c++ extern variable not visible

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 #include 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/…

Accepted Answer

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.)

c++ extern variable not visible

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