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

Question

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?

Answer 1

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.