Using namespace std

If you have seen C++ code before, you may have seen cout being used instead of std::cout. Both name the same object: the first one uses its unqualified name (cout), while the second qualifies it directly within the namespace std (as std::cout).

cout is part of the standard library, and all the elements in the standard C++ library are declared within what is a called a namespace: the namespace std.

In order to refer to the elements in the std namespace a program shall either qualify each and every use of elements of the library (as we have done by prefixing cout with std::), or introduce visibility of its components. The most typical way to introduce visibility of these components is by means of using declarations:

using namespace std;

The above declaration allows all elements in the std namespace to be accessed in an unqualified manner (without the std:: prefix).
With this in mind, the last example can be rewritten to make unqualified uses of cout as: