Forward declaration cause "one or more multiplied defined symbol found"?

Question

I am using forward declaration, and I have already been careful not to have any definition inside header files (only declaration), and also have #pragma once directive before each header file. Yet somehow multiple definition error is still happening. So in GlobalSys.h, I use forward declaration, then i will include this file to any file that needs access to this global variable. In application.h, i initialize this global variable so I have to include EventManager.h or the compiler will complain. Where am I doing wrong?

GlobalSys.h

#pragma once
class EventManager;

namespace GlobalSys {
    EventManager * eventManager;
}

Application.h

#include "GlobalSys.h"
#include "../Event/EventManager.h" 

class Application {

public:
    Application();
};

Application.cpp

#include "Application.h"

Application::Application() {
    GlobalSys::eventManager = new EventManager();
}

Answer 1

and I have already been careful not to have any definition inside header files (only declaration)

No, you defined GlobalSys::eventManager in GlobalSys.h.

Definitions are declarations that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:

• Any declaration with an extern storage class specifier or with a language linkage specifier (such as extern "C") without an initializer

You can change it to declaration with using of extern.

GlobalSys.h

#pragma once
class EventManager;

namespace GlobalSys {
    extern EventManager * eventManager; // declaration
}

then define it in another implementation file, e.g.

GlobalSys.cpp

#include "GlobalSys.h"

namespace GlobalSys {
    EventManager * eventManager; // definition
}