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 {
..
}

While in both cases no memory is being allocated.

Answer 1

The following are declarations:

extern int i;
class A;

And the next two are definitions:

int i;
class A { ... };

The rules are:

• a definition is also a declaration.
• you have to have 'seen' a declaration of an item before you can use it.
• re-declaration is OK (must be identical).
• re-definition is an error (the One Definition Rule).