Use of extern

[bigo005]

I wrote two programs
#include <iostream>
//#include <string>
using namespace std;
int k=50;
int main()
{
cout << k;
}
& second one
#include <iostream>
#include <string>
using namespace std;
extern int k;
int main()
{

cout << k;
}
But in second one i am getting the error that "Undefined reference to k."Did i do any wrong?

[coder777]

Yes, if you write extern the compiler doesn't resolve the variable because extern says that the variable is defined somewhere else. If the variable isn't defined somewhere else the linker will complain

[EDIT]
And no, the second program doesn't take the variable from the first because they're not linked in any way

[monad]

By using extern, you are telling the compiler that you don't want to declare a new variable , but want to refer to a variable that has been declared in another file.

An example:

// main.cpp

void print();
int k;

int main()
{
    k = 50;    
    print();
    return 0;
}

// print.cpp

#include <iostream>

extern int k; // refrences global k in main.cpp

void print()
{
    std::cout << k;
}

Output:

50

Edit:
Fixed some mistakes.

Edit 2:
coder777's post showed up for me after I posted mine. After reading their's, I realized that I should clarify that main.cpp and print.cpp in my example are linked, so they are a part of the same program. It wouldn't work to share variables in this way between separate programs.

[vlad from moscow]

That it wouldl be clear I will show at first a simple example.

void f();

int main()
{
   f();
}

The compiler will issue the same error as in your example: "Undefined reference to f." Why do the compiler issue the error? Because function void f(); is only declared. It is not defined.

Now let return to your example.

extern int k; is only declaration of variable k It is not a definition of the corresponding object. That to make a definition of the variable you should either initialize it in the same statement as for example

extern int k = 0;

Or include another statement which defines the object

extern int k;
int k;