Thread: Having two classes reference each other.

Hello! I'm new to the forums and C++ as well. I've been following through a few books but I haven't found how to do this correctly yet.

I'm programming a console application using Visual C++.

What I am aiming to do is to allow one class to reference another. This is for a game that me and couple of friends are programming.

I found a thread similar to this before I joined. However following through it I haven't produced any reliable results. So, here is my code, split up into 5 files.

The code compiles fine. However, in the linking stage, I recieve several errors:

Code:

Linking...
a.obj : error LNK2005: "public: __thiscall A::A(void)" (??0A@@QAE@XZ) already defined in cppsample.obj
a.obj : error LNK2005: "public: __thiscall A::~A(void)" (??1A@@QAE@XZ) already defined in cppsample.obj
a.obj : error LNK2005: "public: int __thiscall A::getBV(void)" (?getBV@A@@QAEHXZ) already defined in cppsample.obj
b.obj : error LNK2005: "public: __thiscall B::B(void)" (??0B@@QAE@XZ) already defined in cppsample.obj
b.obj : error LNK2005: "public: __thiscall B::~B(void)" (??1B@@QAE@XZ) already defined in cppsample.obj
b.obj : error LNK2005: "public: int __thiscall B::getAV(void)" (?getAV@B@@QAEHXZ) already defined in cppsample.obj
Release/cppsample.exe : fatal error LNK1169: one or more multiply defined symbols found

cppsample.cpp

Code:

//This is the main file.
//Includes (Unused)
/*
These include files are not included but may be included later.
#include <limits>				//Limits
#include <fstream>				//File stream for dealing with files.
*/

//Includes (Standard)
#include <cstdio>				//C Standard Input/Output
#include <cstdlib>				//C Standard Library
#include <iostream>				//Input/Output Stream
#include <string>				//C++ Strings
using namespace std;			//Use Standard Namespace

//Includes (custom)
#include "a.cpp"
#include "b.cpp"

//Defines
#define CURRENT_EXPERIMENT "This program will try to get two classes to reference each other."

//Type Definitions
typedef short unsigned int sui;	//Shorthand form of numbers from 0 to 65,535.
typedef short int si;			//Shorthand form of numbers from -32,768 to 32,767.

int main(int iNumberOfArgs, char* psArgs[])
{
	cout << CURRENT_EXPERIMENT << "\n";
	system("PAUSE");
	A hi;
	cout << hi.iOfA << "\n";
	system("PAUSE");
	return 0;
}

a.cpp

Code:

#include "a.h"
#include "b.h"
A::A()
{
	iOfA=1;
	pToB=0;
}
A::~A()
{
}
int A::getBV()
{
	return pToB->iOfB;
}

b.cpp

Code:

#include "a.h"
#include "b.h"

B::B()
{
	iOfB=2;
	pToA=0;
}
B::~B()
{
}
int B::getAV()
{
	return pToA->iOfA;
}

a.h

Code:

#ifndef A_ADDED
#define A_ADDED
#ifndef B_ADDED
class B;
#endif
class A
{
public:
	A();
	~A();
	int getBV();
	int iOfA;
private:
	B* pToB;
};
#endif

b.h

Code:

#ifndef B_ADDED
#define B_ADDED
#ifndef A_ADDED
class A;
#endif
class B
{
public:
	B();
	~B();
	int getAV();
	int iOfB;
private:
	A* pToA;
};
#endif

>> #include "a.cpp"

You don't include cpp files (unless you are doing template programming, which you aren't). That's your problem. You should be including the header files (a.h and b.h).

>> #ifndef B_ADDED

That is not necessary in your a.h file, and the same is true for the #ifndef A_ADDED in your b.h file. Just use one include guard specific to that particular file.

Thank you, I have solved my problem now.