class Foo{public:	int a;};class Bar : public Foo{public:	int b;};int main (){	printf ("%d, %d, %d\n", sizeof(void*) sizeof(Foo*), sizeof(Bar*));	return 0;}

4, 4, 4

Quote:Original post by Evil Steve
class foo; is a forward declaration; it just tells the compiler that foo is a class, and you'll give it the definition of it later.

Because the compiler doesn't know the classes size or anything, you can only use pointers or references to the class, and not call any class methods until you include the header.

Usually you'd use the forward declaration in the header, so you don't pull foo's header into that file, and then you'd #include "foo.h" in bar.cpp, so you can call it's methods and so on.

class Foo;class Bar{public:    Foo BarMethod( Foo f );   };