Forward Declaration vs Include

Question

Consider the following two scenarios (Edited just to complete the whole question and make it clearer)

Case 1: (doesnt compile as rightly mentioned below)

//B.h
#ifndef B_H
#define B_H
#include "B.h"

class A;

class B { 
        A obj;
        public:
        void printA_thruB();

         };  
#endif

//B.cpp
#include "B.h"
#include <iostream>

void B::printA_thruB(){
        obj.printA();
        }   


//A.h;
#ifndef A_H
#define A_H

#include "A.h"

class A { 
        int a;
        public:
        A();
        void printA();

         };  
#endif   

//A.cpp                           
#include "A.h"                    
#include <iostream>               

A::A(){                           
        a=10;                     
        }                         

void A::printA()                  
{                                 
std::cout<<"A:"<<a<<std::endl;    
}  


//main.cpp
 #include "B.h"
  #include<iostream>
 using namespace std;

 int main()
 {
 B obj;
 obj.printA_thruB();
 }

Case 2: (the only modifications...works without compiliation error)

//B.h

#include "A.h" //Add this line
//class A;     //comment out this line

Let us assume both the A.cpp and B.cpp are complied together. Do the above two scenarios make any differences? Is there a reason to prefer one method over the other?

Edit: So how do I make scenario 1 work.

Answer 1

Case 1 will produce an "incomplete type" error when you compile B.cpp. Because class B contains a class A object, the definition (and in particular the size) of class A is required to be complete before the definition of class B.

Alternatively, you could choose to make some_variable a pointer or reference to class A, and in that case your forward declaration would be sufficient in B.h. You'd still need a full definition of A in B.cpp (assuming you made actual use of the A member functions/data).