From C++ FAQ:
Should I use using namespace std in my code?
Probably not.
People don't like typing std:: over
and over, and they discover that
using namespace std lets the compiler see
any std name, even if unqualified. The
fly in that ointment is that it lets
the compiler see any std name, even
the ones you didn't think about. In
other words, it can create name
conflicts and ambiguities.
https://isocpp.org/wiki/faq/coding-standards#using-namespace-std
