Question

Consider the sample code below :

File1.cpp

#include <iostream>

static int x = 6;                          // line 3

int main()
{
   int x = 10;                             // line 7
   {
      extern int x;                        // line 9
      x = x + 5;
      std::cout << "x = " << x << "\n";
   }
}

The external declaration in line 9 causes x in the following lines to refer to the static x (defined in line 3) instead of the automatic x (defined in line 7). But I get the following compiler warning:

File1.cpp:3: warning: ‘x’ defined but not used

Even with this warning, I get the expected output which is x = 11, which is 5 added to the value of the static variable x.

Why is the compiler giving the above warning? I am using GCC version g++ (GCC) 3.4.6.

Answer 1

Note: It would have been easier if you had numbered all lines.

You are are using an old version of GCC that doesn’t handle extern correctly. If you use a modern version, you’ll see three issues with your code.

First, the local x in main() is not used (warning). Second, the static global x is not used (warning). Third, the external x is not defined(error).

Either you should not declare the global x static, or you should use ::x with no extern specification.