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Re: Making an Existing Class a Singleton

by halley (Prior)
on Jan 23, 2008 at 17:13 UTC ( #663839=note: print w/replies, xml ) Need Help??

in reply to Making an Existing Class a Singleton

You highlight the exact difference between a pattern and a class library. The Gang of Four who published the Design Patterns book decided to describe patterns, or recurring ideas in feature implementation, instead of publishing a book on some great classes to put into any language's built-in class library. Patterns are idioms, not parts.

It seems very neat and tidy to offer a class like Class::Singleton but the devil is in the details, and your exact scenario is where theory meets reality. Don't think "I have to glue on some premade code to adopt a pattern." Think "understanding the standard implementation of a standard pattern will help me effect the same feature in my own code quickly and robustly."

Another poster has already shown you how to make a typical singleton-compliant constructor. Make a hidden module variable that keeps track of which instance is regarded as the master instance, and check that variable before making a possibly redundant instance. That's the singleton pattern. It can be done in your own code in 30 seconds.

You don't need a class library to implement patterns for you, and any class library purporting to implement academic patterns for you is likely going to have some over-engineered non-elegant features bolted on to deal with real-world problems. They'll call it "flexibility."

[ e d @ h a l l e y . c c ]

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[Discipulus]: get them all printed on screen
Discipulus ;=)
[shmem]: silly question, silly answer:
[Discipulus]: i know, i know..
[shmem]: man perlfunc |perl -nle '$ok++ if /Alphabetical Listing of Perl Functions/;if($ok ){$s{$1}++if /^ (\w+)/}}{for(keys %s){$f="prototype \\\&CORE::$_"; eval"$f"||delete $CORE::{$_}}print for sort keys%CORE::'
[shmem]: well, there are some builtins which don't have a prototype...
[Discipulus]: eh eh.. i was looking in toke.c but dunno if is already used
[shmem]: oh the whitespace in the regex got condensed, meh
[shmem]: should be /^ {7}(\w+)/ or such
[shmem]: least for my perldoc on Linux debian 8

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