linker error with classes in c++ [duplicate] - c++

Question

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Closed 10 years ago.
Possible Duplicate:
What is an undefined reference/unresolved external symbol error and how do I fix it?
I have created this code
class Game{
static SDL_Surface* screen;
public:
//Initiate Game(SDL_Graphics, folder for output.....)
static void initialize();
static void initializeScreen();
};
void Game::initializeScreen()
{
Game::screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, 32, SDL_DOUBLEBUF |SDL_HWSURFACE |SDL_SWSURFACE);
SDL_Init(SDL_INIT_VIDEO);
Game::screen == NULL ? printf("SDL_Init failed: %s\n", SDL_GetError()):printf("SDL_Init initialized\n");
SDL_WM_SetCaption("SDL Animation", "SDL Animation");
}
It compiles but I get e linker error, how can I fix this?
1>game.obj : error LNK2001: unresolved external symbol "private: static struct SDL_Surface * Game::screen" (?screen#Game##0PAUSDL_Surface##A)
Edit: This is how I fixed it, in game.cpp
added this
SDL_Surface* Game::screen;
outside of any function*

Answer 1

You have to define static (variable) members (not functions) in seperate source files(*.cpp) and link against them.
for example:
//MyClass.h
class MyClass {
static int x;
};
//MyClass.cpp
#include "MyClass.h"
int MyClass::x;

Answer 2

You need to add the following definition in your cpp file SDL_Surface* Game::screen = NULL.
Here is an example code where you can have a static variable without defining it in the cpp file using a function.
#include <iostream>
struct Lazy {
static int& GetValue() {
static int a = 0;
return a;
}
};
int main() {
std::cout << Lazy::GetValue() << std::endl;
int& a = Lazy::GetValue();
a = 1;
std::cout << Lazy::GetValue() << std::endl;
}