c++ - 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++ programs?

Answer 1

Since the C++ standard has been accepted, practically all of the standard library is inside the std namespace. So if you don't want to qualify all standard library calls with std::, you need to add the using directive.

However,

using namespace std;

is considered a bad practice because you are practically importing the whole standard namespace, thus opening up a lot of possibilities for name clashes. It is better to import only the stuff you are actually using in your code, like

using std::string;