C++ reason for compilation error

[sturk]

Can someone explain why does the code below give a compilation error? Revising for a test. Thanks

What is the output of the following code?
#include <iostream>
namespace first {
   int var = 5;
}
namespace {
   double var = 3.1416;
}
int main(void) {
   using namespace first;
   std::cout << var;
   return 0;
}
Compilation error 

[Peter87]

Error messages can often give you a hint as to what the problem is.

[lastchance]

The unnamed namespace has global scope, so, having also put the 'first' namespace in scope, you will have two conflicting variables var.

Try naming the second namespace something - then there will be no ambiguity and the output will be 5.

[z05DSL3A]

#include <iostream>
namespace first {
   int var = 5;
}
namespace {
   double var = 3.1416;
}
int main(void) {
   using namespace first;
   std::cout << ::var << " "<< first::var;
   return 0;
}