When to use extern in C++

I'm reading "Think in C++" and it just introduced the extern declaration. For example:

extern int x;
extern float y;

I think I understand the meaning (declaration without definition), but I wonder when it proves useful.

Can someone provide an example?

This comes in useful when you have global variables. You declare the existence of global variables in a header, so that each source file that includes the header knows about it, but you only need to “define” it once in one of your source files.

To clarify, using extern int x; tells the compiler that an object of type int called x exists somewhere. It's not the compilers job to know where it exists, it just needs to know the type and name so it knows how to use it. Once all of the source files have been compiled, the linker will resolve all of the references of x to the one definition that it finds in one of the compiled source files. For it to work, the definition of the x variable needs to have what's called “external linkage”, which basically means that it needs to be declared outside of a function (at what's usually called “the file scope”) and without the static keyword.

###header:

#ifndef HEADER_H
#define HEADER_H

// any source file that includes this will be able to use "global_x"
extern int global_x;
 
void print_global_x();

#endif

###source 1:

#include "header.h"

// since global_x still needs to be defined somewhere,
// we define it (for example) in this source file
int global_x;

int main()
{
    //set global_x here:
    global_x = 5;
    
    print_global_x();
}

###source 2:

#include <iostream>
#include "header.h"

void print_global_x()
{
    //print global_x here:
    std::cout << global_x << std::endl;
}

It is useful when you share a variable between a few modules. You define it in one module, and use extern in the others.

For example:

in file1.cpp:

int global_int = 1;

in file2.cpp:

extern int global_int;
//in some function
cout << "global_int = " << global_int;

It's all about the linkage.

The previous answers provided good explanations about extern.

But I want to add an important point.

You ask about extern in C++, not in C and I don't know why there is no answer mentioning the case when extern comes with const in C++.

In C++, a const variable has internal linkage by default (not like C).

So this scenario will lead to linking error:

Source 1 :

const int global = 255; //wrong way to make a definition of global const variable in C++

Source 2 :

extern const int global; //declaration

It needs to be like this:

Source 1 :

extern const int global = 255; //a definition of global const variable in C++

Source 2 :

extern const int global; //declaration

This is useful when you want to have a global variable. You define the global variables in some source file, and declare them extern in a header file so that any file that includes that header file will then see the same global variable.