Correct way to define C++ namespace methods in .cpp file

Question

Probably a duplicate, but not an easy one to search for...

Given a header like:

namespace ns1
{
 class MyClass
 {
  void method();
 };
}

Answer 1

It turns out it's not only "coding-style matter". Num. 2 leads to linking error when defining and initializing a variable declared extern in header file. Take a look at example in my question. Definition of constant within namespace in cpp file