What requires me to declare “using namespace std;”?

Question

This question may be a duplicate, but I can\'t find a good answer. Short and simple, what requires me to declare

using namespace std;

in C+

Answer 1

You should definitely not say:

using namespace std;

in your C++ headers, because that beats the whole point of using namespaces (doing that would constitute "namespace pollution"). Some useful resources on this topic are the following:

1) stackoverflow thread on Standard convention for using “std”

2) an article by Herb Sutter on Migrating to Namespaces

3) FAQ 27.5 from Marshall Cline's C++ Faq lite.