C++ Primer, Fifth Edition

Using Names from the Standard Library

Careful readers will note that this program uses std::cout and std::endl rather than just cout and endl. The prefix std:: indicates that the names cout and endl are defined inside the namespace named std . Namespaces allow us to avoid inadvertent collisions between the names we define and uses of those same names inside a library. All the names defined by the standard library are in the std namespace.

One side effect of the library’s use of a namespace is that when we use a name from the library, we must say explicitly that we want to use the name from the std namespace. Writing std::cout uses the scope operator (the :: operator) to say that we want to use the name cout that is defined in the namespace