If we want to use cin in the standard library, we need to
std::cin
To simplify this work, we can do like this
using namespace::name;
* Headers should not include using Declarations
Why?
The reason is that the contents of a header are copied into the including program‘s text. It‘s likely to encounter unexpected name conflicts.
[Cpp primer] Namespace using Declarations
原文：http://www.cnblogs.com/KennyRom/p/6421672.html