Question

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

I want to know the motivation behind it.

Answer 1

I don’t know exactly, but my guess is that allowing this at class scope could cause confusion:

namespace Hello
{
    typedef int World;
}

class Blah
{
    using namespace Hello;
public:
    World DoSomething();
}

//Should this be just World or Hello::World ?
World Blah::DoSomething()
{
    //Is the using namespace valid in here?
}

Since there is no obvious way of doing this, the standard just says you can’t.

Now, the reason this is less confusing when we’re talking namespace scopes:

namespace Hello
{
    typedef int World;
}

namespace Other
{
    using namespace Hello;
    World DoSomething();
}

//We are outside of any namespace, so we have to fully qualify everything. Therefore either of these are correct:

//Hello was imported into Other, so everything that was in Hello is also in Other. Therefore this is okay:
Other::World Other::DoSomething()
{
    //We're outside of a namespace; obviously the using namespace doesn't apply here.
    //EDIT: Apparently I was wrong about that... see comments. 
}

//The original type was Hello::World, so this is okay too.
Hello::World Other::DoSomething()
{
    //Ditto
}

namespace Other
{
    //namespace Hello has been imported into Other, and we are inside Other, so therefore we never need to qualify anything from Hello.
    //Therefore this is unambiguiously right
    World DoSomething()
    {
        //We're inside the namespace, obviously the using namespace does apply here.
    }
}