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

That's a forward declaration. Consider the following example:

class foo; // you likely need this for the code beneath to compile
class bar {
    void smth( foo& );
};

If you haven't included the definition of class foo in such a way that the compiler sees it before compiling the definition of class bar the code will not compile (the compiler will say it doesn't know what foo means) unless you have the forward declaration.

Answer 2

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();
};

Answer 3

That's a forward declaration. You need it for example if class bar has a pointer to a foo object, but you don't want to include the whole definition of the foo object immediately.