Answer by ricab for How do you implement the Singleton design pattern?

Here is a mockable singleton using CRTP. It relies on a little helper to enforce a single object at any one time (at most). To enforce a single object over program execution, remove the reset (which we find useful for tests).

A ConcreteSinleton can be implemented like this:

class ConcreteSingleton : public Singleton<ConcreteSingleton>{public:  ConcreteSingleton(const Singleton<ConcreteSingleton>::PrivatePass&)      : Singleton<StandardPaths>::Singleton{pass}  {}  // ... concrete interface  int f() const {return 42;}};

And then used with

ConcreteSingleton::instance().f();