What requires me to declare “using namespace std;”?

Question

This question may be a duplicate, but I can\'t find a good answer. Short and simple, what requires me to declare

using namespace std;

in C+

Answer 1

All the files in the C++ standard library declare all of its entities within the std namespace.
e.g: To use cin,cout defined in iostream

Alternatives:

using std::cout;
using std::endl;
cout << "Hello" << endl;

std::cout << "Hello" << std::endl;