linker error with classes in c++

Question

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*

andre · Answer 1 · 2012-11-30T18:13:48.967

1

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;
}

edited Nov 30 '12 at 18:13

answered Nov 30 '12 at 14:11

andre

7,018
4
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75

is it not private by default? – Laggy Nov 30 '12 at 14:14
2

I think your issue is caused because you have not defined `SDL_Surface* screen;` in the cpp file. static variable need to be define like `SDL_Surface* Game::screen = NULL` in you cpp file. – andre Nov 30 '12 at 14:20
this does not work as I cant assign value to a funtion – Laggy Nov 30 '12 at 14:30
@arcana has anwsered the question corectly , you just edited it when you seen that your anwser didnt work – Laggy Nov 30 '12 at 15:46
@Laggy No, I edited it because you did not seem to understand **Lazy initialization**. – andre Nov 30 '12 at 17:40
you gave me a funtion that returns SDL_Surface screen – Laggy Nov 30 '12 at 18:00
@Laggy I updated the code with an example of what I was speaking about. – andre Nov 30 '12 at 18:14

score 1 · Accepted Answer · answered Nov 30 '12 at 14:24

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;

answered Nov 30 '12 at 14:24

Mohamad mehdi Kharatizadeh

364
3
7

i get this error `error C2655: 'Game::screen' : definition or redeclaration illegal in current scope` – Laggy Nov 30 '12 at 15:02
1

thanks for help, I have figured it out, I tried to put it into the function initializeScreen() which didnt work, so I put it outside of the function and it worked :) – Laggy Nov 30 '12 at 15:43

linker error with classes in c++

2 Answers2