Question

I have a LRESULT CALLBACK function in a header file(hook.h) that I both forward declare then define in the file(along with some classes that contain static variables). I then define(implement/create?) the static class variables in the associated .cpp file (hook.cpp).

Finally I include the header file in my stdafx.h file so I can use it in my program.

Because I include the hook.h file twice I get a compilation error that the LRESULT CALLBACK function is defined twice, the error is:

stdafx.obj : error LNK2005: "long __stdcall LowLevelKeyboardProc(int,unsigned int,long)" (?LowLevelKeyboardProc@@YGJHIJ@Z) already defined in main.obj
1>main.obj : error LNK2001: unresolved external symbol "protected: static class LowLevelKeyboardHookEx * LowLevelKeyboardHookEx::instance" (?instance@LowLevelKeyboardHookEx@@1PAV1@A)
1>C:\Users\Soribo\Dropbox\C++ Programming\Visual C++ Programming\Key Cataloguer\Release\Key Cataloguer.exe : fatal error LNK1120: 1 unresolved externals

How can I avoid this issue?

My header file:

#ifndef KEYBOARDHOOK_H
#define KEYBOARDHOOK_H

#include "stdafx.h"

LRESULT CALLBACK KeyboardProc( int code, WPARAM wParam, LPARAM lParam );

class MyClass {
    public:
      static std::string instanceStr;
      // further down this class it refers to the function KeyboardProc() thus need for forward declaration
};

LRESULT CALLBACK KeyboardProc( int code, WPARAM wParam, LPARAM lParam )
{
  // implements function
}

#endif

My hook.cpp file:

#include "stdafx.h"
#include "hook.h"

std::string MyClass::instanceStr = "";

My stdafx.h file:

#pragma once

#include "targetver.h"

#define WIN32_LEAN_AND_MEAN             // Exclude rarely-used stuff from Windows headers
// Windows Header Files:
#include <windows.h>

// C RunTime Header Files
#include <stdlib.h>

// Application Specific Includes
#include <string>
#include "hook.h"       // I think this is the cause of the error because I include this file twice in compilation which means that the LRESULT function is redefined/reimplemented

I have also tried not including the hook.h file in hook.cpp & just including stdafx.h but I get the same issue?

Answer 1

Move your definition of KeyboardProc (or is it LowLevelKeyboardProc?) into hook.cpp. That is, move this code from hook.h into hook.cpp

LRESULT CALLBACK KeyboardProc( int code, WPARAM wParam, LPARAM lParam )
{
  // implements function
}

But leave your “declaration” of KeybaordProc in hook.h. That is, leave this line inside hook.h:

LRESULT CALLBACK KeyboardProc( int code, WPARAM wParam, LPARAM lParam );

That way, any other source file can reference KeyboardProc, but only one definition of the function is linked in.

There are some alternative solutions including some “inline” keywords and possibly a pragma/declspec directive to make it work. Neither seems right for what you want to do.