course, much faster than all of these, but it
268       provides no encapsulation.
269
270       Of course, its not as simple as saying "Class::Accessor is slower than
271       average".  These are benchmarks for a simple accessor.  If your
272       accessors do any sort of complicated work (such as talking to a
273       database or writing to a file) the time spent doing that work will
274       quickly swamp the time spend just calling the accessor.  In that case,
275       Class::Accessor and the ones you write will be roughly the same speed.
276

EXAMPLES

278       Here's an example of generating an accessor for every public field of
279       your class.
280
281           package Altoids;
282
283           use base qw(Class::Accessor Class::Fields);
284           use fields qw(curiously strong mints);
285           Altoids->mk_accessors( Altoids->show_fields('Public') );
286
287           sub new {
288               my $proto = shift;
289               my $class = ref $proto || $proto;
290               return fields::new($class);
291           }
292
293           my Altoids $tin = Altoids->new;
294
295           $tin->curiously('Curiouser and curiouser');
296           print $tin->{curiously};    # prints 'Curiouser and curiouser'
297
298
299           # Subclassing works, too.
300           package Mint::Snuff;
301           use base qw(Altoids);
302
303           my Mint::Snuff $pouch = Mint::Snuff->new;
304           $pouch->strong('Blow your head off!');
305           print $pouch->{strong};     # prints 'Blow your head off!'
306
307       Here's a simple example of altering the behavior of your accessors.
308
309           package Foo;
310           use base qw(Class::Accessor);
311           Foo->mk_accessor(qw(this that up down));
312
313           sub get {
314               my $self = shift;
315
316               # Note every time someone gets some data.
317               print STDERR "Getting @_\n";
318
319               $self->SUPER::get(@_);
320           }
321
322           sub set {
323               my ($self, $key) = splice(@_, 0, 2);
324
325               # Note every time someone sets some data.
326               print STDERR "Setting $key to @_\n";
327
328               $self->SUPER::set($key, @_);
329           }
330

CAVEATS AND TRICKS

332       Class::Accessor has to do some internal wackiness to get its job done
333       quickly and efficiently.  Because of this, there's a few tricks and
334       traps one must know about.
335
336       Hey, nothing's perfect.
337
338   Don't make a field called DESTROY
339       This is bad.  Since DESTROY is a magical method it would be bad for us
340       to define an accessor using that name.  Class::Accessor will carp if
341       you try to use it with a field named "DESTROY".
342
343   Overriding autogenerated accessors
344       You may want to override the autogenerated accessor with your own, yet
345       have your custom accessor call the default one.  For instance, maybe
346       you want to have an accessor which checks its input.  Normally, one
347       would expect this to work:
348
349           package Foo;
350           use base qw(Class::Accessor);
351           Foo->mk_accessors(qw(email this that whatever));
352
353           # Only accept addresses which look valid.
354           sub email {
355               my($self) = shift;
356               my($email) = @_;
357
358               if( @_ ) {  # Setting
359                   require Email::Valid;
360                   unless( Email::Valid->address($email) ) {
361                       carp("$email doesn't look like a valid address.");
362                       return;
363                   }
364               }
365
366               return $self->SUPER::email(@_);
367           }
368
369       There's a subtle problem in the last example, and its in this line:
370
371           return $self->SUPER::email(@_);
372
373       If we look at how Foo was defined, it called mk_accessors() which stuck
374       email() right into Foo's namespace.  There *is* no SUPER::email() to
375       delegate to!  Two ways around this... first is to make a "pure" base
376       class for Foo.  This pure class will generate the accessors and provide
377       the necessary super class for Foo to use:
378
379           package Pure::Organic::Foo;
380           use base qw(Class::Accessor);
381           Pure::Organic::Foo->mk_accessors(qw(email this that whatever));
382
383           package Foo;
384           use base qw(Pure::Organic::Foo);
385
386       And now Foo::email() can override the generated
387       Pure::Organic::Foo::email() and use it as SUPER::email().
388
389       This is probably the most obvious solution to everyone but me.
390       Instead, what first made sense to me was for mk_accessors() to define
391       an alias of email(), _email_accessor().  Using this solution,
392       Foo::email() would be written with:
393
394           return $self->_email_accessor(@_);
395
396       instead of the expected SUPER::email().
397

AUTHORS

399       Copyright 2007 Marty Pauley <marty+perl@kasei.com>
400
401       This program is free software; you can redistribute it and/or modify it
402       under the same terms as Perl itself.  That means either (a) the GNU
403       General Public License or (b) the Artistic License.
404
405   ORIGINAL AUTHOR
406       Michael G Schwern <schwern@pobox.com>
407
408   THANKS
409       Liz and RUZ for performance tweaks.
410
411       Tels, for his big feature request/bug report.
412