Creating a C++ namespace in header and source (cpp)

Is there any difference between wrapping both header and cpp file contents in a namespace or wrapping just the header contents and then doing using namespace in the cpp file?

By difference I mean any sort performance penalty or slightly different semantics that can cause problems or anything I need to be aware of.

Example:

// header
namespace X
{
  class Foo
  {
  public:
    void TheFunc();
  };
}

// cpp
namespace X
{
  void Foo::TheFunc()
  {
    return;
  }
}

VS

// header
namespace X
{
  class Foo
  {
  public:
    void TheFunc();
  };
}

// cpp
using namespace X;
{
  void Foo::TheFunc()
  {
    return;
  }
} 

If there is no difference what is the preferred form and why?

The difference in "namespace X" to "using namespace X" is in the first one any new declarations will be under the name space while in the second one it won't.

In your example there are no new declaration - so no difference hence no preferred way.

Namespace is just a way to mangle function signature so that they will not conflict. Some prefer the first way and other prefer the second version. Both versions do not have any effect on compile time performance. Note that namespaces are just a compile time entity.

The only problem that arises with using namespace is when we have same nested namespace names (i.e) X::X::Foo. Doing that creates more confusion with or without using keyword.