Why should we use "using namespace std:"? Is it necessary

Question

In certain books they don't mention about this function

Answer 1

using or not using is of your choice.

if you not use using namespace std statement then while using methods of std namespace you have to explicitly define namespace name with scope resolution operator.
example:
std::cout

Answer 2

You can use them Specifically too
like
using std::cout;

Now you can use cout normally but you have to use cin like std::cin>>value;

Answer 3

Are there any other functions like this in C++