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Class::Default - Static calls apply to a default instantiation


  # Create the defaulted class
  package Foo::Base;
  use base 'Class::Default';
  sub new { bless {}, $_[0] }
  sub show {
      my $self = shift->_self;  
  # Do something to the default object
  package main;
  print Foo::Bar->show;
  # Prints 'Foo::Bar=HASH(0x80d22f8)'


Class::Default provides a mechanism to allow your class to take static method calls and apply it to a default instantiation of an object. It provides a flexibility to an API that allows it to be used more confortably in different situations.

A good example of this technique in use is When you use a static method, like CGI-header>, your call is being applied to a default instantiation of a CGI object.

This technique appears to be especially usefull when writing modules that you want to be used in either a single use or a persistant environment. In a CGI like environment, you want the simplicity of a static interface. You can call Class-method> directly, without having to pass an instantiation around constantly.


Class::Default provides a couple of levels of control. They start with simple enabling the method to apply to the default instantation, and move on to providing some level of control over the creation of the default object.

Inheriting from Class::Default

To start, you will need to inherit from Class::Default. You do this in the normal manner, using something like use base 'Class::Default', or setting the @ISA value directly. Class::Default does not have a default constructor or any public methods, so you should be able to use it a multiple inheritance situation without any implications.

Making method work

To make your class work with Class::Default you need to make a small adjustment to each method that you would like to be able to access the default object.

A typical method will look something like the following

  sub foobar {
      my $self = shift;
      # Do whatever the method does

To make the method work with Class::Default, you should change it to the following

  sub foobar {
      my $self = shift->_self;
      # Do whatever the method does

This change is very low impact, easy to use, and will not make any other differences to the way your code works.

Control over the default object

When needed, Class::Default will make a new instantation of your class and cache it to be used whenever a static call is made. It does this in the simplest way possible, by calling Class-new()> with no arguments.

This is fine if you have a very pure class that can handle creating a new object without any arguments, but many classes expect some sort of argument to the the constructor, and indeed that the constructor that should be used it the new method.

Enter the _create_default_object method. By overloading the _create_default_object method in your class, you can custom create the default object. This will used to create the default object on demand, the first time a method is called. For example, the following class demonstrate the use of _create_default_object to set some values in the default object.

  package Slashdot::User;
  use base 'Class::Default';
  # Constructor
  sub new {
        my $class = shift;
        my $name = shift;
        my $self = {
                name => $name,
                favourite_color => '',
        return bless $self, $class;
  # Default constructor
  sub _create_default_object {
        my $class = shift;
        my $self = $class->new( 'Anonymous Coward' );
        $self->{favourite_color} = 'Orange';
        return $self;
  sub name {
  sub favourite_color {

That provides a statically accessible default object that could be used as in the following manner.

  print "The default slashdot user is " . Slashdot::User->name
      . " and they like the colour " . Slashdot::User->favourite_color;

Remember that the default object is persistant, so changes made to the statically accessible object can be recovered later.

Getting access to the default object

There are a few ways to do this, but the easiest way is to simple do the following

  my $default = Slashdot::User->_get_default;



Used by methods to make the method apply to the default object if called statically without affecting normal object methods.


The _class method provides the opposite of the _self method. Instead of always getting an object, _class will always get the class name, so a method can be guarenteed to run in a static context. This is not essential to the use of a Class::Default module, but is provided as a convenience.


Used to get the default object directly.


To be overloaded by your class to set any properties to the default object at creation time.


No known bugs, but suggestions are welcome


Bugs should be reported via the CPAN bug tracker at

For other issues, contact the author


Adam Kennedy <>

SEE ALSO, Class::Singleton


Copyright (c) 2002 - 2006 Adam Kennedy.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included with this module.