Question

I have a class that I only wish clients create them one object per process. Instead of singleton, a better way (I believe) is to tell the clients to only create them in main(). So a natural enforcement is to make the constructor private and main() as a friend.

It works this way:

class A { friend int main(int, char**); A() {} };
int main(int, char **) { A a; }

But it breaks when I need to put class A in a namespace:

namepace ns { class A { friend int main(int, char**); A() {} }; }
int main(int, char **) { ns::A a; }

The problem is scoping: the compiler now thinks

friend int main 

means a function named main() in namespace ns. So the real main() becomes irrelevant.

So the question is: how to fix this? Of course I’ll have to put class A in a namespace.

Answer 1

You need to declare main in the global namespace before the class definition, since friend declarations can only introduce names in the surrounding namespace:

int main(int, char**);

and qualify the name when referring to it inside the namespace:

namepace ns { class A { friend int ::main(int, char**); A() {} }; }
//                                 ^^