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."

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