problem with classes interaction

singersinger

hey you guys, the problem is that i have 2 classes (say A,B) , class A have a pointer on class B class A { B* b;}; and class B have a member variable for type class A class B {A a;}; this is seems to be a problem and the compiler retrun an error say 'a' uses undefined class 'A' although i am declaring the header of A #include "A.h" class B {A a;}; anybody have any idea abt wat might be the problem here thnx 4 ur time and concern learn, learn, and learn

Nibu babu thomas

singersinger wrote:

this is seems to be a problem and the compiler retrun an error say 'a' uses undefined class 'A' although i am declaring the header of A

Mr Compiler is right. You need to forward declare class B, which tells the compiler that there is a class called B, and he needn't worry about it.

class B;

class A
{ B* b;};

class B
{A a;};

Nibu thomas A Developer Programming tips[^] My site[^]

wheelerbarry

Your problem is that a is not defined yet, as the compiler correctly says. The first thing it does when compiling A is go to B and compile that, and thus it does not know that class A exists yet. THe solution to this is to add the prototype to A before including B. e.g. class A; #inlcude "B.h" class A { B* b;};

singersinger

guys, thnx alot 4 ur fast reply but still this doesn't solve the problem, i don't know why :( --------------------- learn, learn, and learn

Maximilien

wheelerbarry wrote:

class A;

#inlcude "B.h"

class A
{ B* b;};

with this, you will have include problems. in B.h you will need to have include guards

// B.h
#ifndef _B_H_
#define _B_H_
class B
{
// ...
};
#endif

and in A.h it is good practice ( IMO, YMMV ) to also add include guards :

// A.h
#ifndef _A_H_
#define _A_H_

#ifndef _B_H_
#include "b.h"
#endif

class A
{
B* b;
};
#endif

IMO, using forward declaration is the best approach to that problem unless B is not a pointer.

Maximilien Lincourt Your Head A Splode - Strong Bad

David Crow

// A.h
#pragma once

class B;

#include "B.h"

class A
{
B* b;
};

// B.h
#pragma once

#include "A.h"

class B
{
A a;
};

// main.cpp
#include "B.h"

void main( void )
{
B bb;
}

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