C++: Namespaces — How to use in header and source files correctly?

Question

Consider a pair of two source files: an interface declaration file (*.h or *.hpp) and its implementation file (*.cpp).

Let the

Answer 1

I'd like also to add that if you decide due to some reason to implement a template specialization in a cpp file and just rely on using namespace you will run into the following problem:

// .h file
namespace someNameSpace
{
  template
    class Demo
    {
      void foo();
    };
}

// .cpp file
using namespace someNameSpace;

template
void Demo::foo(){}

// this will produce
// error: specialization of 'template void someNameSpace::Demo::foo()' in different namespace [-fpermissive]
template<>
void Demo::foo(){}

Otherwise if you apply #2 method this will be fine.