Global Variables in Header file causing redefinition error c++

Question

I have a header file which holds all my global variables(and a cpp file to declare them) and I use the #ifndef #define #endif tags, but I still get redefinition errors

I have a total of 3 header files and 4 cpp files, and all of the header / main.cpp contains the globalvar.h header file.

Here is the code:

GlobalVar.h

#ifndef GLOBALVAR_H
#define GLOBALVAR_H
#include "SDL.h"

extern const int SCREEN_WIDTH = 960;
extern const int SCREEN_HEIGHT = 960;
extern const int SCREEN_BPP = 32;

extern const int FRAMES_PER_SECOND = 30;
//tiles attribute
extern const int TILE_WIDTH = 64;
extern const int TILE_HEIGHT = 64;
extern const int TOTAL_TILES = 150;
extern const int TOTAL_SPRITES = 64;
//tile sprites
extern SDL_Rect clip[144];
//Images / backgrounds
extern SDL_Surface* screen;
extern SDL_Surface* background;
extern SDL_Surface* Ike;
extern SDL_Surface* thetiles;
extern SDL_Event event;

#endif

GlobalVar.cpp

#include "GlobalVar.h"

const int SCREEN_WIDTH = 960;
const int SCREEN_HEIGHT = 960;
const int SCREEN_BPP = 32;

const int FRAMES_PER_SECOND = 30;
//tiles attribute
const int TILE_WIDTH = 64;
const int TILE_HEIGHT = 64;
const int TOTAL_TILES = 150;
const int TOTAL_SPRITES = 64;
//tile sprites
SDL_Rect clip[144];
//Images / backgrounds
SDL_Surface* screen;
SDL_Surface* background;
SDL_Surface* Ike;
SDL_Surface* thetiles;
SDL_Event event;

Answer 1

You have two options for how to deal with the constants which cause the trouble.

Option 1

Remove the extern from the header:

#ifndef GLOBALVAR_H
#define GLOBALVAR_H
#include "SDL.h"

const int SCREEN_WIDTH = 960;
const int SCREEN_HEIGHT = 960;
const int SCREEN_BPP = 32;

const int FRAMES_PER_SECOND = 30;
const int TILE_WIDTH = 64;
const int TILE_HEIGHT = 64;
const int TOTAL_TILES = 150;
const int TOTAL_SPRITES = 64;

extern SDL_Rect clip[144];
extern SDL_Surface* screen;
extern SDL_Surface* background;
extern SDL_Surface* Ike;
extern SDL_Surface* thetiles;
extern SDL_Event event;

#endif

If you do this, you must not define the variables in GlobalVar.cpp.

Option 2

Remove the initializers from the header:

#ifndef GLOBALVAR_H
#define GLOBALVAR_H
#include "SDL.h"

extern const int SCREEN_WIDTH; // = 960;
extern const int SCREEN_HEIGHT; // = 960;
extern const int SCREEN_BPP; // = 32;

extern const int FRAMES_PER_SECOND; // = 30;
extern const int TILE_WIDTH; // = 64;
extern const int TILE_HEIGHT; // = 64;
extern const int TOTAL_TILES; // = 150;
extern const int TOTAL_SPRITES; // = 64;

extern SDL_Rect clip[144];
extern SDL_Surface* screen;
extern SDL_Surface* background;
extern SDL_Surface* Ike;
extern SDL_Surface* thetiles;
extern SDL_Event event;

#endif

Now you do need to define and initialize the constants in GlobalVar.cpp.

This disadvantage of this is that you cannot use names such as SCREEN_WIDTH in contexts that require a compile-time integer constant, such as the dimensions of an array or the case clauses of a switch statement.

So, option 1 is the technique which is used more often.

Answer 2

You should only give the constants values in one place.

You an either keep the extern declarations in the header (without the values) and have the values in the cpp file, or remove the extern keyword and define the values in the header only.