I made 2 projects, the first one in C and the second one in C++, both work with same behavior.
C project:
header.h
int varGlobal=7;
main.c
#include <stdio.h>
#include <stdlib.h>
#include "header.h"
void function(int i)
{
static int a=0;
a++;
int t=i;
i=varGlobal;
varGlobal=t;
printf("Call #%d:\ni=%d\nvarGlobal=%d\n\n",a,i,varGlobal,t);
}
int main() {
function(4);
function(6);
function(12);
return 0;
}
C++ project:
header.h
int varGlobal=7;
main.cpp
#include <iostream>
#include "header.h"
using namespace std;
void function(int i)
{
static int a=0;
int t=i;
a++;
i=varGlobal;
varGlobal=t;
cout<<"Call #"<<a<<":"<<endl<<"i="<<i<<endl<<"varGlobal="<<varGlobal<<endl<<endl;
}
int main() {
function(4);
function(6);
function(12);
return 0;
}
I read that global variables are extern by default and in C and static by default in C++; so why does the C++ code works?
I mean int varGlobal=7; is same as static int varGlobal=7; and if it's static then it can be used only in the file it was declared, right?
Global variables are not extern
nor static
by default on C and C++.
When you declare a variable as static
, you are restricting it to the current source file. If you declare it as extern
, you are saying that the variable exists, but are defined somewhere else, and if you don't have it defined elsewhere (without the extern
keyword) you will get a link error (symbol not found).
Your code will break when you have more source files including that header, on link time you will have multiple references to varGlobal
. If you declare it as static
, then it will work with multiple sources (I mean, it will compile and link), but each source will have its own varGlobal
.
What you can do in C++, that you can't in C, is to declare the variable as const
on the header, like this:
const int varGlobal = 7;
And include in multiple sources, without breaking things at link time. The idea is to replace the old C style #define
for constants.
If you need a global variable visible on multiple sources and not const
, declare it as extern
on the header, and then define it, this time without the extern keyword, on a source file:
Header included by multiple files:
extern int varGlobal;
In one of your source files:
int varGlobal = 7;