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

It's used whenever you're using something that is declared within a namespace. The C++ standard library is declared within the namespace std. Therefore you have to do

using namespace std;

unless you want to specify the namespace when calling functions within another namespace, like so:

std::cout << "cout is declared within the namespace std";

You can read more about it at http://www.cplusplus.com/doc/tutorial/namespaces/.