Question

There are few global variables which are static in one of the header files. I see these variables are used in the associated .cc files. So, looks like this has no issues.

My questions are:

• Whats the difference between including a global variable vs static global variable ?
  I know static global doesnt have visibility outside its file. But dont know how this would work when it comes as part of a .h which is #included.

• I wrote a sample program, and tried the same thing. But, I get compilation error the moment I make the variable static. When it is just global, it is fine.
  So, is there something which I am missing on a regular g++ build ? (Please note, the initial case was on our official code base which has enough makefiles, .h files and all).

Thanks for the help !

Here is my sample program :

.h file:

#include <iostream>

typedef unsigned int uint;

static const int appk=189;

class abc1
{
    public:
        abc1(int x);
        virtual void printVal();

};

.cc file:

#include "abc1.h"

extern int appk;

abc1::abc1(int x)
{

}

void abc1::printVal()
{
    printf("abc1 print: %d\n", appk);
}

Answer 1

(1) If you put a global variable in a .h file and include it in various .cpp/.cc files then it will be defined multiple times for every file. So you are most like to get a linker error.
To overcome that, mostly you are likely to use extern keyword:

// myfile.h
extern int i;

and define that in only one translation unit:

// somefile.cc
int i;

(2) If you put a static global in a .h file and include it, then you will not get any error, because for every different translation unit, there will be a different copy for that static global variable.

// myfile.h
static int i;  // creates a unique and unrelated copy in all .cc file where included

However, such usage is deprecated; instead of that it’s better to use unnamed namespace:

namespace {
  int i;
}

From your question, I don’t see that you should get any linker error for static global.