class foo; in header file

Question

Is some one able to explain why header files have something like this?

class foo; // This here?
class bar
{
   bar();

};

Do you need an in

Answer 1

Just curious, why do we need the term forward declaration at all? Isn't a forward declaration simply a declaration (as opposed to a definition)?

class X;  // declaration

class X   // definition
{
    int member;
    void function();
};