Function inside namespace in header file, declaration with 'using' in cpp file not compiling [duplicate]

In myfunction.h, I declare the function myFunction() inside the namespace MyNamespace:

namespace MyNamespace{
  void myFunction();
}

In myfunction.cpp, I include the implementation:

#include "myfunction.h"

void MyNamespace::myFunction(){
   std::cout<<"My function"<<std::end;
}

Everything compiles fine.

If instead in the .cpp, I write:

using namespace MyNamespace;

void myFunction(){
   std::cout<<"My function"<<std::end;
}

I get an "Undefined Symbol" error during linking. Why?

If instead of a function, I declare a class in the header file, and the implementation of a method of that class in the .cpp file, I have no issue.