How does getInstance() work?

Question

Recent I read some C++ code using extensively following getInstance() method:

class S
{
    private:
        int some_int = 0;
    public:
        static S& getInstance()
        {
            static S instance; / (*) /
            return instance;
        }
};

From how this code fragment being used, I learned the getInstance() works like return this, returning the address(or ref) of the instance of class S. But I got confused.

1) where does the static variable S defined in line(*) allocated in memory? And why it can work like return this?

2) what if there exist more than one instance of class S, whose reference will be returned?

Answer 1

This is the so-called Singleton design pattern. Its distinguishing feature is that there can only ever be exactly one instance of that class and the pattern ensures that. The class has a private constructor and a statically-created instance that is returned with the getInstance method. You cannot create an instance from the outside and thus get the object only through said method.

Since instance is static in the getInstance method it will retain its value between multiple invocations. It is allocated and constructed somewhen before it's first used. E.g. in this answer it seems like GCC initializes the static variable at the time the function is first used. This answer has some excerpts from the C++ standard related to that.