Error: invalid use of member in static member function - c++

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.

Answer 3

I'm not convinced that the code abstract you show us is an accurate characterization of your problem.
Your header should not include using namespace std; — it doesn't use or declare anything from the std namespace, and specifying using namespace std; is generally regarded as 'not a good idea', doubly so when it appears in a header file.
It also isn't clear that your header needs to include SDL/SDL.h. If the Uint8 type is easily isolated (not necessarily valid), then your header file can simply use a forward declaration of the SDL_Surface class. (Your implementation code will need to include SDL/SDL.h; but you should not burden the users of your wrapper class with unnecessary #include directives when simple forward declarations would suffice.)
This code is self-contained (does not need any headers), but more or less simulates what you could use, and it compiles OK:
#ifndef WRAPPER_HPP
#define WRAPPER_HPP
typedef unsigned char Uint8;
class SDL_Surface;
class Wrapper
{
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
//#include <SDL/SDL.h>
typedef unsigned short Uint16;
class SDL_Surface
{
public:
Uint8 *pixels;
Uint16 pitch;
struct
{
Uint8 BytesPerPixel;
} *format;
};
// End of SDL/SDL.h
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;
}
It also compiles without warnings. The (Uint8 *) cast (copied from the original) is unnecessary. With the class definition given, it is superfluous; if you are needing to use a cast because the type of the pixels member of SDL_Surface actually isn't Uint8, are you sure it is a good idea? And can't you use reinterpret_cast<Uint8>(screen->pixels) instead to make it clearer?
Can you reduce your problem to code analogous to this that still shows the actual error?