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

Are there any differences between these two practices

Yes. #1 and #2 are examples of a using-directive and a namespace definition respectively. They are effectively the same in this case but have other consequences. For instance, if you introduce a new identifier alongside MyClass::foo, it will have a different scope:

#1:

using namespace MyNamespace;
int x;  // defines ::x

#2:

namespace MyNamespace {
  int x;  // defines MyNamespace::x
}

is one considered better than the other?

#1 Pros: a little more terse; harder to accidentally introduce something into MyNamespace unwittingly. Cons: may pull in existing identifiers unintentionally.

#2 Pros: more clear that definitions of existing identifiers and declarations of new identifiers both belong to MyNamespace. Cons: easier to unintentionally introduce identifiers to MyNamespace.

A criticism of both #1 and #2 is that they are referring to an entire namespace when you probably only care about the definition of members of MyNamespace::MyClass. This is heavy-handed and it communicates the intent poorly.

A possible alternative to #1 is a using-declaration which includes only the identifier you're interested in:

#include "MyClass.h"
using MyNamespace::MyClass;

int MyClass::foo() { ... }