Question

I have three files, a main .cpp file:

#include <stdio.h>
#include "myClass.h"



int main()
{

    myClass mvar;

    tryVar = 23; // why does this not work?

    printf("%d ", mvar.readTryVar()); // This writes out 0, why??

    return 0;
}

a myClass.cpp file

#include "myClass.h"


myClass::myClass(void)
{
}


myClass::~myClass(void)
{
}

void myClass::setTryVar()
{
    tryVar = 23334;
}

int myClass::readTryVar()
{
    return tryVar;
}

and a myClass.h file

#pragma once

static int tryVar;

class myClass
{
public:
    myClass(void);
    ~myClass(void);


    void setTryVar();

    int readTryVar();
};

They’re very simple files, however I can’t understand why the static variabile isn’t set in the main function and I need to set it through the myClass functions.

I think that I don’t know very well how the “translation units” are created, I know that the “include” directive simply copies the content of the header file into the .cpp file before the actual compilation.. then why isn’t the static variable visible?

Answer 1

static has multiple meanings. Outside a class, it declares a variable that is unique to every translation unit, so main.cpp and myClass.cpp have their own copies.

To accomplish what you want, you need an extern variable:

//myClass.h
extern int tryVar;

//myClass.cpp
int tryVar = 0;  //definition needed for extern variable