CODEWITHSUNDEEP
c++
roohan
roohan

Reputation: 751

C++ Extern Class Declaration

I wrote a little test program to show here. Here is the source code.

main.cpp:

#include "core.h"

Core core;

int main()
{
  core.coreFunction();
}

core.h:

#ifndef CORE_H__
#define CORE_H__

#include "definitions.h"
#include "window.h"

class Core
{
public:
  Window window;
  void coreFunction()
  {
    window.windowFunction();
  }
};
extern Core core;

#endif

definitions.h

#ifndef DEFINITIONS_H__
#define DEFINITIONS_H__

class Core;
class Window;

#endif

window.h

#ifndef WINDOW_H__
#define WINDOW_H__

class Window
{
public:

   void windowFunction()
   {
     core.coreFunction();
   }
};

#endif

With this test program I get the following error: window.h(10): error C2065: 'core' : undeclared identifier. I hope this clarifies my problem a little bit. Please ignore that these functions make no sense its just for showing what I did because my original code is way too long to post here.

Upvotes: 7

Views: 83202

Answers (3)

vz0
vz0

Reputation: 32923

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();
}

Upvotes: 9

Mooing Duck
Mooing Duck

Reputation: 66922

Either you forgot to define core's default constructor, or core cannot be trivially default constructed (due to having a base or member that does not have a default constructor.

To core, add:

class Core {
    Window window;

    Core() {} //add this line
    void someFunction()
    {
        window.doSomething();
    }
}

To window, add:

class Window
{
    Window() {} //add this line
    void someFunction()
    {
        core.doSomething();
    }
}

If either fails to compile, you'll have pinned down the problem a little more

EDIT:

Well now that the error message was clarified, I see the error right off. Window.h requires Core core to be defined, and Core.h requires Window to be defined. The solution is to do as vz0 suggested from the get go. Move the definition of Window::someFunction to window.cpp (and I feel the need to go apologize to vz0)

Upvotes: 0

JKor
JKor

Reputation: 3832

I think I found your problem. In the definition header file the "Core" class is declared as "core". Remember, caps makes a big difference.

Upvotes: 0

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