C++ class forward declaration

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) ;   
   }