Question

i am trying to update static int

test.h

static int Delay = 0;
void UpdateDelay();

test.cpp

#include "test.h"

void UpdateDelay(){
Delay = 500;
}

Main.cpp

    #include "test.h"

    int main(){
    UpdateDelay();
    std::cout << Delay << std::endl;
    return 0;

}

output should be : 500
but it shows : 0

Thx

Answer 1

A global variable declared as static has internal linkage. This means that each translation unit (i.e. .cpp file) gets a private copy of that variable.

Changes done to a private copy of one translation unit won’t have any effect on private copies of the same variable held by different translation units.

If you want to share one global variable, provide one definition for it in a single translation unit, and let all other translation unit refer it through a declaration that specifies the extern keyword:

test.h

extern int Delay;
void UpdateDelay();

test.cpp

#include "test.h"

void UpdateDelay(){
    Delay = 500;
}

main.cpp

#include "test.h"

int Delay = 0; // Do not declare this as static, or you will give
               // internal linkage to this variable. That means it
               // won't be visible from other translation units.

int main(){
    UpdateDelay();
    std::cout << Delay << std::endl;
    return 0;
}