Is there any possible use of using extern variable in a file that is not included in any other file?

Question

I've encountered many examples on the web, that have extern int x in main.c, where the main function lies.

The only use of extern I'm aware of, is to make a declaration in another file, and use it in another file, after defining it.

Like : a.h : extern int x;

a.c : int x = 5;

main.c : #include "a.h" // and start using x

The 1st case seems redundant to me.

So, Is there any possible use of using an extern variable in a file that is not included in any other file?

Answer 1

extern tells the compiler that x exists in a different module and should be linked from elsewhere. Putting it in main.c directly just avoids pulling in a header (which would be included in-line anyways)

Just like in a header, x still needs to exist in another .c module where it isn't defined extern.

Answer 2

The use of extern causes the object to have external linkage; to instantiate a template with a object (and not a value or a type), the object has to have external linkage (at least in C++03). Most objects defined in namespace scope have global linkage, but const objects don't. So you have something like:

template <char const* n>
class Toto { /* ... */ };

char const n1[] = "abc";
Toto<n1> t1;    //  Illegal...

extern char const n2[] = "xyz";
Toto<n2> t2;    //  Legal...

It's sort of a special case, but it has led me (once or twice) to use extern in an unnamed namespace in a source file.

Answer 3

Of course. Using extern in a file lets you use that variable in that file. It doesn't have to be included anywhere else.

Answer 4

extern variable has basically two functions one is to use the variable in the other file and the other is to access global variables as in the following code.

int x=10;
int main()
{
     int x=20;
     cout<<x;             //refers to x=20
     if(x==20)
     {
            extern int x;
            cout<<x;      //refers to the global x that is x=10
     }
 }