Question

I am having trouble understanding namespaces. A.cpp:

#include <iostream>
namespace A { int pause = 8; }
int main() {
    std::cout << A::pause << std::endl;
    return 0;
}

And it prints 8. However, now I add using namespace A and change A::pause to just pause. A.cpp:

#include <iostream>
namespace A { int pause = 8; }
using namespace A;
int main() {
    std::cout << pause << std::endl;
    return 0;
}

Now, I get the compile errors:

A.cpp: In function ‘int main()’:
A.cpp:5: error: reference to ‘pause’ is ambiguous
/usr/include/unistd.h:507: error: candidates are: int pause()
A.cpp:2: error:                 int A::pause
A.cpp:5: error: reference to ‘pause’ is ambiguous
/usr/include/unistd.h:507: error: candidates are: int pause()
A.cpp:2: error:                 int A::pause

Can someone explain what went wrong? I thought using namespace A allows me to omit the A::, similarly to how using namespace std allows you to omit std::. I tried moving the line in main()but I get the same error messages. Note, I purposefully chose the variable name pause as it apparently conflicts with the pause() declared in iostream. Any feedback is appreciated. Thanks!

Answer 1

The compiler is telling you what the problem is. It has two possible routes to resolve pause – one is via A::pause and the other is via a function pause() which is defined in <unistd.h> and exists outside any namespace. The compiler can’t decide which to use so you have to help it.

If you chose a less generic name for your variable inside the A namespace, e.g. pause_val, you could output it without the namespace scoping

#include <iostream>
namespace A { int pause_val = 8; }
using namespace A;
int main() {
    std::cout << pause_val << std::endl;
    return 0;
}