Why can't I multi-declare a class

Question

I can do this

extern int i;
extern int i;

But I can\'t do the same with a class

class A {
..
}
class A {
..
}

Answer 1

you can do

class A;

as often as you want and then in one file define it with

class A { ... }

Example for this:
classB.h:

class A;
class B { A *a; }

classA.h:

class B;
class A { B *b; }