Using a class/struct/union over multiple cpp files C++

I am trying to create a class in C++ and be able to access elements of that class in more than one C++ file. I have tried over 7 possible senarios to resolve the error but have been unsuccessful. I have looked into class forward decl开发者_StackOverflow社区aration which doesen't seem to be the answer (I could be wrong).

//resources.h
class Jam{
public:
int age;
}jam;

//functions.cpp
#include "resources.h"
void printme(){
std::cout << jam.age;
}

//main.cpp
#include "resources.h"
int main(){
printme();
std::cout << jam.age;
}

Error 1 error LNK2005: "class Jam jam" (?jam@@3VJam@@A) already defined in stdafx.obj

Error 2 error LNK1169: one or more multiply defined symbols found

I understand the error is a multiple definiton because I am including resources.h in both CPP files. How can I fix this? I have tried declaring the class Jam in a CPP file and then declaring extern class Jam jam; for each CPP file that needed to access the class. I have also tried declaring pointers to the class, but I have been unsuccessful. Thank you!

You're declaring the structure Jam and creating a variable called jam of this type. The linker is complaining that you've got two (or more) things called jam, because your header causes each .cpp file to declare one of its own.

To fix this, change your header declaration to:

class Jam{
public:
int age;
};

extern Jam jam;

And then place the following line in one of your .cpp sources:

Jam jam;

You need to separate your definition from your declaration. Use:

//resources.h
class Jam{
public:
int age;
};
// Declare but don't define jam
extern Jam jam;

//resources.cpp
// Define jam here; linker will link references to this definition:
Jam jam;

The variable jam is defined in the H file, and included in multiple CPP classes, which is a problem.

Variables shouldn't be declared in H files, in order to avoid precisely that. Leave the class definition in the H file, but define the variable in one of the CPP files (and if you need to access it globally - define it as extern in all the rest).

For example:

//resources.h
class Jam{
public:
int age;
};
extern Jam jam; // all the files that include this header will know about it

//functions.cpp
#include "resources.h"
Jam jam; // the linker will link all the references to jam to this one
void printme(){
std::cout << jam.age;
}

//main.cpp
#include "resources.h"
int main(){
printme();
std::cout << jam.age;
}

As a first step you should separate the definition of the class and declaration of an instance. Then use extern in resources.h and declare the instance in a CPP.

Like this:

//resources.h
class Jam{
public:
    int age;
};

extern Jam jam;

//functions.cpp
#include "resources.h"
void printme(){
    std::cout << jam.age;
}

//main.cpp
#include "resources.h"
Jam jam;

int main(){
    printme();
    std::cout << jam.age;
}

Use the pragma once directive, or some defines to make sure a header doesn't get included more than once in your code.

Example using pragma directive (example.h):

#pragma once
// Everything below the pragma once directive will only be parsed once
class Jam { ... } jam;

Example using defines (example.h):

#ifndef _EXAMPLE_H
#define _EXAMPLE_H
// This define makes sure this code only gets parsed once
class Jam { ... } jam;
#endif