Exactly as you would invoke a public function of a class with a non-private constructor, i.e.
Code:
A* foo = whatever(); // this function returns a pointer to an object of class A
foo.fun(0); // this invokes A::fun(int).
Of course, if the code you posted is all the code you have, then the answer is that you can't, because you have no way of constructing an object of class A.