[c++] 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?

It's all about the linkage.

The previous answers provided good explainations 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 about 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 need 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

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;

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.