Es necesario using namespace std?

Question

En la universidad no me enseñan eso

Answer 1

No it's not, it's also a bad practice generally unless you have a precise reason to do it.
In small exercises it's useful however, in real programming not.

There are many discussions about but I suggest you to keep using scope resolution :: to access namespaces, when you 'll feel you must use "using namespace directive you will be experienced enough to know how to avoid it :)