NAME
Class::SelfMethods - a Module for supporting instance-defined methods
SYNOPSIS
use Class::SelfMethods;
package MyClass;
@ISA = qw(Class::SelfMethods);
use strict;
sub _friendly {
my $self = shift;
return $self->name;
}
package main;
no strict;
my $foo = MyClass->new( name => 'foo' );
my $bar = MyClass->new( name => 'bar', friendly => 'Bar');
my $bas = MyClass->new( name => 'bas',
friendly => sub {
my $self = shift;
return ucfirst($self->_friendly);
}
);
print $foo->friendly, "\n";
print $bar->friendly, "\n";
print $bas->friendly, "\n";
$bas->friendly_SET('a reset friendly');
print $bas->friendly, "\n";
$bas->friendly_SET( sub { my $self = shift; return uc($self->_friendly) });
print $bas->friendly, "\n";
$bas->friendly_CLEAR;
print $bas->friendly, "\n";
DESCRIPTION
Development of this module has largely lapsed due to the superior
performance and feature set of "Class::Prototyped". If you haven't
written code that depends upon "Class::SelfMethods", I strongly urge you
to look at "Class::Prototyped" first.
"Class::SelfMethods" merges some features of other Object Oriented
languages to build a system for implementing more flexible objects than
is provided by default in Perl.
The core features I was looking for when I wrote "Class::SelfMethods"
were:
Class-based inheritance hierarchy
I wanted to retain Perl's normal class-based inheritance hierarchy
rather than to write (or use) a completely prototype based system.
If you are looking for a purely prototype based system, see Sean M.
Burke's "Class::Classless". My reasoning on this is that it is
easier in file based languages (as opposed to world based languages
like Self) to code class based inheritance hierarchies (which are
largely static) than to code object based inheritance hierarchies
(since objects in such languages have a dynamicism that is not
granted to classes).
Instance-defined method overriding
I wanted instances to be able to override their class-defined
methods. In the example above, the "$bas" object has its own
"friendly" method. Instance-defined methods are passed the exact
same parameter list as class-defined methods.
Subroutine/Attribute equivalence
Borrowing from Self, I wanted to be able to treat methods and
attributes similarly. For instance, in the above example the "$bar"
object has an attribute "friendly", whereas the "$bas" object has a
method "friendly", and the "$foo" object uses the class-defined
method. The calling syntax is independent of the implementation.
Parameters can even be passed in the method call and they will
simply be ignored if the method is implemented by a simple attribute
In addition to those core features, I (and Damian) had a wish list of
additional features:
Simple syntax
I wanted the system to be reasonable easy to use for both
implementers of classes and users of objects. Simple syntax for
users is more important than simple syntax for implementers.
Full support for "SUPER" type concepts
I wanted instance-defined methods to be able to call the
class-defined methods they replace.
Support for calling methods at instantiation time
In some circumstances, rather than deal with multiple inheritance it
is easier to have a class-defined object method that sets up the
various instance-defined methods for a given object. To support
this, the "new" method allows deferred method calls to be passed in
as parameters.
Modifying objects post-instantiation
I originally had no need for modifying objects post-instantiation,
but Damian Conway thought it would be a Good Thing (TM) to support.
Being so very good at these sorts of thing, he instantly came up
with a good general syntax to support such. Method calls that end in
a "_SET" result in the first parameter being assigned to the
attribute/method. I noticed one remaining hole and added support for
"_CLEAR".
HOW TO
Write A Class
Your class should inherit from "Class::SelfMethods". The class-defined
instance methods should be defined with a leading underscore and should
be called without a leading underscore. Don't do anything silly like
writing methods whose proper names have a leading underscore and whose
definitions have two leading underscores - that's just asking for
trouble.
Do not, of course, make use of attributes that have leading underscores
- that's also just asking for trouble. Also, do not access attributes
directly (i.e. "$self->{foo}"). That will prevent people who use your
class from substituting a method for an attribute. Instead, always read
attributes by making the corresponding method call ("$self->foo").
If you need to call "SUPER::methodname", call "SUPER::_methodname".
Create An Instance
The default "new" method uses named parameters. Unless you are
certifiable, you will too. To specify attributes, simply use the syntax
"name => 'value'" and to specify a method use "name => sub { my $self =
shift; . . . }". Note that methods and attributes are interchangeable.
Modify An Instance
Method calls that end in a "_SET" will result in their first parameter
being assigned to the appropriate attribute/method. For instance, in the
"SYNOPSIS" I use "$foo->friendly_SET" to specify both a value and a
method for "friendly". Method calls that end in a "_CLEAR" will delete
that attribute/method from the object. The "can" method will behave just
like "UNIVERSAL::can" - it returns a code reference that will
interoperate with the associated object properly using the
"$obj->$coderef()" syntax. For examples of usage, see "test.pl".
Installation instructions
Standard module installation procedure.
INTERNALS
can
This implementation of "can" is the heart of the system. By making "can"
responsible for almost everything relating to accessing the objects, the
code for deciding how to respond to the various situtations is kept in
one place.
In order to get major speed improvements (a factor of 2 to 3 for
attribute retrieval and method calls), extensive symbol table
manipulation was used to build methods on the fly that react
appropriately.
The three types of methods are "_SET" methods, "_CLEAR" methods, and
"normal" methods. The first two are fairly straight forward as far as
implementation goes. First "UNIVERSAL::can" is called to determine
whether an appropriate entry has been made in the package symbol table.
If not, an anonymous subroutine (actually, a closure in this case
because "$func" is a lexically scoped variable defined outside the
anonymous subroutine and referenced from within) is created and assigned
into the package symbol table. In either case, a reference to the
appropriate closure is returned (normal "can" behavior is to return a
reference to the code or "undef" if the method call is not legal).
The "normal" methods are somewhat trickier. The outer "if" statement
exists to ensure that "can" returns "undef" for illegal method calls
(remember that there may be situations where "$self->can($func)" should
return false even though "UNIVERSAL::can($self, $func)" returns true).
It then checks whether an appropriate entry has been made in the package
symbol table. If not, it builds a closure that will do the trick.
Remember that the closure could get called on an object that is in any
of the four possible states - attribute, instance method, inherited
method, or illegal. The closure includes the logic to test for instance
methods and attributes, but if neither are present it will make the call
to "_method" regardless of whether or not there is an inherited method
with the proper name. It relies on "AUTOLOAD" to properly deal with
unhandled "_method" calls.
AUTOLOAD
"AUTOLOAD" gets called the first time a given method call is made. It
first strips off the package name from the function call to extract the
actual function name. It then checks to see if the function name starts
with an underscore. If it does, it's a failed call from the "normal"
method closure, so "AUTOLOAD" calls "croak" to die with the appropriate
error message. Notice that the underscore has been stripped off, so it
will "die" failing to find "method".
"AUTOLOAD" then calls "can", which will return a reference to the
appropriate "CODE" entity if the method call is supported. At the same
time, "can" puts an entry into the symbol table for "Class::SelfMethods"
to support future calls to that method. "AUTOLOAD" jumps to that "CODE"
entity if a valid entity was return. Otherwise, execution continues on
to another "croak" call.
new
The "new" method supplied in "Class::SelfMethods" provides one
interesting twist on an otherwise standard named parameters constructor.
It strips out any passed parameters that have leading underscores and
stores them away. It then creates the hash ref from the remaining
parameters and blesses it appropriately. Finally, it takes the stored
parameters that have leading underscores and makes the matching method
calls - the key is used for the method name and the value is
dereferenced to an array and passed as parameters.
AUTHOR
Toby Ovod-Everett, tovod-everett@alascom.att.com
CREDITS
Damian Conway, damian@cs.monash.edu.au
Responsible for accessor methods, module name, constructive
criticism and moral support. After I responded to Sean's suggestion
of implementing a "can" method, Damian completely rewrote my first
attempt by routing everything through "can". He also was the first
to point out direct symbol table manipulation by implementing it for
the "_SET" and "_CLEAR" methods. I rebutted his routing everything
through "can" by doing performance testing. He agreed that the
performance issues were a problem, but suggested retaining the
direct symbol table for the accessor methods. It was then that the
lightbulb went off and I realized that a properly written closure
could be used for the normal method calls. Damian's criticisms kept
me on track and from making a fool of myself, and the result is some
very fast (and I hope safe:) code.
I first started writing to Damian as a result of an excellent book
he wrote, Object Oriented Perl. I highly recommend it - get it, read
it.
Sean M. Burke, sburke@netadventure.net
Suggested implementing a "can" method. Sean was/is responsible for
"Class::Classless". If you need a full-featured purely prototype
based object system, check it out.