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

The clearest is the option you didn't show:

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

It's also very verbose; too much so for most people. Since using namespace is a recepe for name conflicts, at least in my experience, and should be avoided except in very limited scopes and places, I generally use your #2.