Question

Let’s say I have 2 classses:

// a.h
#ifndef A_H
#define A_H

#include "b.h"

class A {
  public: void a() {
    B* b = new B(this);
  }
}

#endif


// b.h
#ifndef B_H
#define B_H

#include "a.h"

class B {
  public: B(A* a) {
     // ...
  }
}

#endif

This code will raise error A has not been declared because class A references class B, which in turn references class A that hasn’t been declared in a.h yet.

So how can I pass an instance of A to B?

Answer 1

You need to

1. change the pass-by-value parameter of the B constructor to pointer or reference*
2. move the definition of B::B to the cpp file (this removes any direct dependency on the definition of A, and allows you to

3. forward declare A in B.h :

   // b.h
   #ifndef B_H
   #define B_H
   
   class A;
   
   class B {
     public: B(const A& a) {
        // ...
     }
   }
   
   #endif
   
   // b.cpp
   
   #include "b.h"
   #include "a.h"
   
   public B::B(const A& a) {
     // ...
   }
   

Note that I added a const qualifier for the A& constructor parameter, as I assume you don’t want to modify a in the constructor of B.

Of course, you can also play this the other way around, by moving the definition of A::a and the corresponding #include "b.h" into a.cpp.

*_{You should most likely do this anyway, as it is highly unlikely you wanted to pass an A object by value there. Passing an object by value means implicitly creating a copy of the object and pushing that onto the stack. Which}

• _{means that changes made on the parameter object will not affect the original object, which is usually not what you would desire, thus is a source of bugs,}
• _{includes creating and destroying a temporary,}
• _{usually consumes much more memory than a reference / pointer,}
• _{opens the door to object slicing bugs.}