Error: invalid use of member in static member function

Question

I have two classes, and this is the header of one of them:

#ifndef WRAPPER_HPP
#define WRAPPER_HPP

#include <SDL/SDL.h>

using namespace std;

class Wrapper
{
  private:
    //SDL_Surface *screen;

  public:
    static SDL_Surface *screen;

    static void set_screen(SDL_Surface *_screen);
    static void set_pixel(int x, int y, Uint8 color);
    static void clear_screen(int r, int g, int b);
    static SDL_Surface* load_image(char path[500]);
    static void draw_image(SDL_Surface *img, int x, int y, int width, int height);
    static void draw_line(int x1, int y1, int x2, int y2, Uint8 color);
};

#endif

I am calling Wrapper::set_screen(screen) from another file and I get this error:

In file included from /home/david/src/aships/src/Wrapper.cpp:6:0:
/home/david/src/aships/src/Wrapper.hpp: In static member function ‘static void Wrapper::set_screen(SDL_Surface*)’:
/home/david/src/aships/src/Wrapper.hpp:11:18: error: invalid use of member ‘Wrapper::screen’ in static member function
/home/david/src/aships/src/Wrapper.cpp:10:3: error: from this location

I also get a similar error for the definition of every single function on Wrapper.cpp, for example:

void Wrapper::set_pixel(int x, int y, Uint8 color)
{
  /* Draws a pixel on the screen at (x, y) with color 'color' */
  Uint8 *p;
  p = (Uint8 *) screen->pixels + y * screen->pitch + x * screen->format->BytesPerPixel;
  *p = color;
}

On compile:

/home/david/src/aships/src/Wrapper.hpp: In static member function ‘static void Wrapper::set_pixel(int, int, Uint8)’:
/home/david/src/aships/src/Wrapper.hpp:11:18: error: invalid use of member ‘Wrapper::screen’ in static member function
/home/david/src/aships/src/Wrapper.cpp:17:17: error: from this location

I know it's related to the class being static and thus the variable Wrapper.screen is not accessible or something, but I'm not sure of how to fix it. Any ideas?

Answer 1

You are using a static variable

static SDL_Surface *screen;

in your code.

In C++ when you declare a static variable in the .h (or .hpp) you are creating a variable that is general (static) to the class. Thus, to use it in another file you have to redeclare it (which I'm guessing you didn't) to create a variable in that file referencing the static one. In your case put this:

SDL_Surface* Wrapper::screen;

in the .cpp file.

I'm not sure the theory is well explained, but it works like that.

Answer 2

Your class and member (screen) are not static, which means they don't actually exist. You can't access a non static member in a static function.

Try to make your data members to be static.