NAME

Class::AutoloadCAN - Make AUTOLOAD, can and inheritance cooperate.

SYNOPSIS

package Foo;
use Class::AutoloadCAN;

sub CAN {
  my ($starting_class, $method, $self, @arguments) = @_;
  return sub {
    my $self = shift;
    print join ", ", $method, @_;
    print "\n";
  };
}

# And this prints the famous greeting.
Foo->hello("world");

DESCRIPTION

This module solves a fundamental conflict between AUTOLOAD, can and inheritance. The problem is that while you can implement anything in AUTOLOAD, UNIVERSAL::can is not aware that it is there. Attempting to modify UNIVERSAL::can to document those methods is very hard. And if a parent class uses AUTOLOAD then subclasses have to do a lot of work to make their AUTOLOADs cooperate with the parent one. It is harder still if 2 parent classes in a multiple inheritance tree wish to cooperate with each other. Few try to do this, which may be good since those who try usually get it wrong. See http://www.perlmonks.org/?node_id=342804 for a fuller discussion.

With this module instead of writing AUTOLOADs, you write CANs. Based on what they return, Class::AutoloadCAN will decide whether you handle the call or it needs to search higher up the inheritance chain.

Here are the methods and functions that matter for the operation of this module.

AUTOLOAD

An AUTOLOAD will be installed in every package that uses this module. You can choose to have it installed in other packages. If you write your own AUTOLOADs, you can easily break this module. So don't do that. Write CANs instead.

can

UNIVERSAL::can will be modified to be aware of the functions provided dynamically through this module. You are free to override can in any subclass and this module will not interfere. I have no idea why you would want to, though.

CAN

If there is a method named CAN in a class that inherits from one that Universal::AutoloadCAN was installed to, it may be called in deciding how a method is implemented. It will be passed the class that the method search started in, the method name, the object called, and the arguments to the function. It is expected to do nothing but return a subroutine reference if it implements that method on that object, or undef otherwise.

If that subroutine is actually called, it will be passed all of the usual arguments that a method call gets, and the AUTOLOAD that found it will erase itself from the callstack.

Class::AutoloadCAN::import

If the import method for Class::AutoloadCAN is called with no arguments it installs an AUTOLOAD in the calling class. If it is called with arguments, it installs an AUTOLOAD in those classes as well. Use with caution: this is a convenience feature that is not expected to be used very often.

SUGGESTION

Many people use AUTOLOAD to implement large numbers of fixed and straightforward methods. Such as accessors. If you are doing this, then I suggest implementing them by typeglobbing closures instead of by using AUTOLOAD or this module. Here is a simple example:

package Parent;
use strict;

sub make_accessors {
  my ($class, @attributes) = @_;
  foreach my $attribute (@attributes) {
    no strict 'refs';
    *{"$class\::$attribute"} = sub {
      my $self = shift;
      if (@_) {
        $self->{$attribute} = shift;
      }
      return $self->{$attribute};
    };
  }
}


package Child;
our @ISA = 'Parent';
__PACKAGE__->make_accessors(qw(this that the other));

This approach is simpler, often faster, and avoids some of the problems that AUTOLOAD has, like mistaking function calls as method calls.

BUGS AND LIMITATIONS

There are many other issues with AUTOLOAD that this module does not address. Primary among them is the fact that if you call a function that does not exist in a package that inherits from one with an AUTOLOAD, Perl will do a method search for that AUTOLOAD. This is why this module does not install AUTOLOAD in UNIVERSAL by default, and it is strongly suggested that you not do so either.

Also many people like to lazily install AUTOLOADed methods in the local package so that they will be found more quickly in the future. This module won't do that for you, but you can easily do that from within CAN. The reason that this module doesn't do that is that some useful CANs may decide whether to support a method on an object by object basis.

ACKNOWLEDGEMENTS

My thanks to various people on Perlmonks for conversations that clarified what problems AUTOLOAD raises, and convinced me that it would be good to have a solution to them.

AUTHOR AND COPYRIGHT

Ben Tilly (btilly@gmail.com).

Copyright 2005. This may be copied, modified and distributed on the same terms as Perl.