Why shouldn't I put “using namespace std” in a header?

Question

Someone once hinted that doing this in a header file is not advised:

using namespace std;

Why is it not advised?

Could it cause lin

Answer 1

If the file gets included elsewhere the compilation unit will implicitely get the using directive. This can lead to confusing errors when names overlap.

Answer 2

Because it forces anyone who uses your header file to bring the std namespace into global scope. This could be a problem if they have a class that has the same name as one of the standard library classes.