C++ INCLUDE Rule : Use forward declaration when possible

Suppose you want to define a new class B that uses objects of class A.

1. B only uses references or pointers to A. Use forward declaration then : you don't need to include <A.h>. This will in turn speed a little bit the compilation.

     class A ;
     
     class B {
       private:
         A* fPtrA ;
       public:
         void mymethod(const& A) const ;
     } ;
     

2. B derives from A or B explicitely (or implicitely) uses objects of class A. You then need to include <A.h>

     #include <A.h>
     
     class B : public A {
     
     } ;
     
     class C {
       private:
         A fA ;
       public:
         void mymethod(A par) ;   
     }