C++: Has Not Been Declared

Question

I have seen this type of error everywhere and, although I have looked at the answers, none seem to help.

I get the following error with the following piece of code:<

Answer 1

Replace the include with a forward declaration:

//B.h
class A;
class B{
    public:
         static bool doX(A *a);
};

Include files only when you have to.

Also, use include guards. This will prevent other nasty issues like re-definitions & such.