c++ extern variable not visible - c++

Question

I use extern variable for my application class so i can forward class function to glutDisplayFunction(funcPtr).
main.cpp:
#include "main.hpp"
int main(int argc, char** argv)
{
gApp = new GameApp();
return 0;
}
main.hpp:
#ifndef MAIN_HPP
#define MAIN_HPP
#include "GameApp.hpp"
#endif
GameApp.hpp:
#include <GL/gl.h>
#include <GL/freeglut.h>
class GameApp
{
public:
int running;
GameApp();
virtual ~GameApp();
void resize(int width, int height);
void init(int argc, char** argv, int width, int height);
void draw();
void update();
void key_input(unsigned char key, int x, int y);
};
extern GameApp *gApp;
void display_clb()
{
if (!gApp)
{
return;
}
gApp->draw();
}
This is the output:
g++ -o dist/Debug/GNU-Linux-x86/gravity build/Debug/GNU-Linux-x86/main.o build/Debug/GNU-Linux-x86/GBody.o build/Debug/GNU-Linux-x86/GameApp.o build/Debug/GNU-Linux-x86/GBodyList.o -lm -lGL -lglfw -lGLU -lglut
build/Debug/GNU-Linux-x86/main.o: In function `main':
/home/viktor/Documents/cpp/Gravity/main.cpp:6: undefined reference to `gApp'
/home/viktor/Documents/cpp/Gravity/main.cpp:7: undefined reference to `gApp'
/home/viktor/Documents/cpp/Gravity/GameApp.cpp:13: undefined reference to `gApp'
/home/viktor/Documents/cpp/Gravity/GameApp.cpp:18: undefined reference to `gApp'
build/Debug/GNU-Linux-x86/GameApp.o: In function `display_clb()':
/home/viktor/Documents/cpp/Gravity/GameApp.cpp:23: undefined reference to `gApp'
build/Debug/GNU-Linux-x86/GameApp.o:/home/viktor/Documents/cpp/Gravity/GameApp.cpp:28: more undefined references to `gApp' follow
collect2: ld returned 1 exit status
make[2]: *** [dist/Debug/GNU-Linux-x86/gravity] Error 1
make[2]: Leaving directory `/home/viktor/Documents/cpp/Gravity'
make[1]: *** [.build-conf] Error 2
make[1]: Leaving directory `/home/viktor/Documents/cpp/Gravity'
make: *** [.build-impl] Error 2
I expected gApp to be visible in my main.cpp and in GameApp class.

Answer 1

That's not a compile error, it is a link error. You variable declaration is visible just fine in main.cpp, but you haven't defined it anywhere - i.e. you don't allocate space for that variable anywhere.
You'll need one (and exactly one) C++ file that defines that variable. Possibly your main.cpp:
GameApp *gApp;
(You could initialize it too right there, but that is not necessary in this case.)

Answer 2

This tells the compiler there is a variable named gApp but it is defined somewhere else:
extern GameApp *gApp;
because that definition does not exist, the linker fails.
Add the following to another (and only one) source file:
GameApp *gApp;

Answer 3

With extern, you tell the compiler that the variable exists, but it is located somewhere else. The compiler believes you the variable exists, and
All you have to do is create the actual variable somewhere in the source. You can do this by simply adding something like GameApp *gApp; somewhere. For example in your cpp file.

Answer 4

Same as the previous answers from other guys, you announced the existence of gApp, but you did not actually provide it.
Add one more word: I suggest you put the definition of gApp in a "GameApp.cpp" file(not GameApp.hpp), and put the declaration of it in a "GameApp.h" file.