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

Firstly, the using directive is never required in C since C does not support namespaces at all.

The using directive is never actually required in C++ since any of the items found in the namespace can be accessed directly by prefixing them with std:: instead. So, for example:

using namespace std;
string myString;

is equivalent to:

std::string myString;

Whether or not you choose to use it is a matter of preference, but exposing the entire std namespace to save a few keystrokes is generally considered bad form. An alternative method which only exposes particular items in the namespace is as follows:

using std::string;
string myString;

This allows you to expose only the items in the std namespace that you particularly need, without the risk of unintentionally exposing something you didn't intend to.