C++ Extern Class Declaration

Question

Edit: Ok I wrote a little test program to show here. Here is the Source Code.

main.cpp:

#include \"core.h\"

Core core;

int main()
{
  core.coreFunc         


        

Answer 1

You are including the window.h header before the "extern Core core;" line. Try adding that line just before the class Window line on the window.h header:

window.h

#ifndef WINDOW_H__
#define WINDOW_H__

extern Core core;

class Window
{...}

Instead of using Core as a global variable, you can move core as a static member of the Core class. This is called the Singleton pattern.

main.cpp

#include "core.h"

int main()
{
  Core* core = Core::getInstance();

  core->coreFunction();
}

core.h

#include "window.h"

class Core
{
public:
  static Core* getInstance() { return &coreInstance; }
  void someFunction();

private:
  static Core coreInstance;
  Window window;
};

core.cpp

#include "core.h"

Core Core::coreInstance;

void Core::someFunction()
{
  window.doSomething();
}

window.h

class Window
{
  void someFunction();
};

window.cpp

#include "window.h"
#include "core.h"

void Window::someFunction()
{
  Core* core = Core::getInstance();

  core->doSomething();
}