Question

Possible Duplicate:
Ordering of using namespace std; and includes?

I heard that it is strongly prohibited to use “using namespace xy” before “#include “. E.g.

#include <iostream>

using namespace std;

int main() {
...
}

What’s the technical reason for that? I tried following and it worked without any problems:

using namespace std;

#include <iostream>

int main() {
....
}

Answer 1

No, it is not strongly prohibited (otherwise it would be a compilation or preprocessing error).

The using keyword puts all functions and variable into the current namespace, and it is discouraged to use it in the header files.