Question

What is the difference between private static variable and private instance variable in a singleton class?

I see no semantic difference.

EDIT: Not asking if the variable holding the instance of itself should be static, but other data members.

Answer 1

The way I see it, there is only “no semantic difference” if you assume that the singleton is implemented using a static reference to the single instance. The thing is, static is just one way to implement a singleton — it’s an implementation detail. You can implement singletons other ways, too.