NAME

Singleton - A class which can instantiated only once.

SYNOPSIS

 package Testme;
 use ResourcePool::Singleton;
 use Data::Dumper;
 
 push @ISA, "ResourcePool::Singleton";
 
 sub new($@) {
    my $proto = shift;
    my $class = ref($proto) || $proto;
    my $d = Data::Dumper->new([@_]);
    $d->Indent(0);
    my $key = $d->Dump();
    my $self;
    
    $self = $class->SUPER::new("Testme". $key);
    if (!exists($self->{CNT})) {
       $self->{CNT} = $_[0];
       bless($self, $class);
    }
    return $self;
 }      
 
 sub next($) {
     my ($self) = @_;
     return $self->{CNT}++;
 }
 

DESCRIPTION

The Singleton class, or clasess derived from this class, can be instantiated only once. If you call the constructor of this class the first time, it will perform an normal object construction and return a reference to a blessed value. But it will also store this reference in a global hash.

On further calls of this constructor the Singleton class will just return the stored reference instead of creating a new one.

This is very useful if the construction of an object is very expansive but it is required to be constructed at different places in your program. A special application for this feature is a Apache/mod_perl environment.

The Singleton class can not check if the stored object references are still valid, therfore it might return references to objects which have already been destroyed. If you need a persistant object which gets recreated on failure you should consider to use the ResourcePool and/or the LoadBalancer modules.

Singleton->new($$)

The construcotr takes one argument which is a key to the object wich will be created. You have to build a key which is unique for your needs. In most cases it's most appropriate to use the Data::Dumper like shown above to construct such a key.

AUTHOR

    Copyright (C) 2002 by Markus Winand <mws@fatalmind.com>

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