C++ Quiz - Singletons

Question

I'll soon be posting an article on my blog, but I'd like to verify I haven't missed anything first.

Find an example I've missed, and I'll cite you on my post.

The topic is failed Singleton implementations: In what cases can you accidentally get multiple instances of a singleton?

So far, I've come up with:

• Race Condition on first call to instance()
• Incorporation into multiple DLLs or DLL and executable
• Template definition of a singleton - actually separate classes

Any other ways I'm missing - perhaps with inheritance?

Answer 1

If you use a static instance field that you initialize in your cpp file, you can get multiple instances (and even worse behavior) if the initialization of some static/global tries to get an instance of your singleton. This is because the order of static initialization across compilation units is undefined.

Answer 2

Inheritance shouldn't be an issue as long as the ctor is private.

However, if you don't disallow the copy constructor, users may [un]intentionally copy the singleton instance. Privately inheriting from boost::noncopyable is the easiest way to prevent this.