Question

I know what is going on, but I don’t know how to fix this:

main.cpp

#include "Win32.h"

int main () {
    return 0;
}

Win32.h

#include <windows.h>
#include <map>

#ifndef WIN32_H_
#define WIN32_H_

namespace W32 {

class Win32;                        // Pre-Declaration
std::map<HWND, Win32 *> windowMap;  // Handle to Class instance mapping

class Win32 {
    public:

        Win32();
        virtual ~Win32();

    protected:

    private:

};  // Class Win32

} // namespace W32

#endif // WIN32_H_

Win32.cpp

#include "Win32.h"


namespace W32 {

Win32::Win32() {
}

Win32::~Win32() {
}

} /* namespace W32 */

Error Messages:

    src\Win32.o: In function `Win32':
    D:\Dev\Projects\Eclipse\OpenGL3\Debug/../src/Win32.cpp:7: multiple definition of `W32::windowMap'
    src\main.o:D:\Dev\Projects\Eclipse\OpenGL3\Debug/../src/main.cpp:14: first defined here

Ok, I get it that std::map<HWND, Win32 *> windowMap; is appearing in more the one file, and that because it is included in more then one file (main.cpp/Win32.cpp) it is causing it to be redefined. I am still somewhat new to std::map. What I need to do is prototype windowMap, but what I don’t know is how? I thought this was when I grabbed this piece of code. The Win32 class needs to be able to use it, but it will have to be declared in order to do so, but what I have is not the way to do it, I am at a loss on what to look for to get the right information on how to properly forward declare std::map<HWND, Win32 *> windowMap.

Answer 1

std::map<HWND, Win32 *> windowMap; 

is a definition, so you’re breaking the one definition rule. You need to make the variable extern:

extern std::map<HWND, Win32 *> windowMap; 

and define it in a single implementation file:

Win32.h

#include <windows.h>
#include <map>

#ifndef WIN32_H_
#define WIN32_H_

namespace W32 {
   class Win32;                        // Pre-Declaration
   extern std::map<HWND, Win32 *> windowMap;  // Handle to Class instance mapping
   //...
};  // Class Win32

} // namespace W32

#endif // WIN32_H_

Win32.cpp

#include "Win32.h"
namespace W32 {
   std::map<HWND, Win32 *> windowMap;  // Handle to Class instance mapping
   //...
} /* namespace W32 */