Variable in header file not declared in scope

Question

I'm attempting to make a simple game, I have my main .cpp file which includes a header file (let's call it A) which includes all the other header files (let's call them B). In one of those B header files I have included the A file to access the programRunning boolean which is defined in it. None of the B header files, despite including the A file which defines the variable, seem to be able to use it. I am really confused by this and would really appreciate some help. Below is the code I have used:

pong_header.h (the A header file as described above)

#ifndef PONG_HEADER_H
#define PONG_HEADER_H

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include <stdio.h>

#include "pong_graphics.h"
#include "pong_core.h"
#include "pong_entity.h"
#include "pong_event.h"

bool programRunning;

#endif

pong_event.h (one of the B header files)

#ifndef PONG_EVENT_H
#define PONG_EVENT_H

#include "pong_header.h"


void Pong_handleEvents(SDL_Event event)
{
    while(SDL_PollEvent(&event))
    {
        switch(event.type)
        {
        case SDL_QUIT:
            programRunning = true;
            break;
        case SDL_KEYDOWN:
            switch(event.key.keysym.sym):
            case SDLK_ESCAPE:
                programRunning = false;
                break;
            break;

        default:
            break;
        }
        Pong_handleEntityEvents(event)
    }
}

The other B files access programRunning in the same way.

The exact error Code::Blocks gives me is as follows Pong\pong_event.h|20|error: 'programRunning' was not declared in this scope

Solution

The issue is that pong_header.h includes pong_event.h before it declares programRunning, so when pong_header.h tries to include pong_event.h, the include guards prevent it. The fix is to simply move the bool programRunning declaration to the top of pong_event.h.

Now, this will result in another issue - every .cpp file that includes any of these headers will get their own copy of programRunning, which will either cause a link error (multiple definition of programRunning), or it will compile, but won't run the way you expect.

What you want to do is declare it as extern, i.e.

extern bool programRunning;

Then, in one of your .cpp files (preferrably whichever has int main), you actually declare it (i.e. without extern):

bool programRunning;