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 \s-1AUTOLOAD\s0, can and inheritance. The problem is that while you can implement anything in \s-1AUTOLOAD\s0, 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 \s-1AUTOLOAD\s0 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. An \s-1AUTOLOAD\s0 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. 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.

If there is a method named \s-1CAN\s0 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 \s-1AUTOLOAD\s0 that found it will erase itself from the callstack. If the import method for Class::AutoloadCAN is called with no arguments it installs an \s-1AUTOLOAD\s0 in the calling class. If it is called with arguments, it installs an \s-1AUTOLOAD\s0 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 \s-1AUTOLOAD\s0 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 \s-1AUTOLOAD\s0 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 \s-1AUTOLOAD\s0 has, like mistaking function calls as method calls.

BUGS AND LIMITATIONS

There are many other issues with \s-1AUTOLOAD\s0 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 \s-1AUTOLOAD\s0, Perl will do a method search for that \s-1AUTOLOAD\s0. This is why this module does not install \s-1AUTOLOAD\s0 in \s-1UNIVERSAL\s0 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 \s-1CAN\s0. 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 \s-1AUTOLOAD\s0 raises, and convinced me that it would be good to have a solution to them.

AUTHOR AND COPYRIGHT

Ben Tilly ([email protected]).