Qt : multiple defined symbols found

Question

This is a Qt project which, once built, results in a dll AnTS_Core.dll so I have:

AnTs_Core.cpp

#include <windows.h>
#include "Globals.h" // my global values
extern "C"
{
    __declspec(dllexport) void load();
}
void load()
{

    mainDispatcher = new Dispatcher();
}

The Global header file which contains all main objects as global(because I want to call an object method from an other object):

Globals.h:

#ifndef GLOBALS_H
#define GLOBALS_H
#include "AnTS_Types.h"
#include "Dispatcher.h"
#ifdef __cplusplus
extern "C"
{
#endif
    Dispatcher *mainDispatcher;
#ifdef __cplusplus
}
#endif
#endif // GLOBALS_H

The Dispatcher : header file

#ifndef DISPATCHER_H
#define DISPATCHER_H
#include "AnTS_Types.h"
#include "Device.h"
#include <list>
#include <windows.h>
class Dispatcher
{
public:
    Dispatcher();
    ~Dispatcher();
private:
    std::list<Device*> _devices;
};
#endif

Dispatcher.cpp :

#include "Dispatcher.h"
#include <algorithm>
#include <iostream>
#include <cstdio>
#include <string.h>
#include <dirent.h>
#include <regex>
#include "Device/DEV_Struct.h"
Dispatcher::Dispatcher()
{
}

and the device (Dispatcher contains a list of devices)

Device.h

#ifndef DEVICE_H
#define DEVICE_H

#include <windows.h>
#include "Device/DEV_Struct.h"
#include "AnTS_Types.h"
#define ANTS_DEVICE_NAME_LENGHT 64
class Device
{
public:
    Device(char*);
    ~Device();
};
#endif // DEVICE_H

Device.cpp

#include "../Includes/Device.h"
#include <string.h>
#include <iostream>
#include <cstdio>
#include "Globals.h"

Device::Device(char* dllPath)
{
}

The errors are:

LNK2005 _mainDispatcher already defined in AnTS_Core.cpp.obj

LNK1169 one or more multiply defined symbols found

The errors disappear when I comment the line #include "Globals.h" in Device.cpp. But I want to access to global variables (to an other Dispatcher for example or to another object) from the device.cpp file.

score 1 · Accepted Answer · edited May 23 '17 at 11:43

So, this is a classical declaration vs definition issue - you have defined the variable mainDispatcher in the header and thus every compilation unit that includes this header winds up with a definition, where what you want is to declare the variable in the header as extern (this will only inform every compilation unit that includes the header that such variable exists):

#ifndef GLOBALS_H
#define GLOBALS_H
#include "AnTS_Types.h"
#include "Dispatcher.h"
#ifdef __cplusplus
extern "C"
{
#endif
    extern Dispatcher *mainDispatcher;
#ifdef __cplusplus
}
#endif
#endif // GLOBALS_H`

And you should place the actual definition Dispatcher* mainDispatcher in one of your .cpp files.

Ahh thank you very much ! I had already tried the "extern" but I didn't place the definition in a .cpp file. — lctdim, Apr 30 '15 at 11:11

score 0 · Answer 2 · answered Apr 30 '15 at 10:17

You have Dispatcher *mainDispatcher; in your Globals.h, this way every compilation unit which includes this header creates its own instance of this symbol. Declare extern Dispatcher *mainDispatcher; in Globals.h, and add Dispatcher *mainDispatcher; in AnTs_Core.cpp. This way you will have one symbol for AnTs_Core.cpp compilation unit, but others will see it through extern declaration.

Qt : multiple defined symbols found

2 Answers2