Do you prefer explicit namespaces or 'using' in C++?

Question

When using C++ namespaces, do you prefer to explicitly name them, like this:

std::cout << "Hello, world!\n";

Or do you prefer using namespace:

using namespace std;
cout << "Hello, world!\n";

And if if you prefer the latter, do you declare your usings at file or function scope?

Personally I prefer to explicitly name them - it's more typing but when using a mixture of namespaces (e.g. std and boost) I find it more readable.

Answer 1

I use always explicit ones. Writing std does not hurt me and I clearly see where is it from. It's useful when you have some legacy project to take care of with it's own "strings", "vectors" etc to maintain. The more information the code carries with it, the better.

Answer 2

Extra typing is not the issue here. The problem with explicitly qualified names is the visual clutter. Let's face it, C++ syntax is untidy. No need to make this worse by needlessly making names longer and sprinkling the code generously with ::s.

I'm with Jeff Atwood: The Best Code is No Code At All. This is so true.

Namespace imports are a great way of reducing clutter with no drawback: As long as the scope of opened namespaces is reduced to a single compilation unit¹, name conflicts, should they appear, can be resolved easily.

Why explicit names should (in general) be more readable has always been a mystery to me. The readers should generally know the code good enough to be able to deduce semantics. If they aren't, the code needs fixing anyway.

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¹⁾ Corollary: no using in headers!