Header files inclusion / Forward declaration

Question

In my C++ project when do I have to use inclusion (#include \"myclass.h\") of header files? And when do I have to use forward declaration of the class (cl

Answer 1

As a beginner, you should always #include header files when you need to use the types or functions they contain - do not try to "optimise" your build by forward declaring things - this is hardly ever necessary, even on large projects, provided the project is well architected.

The only time you absolutely need a forward declaration is in situations like this:

struct A {
   void f( B b );
};

struct B {
   void f( A a );
};

where each struct (or class) refers to the type of the other. In this case, you need a forward declaration of B to resolve the issue:

struct B;   // forward declaration

struct A {
   void f( B b );
};

struct B {
   void f( A a );
};