Why “using namespace X;” is not allowed inside class/struct level?

Question

class C {
  using namespace std;  // error
};
namespace N {
  using namespace std; // ok
}
int main () {
  using namespace std; // ok
}

Edi

Answer 1

This is probably disallowed because of openness vs closedness.

• Classes and structs in C++ are always closed entities. They are defined in exactly one place (although you can split declaration and implementation).
• namespaces can be opened, re-opened and extended arbitrarily often.

Importing namespaces into classes would lead to funny cases like this:

namespace Foo {}

struct Bar { using namespace Foo; };

namespace Foo {
using Baz = int; // I've just extended `Bar` with a type alias!
void baz(); // I've just extended `Bar` with what looks like a static function!
// etc.
}