Do I have to put code from .cpp in a namespace from corresponding .h or it's enough to just write using declaration?

//file .h
namespace a
{
/*interface*/
class my
{
};
}

//file .cpp

using a::my; // Can I just write in this file this declaration and
             // after that start to write implementation, or
             // should I write:

namespace a //everything in a namespace now
{
//Implementation goes here
}

Thanks.