在多个cpp文件上使用类/结构/联合C ++

Question

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 declaration 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!

Answer 1

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;

Answer 2

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;

Answer 3

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;
}

Answer 4

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;
}

Answer 5

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