What is namespace in C++ ?

A namespace is a scope. Namespaces are used to make logical groupings and to avoid potential naming conflicts within C++ applications. Use of namespaces incurs no overhead to application size or performance. To use the contents of a namespace, its contents or those parts of it that are required must be brought into current scope which can be achieved in a number of different ways:

eg. To bring an entire namespace into current scope, use the 'using' keyword, eg:

using namespace std;

This brings the whole of the standard template library (STL) into current scope. This is generally only used in examples; bringing the entire contents of such a namespace into current scope in this fashion defeats the purpose of namespaces. To bring a single namespace member into scope, the using keyword can be used to explicitly refer to that item, eg:

using std::string;

This brings the STL 'string' type into current scope. Alternatively you can simply declare your variable with full namespace syntax within code. eg:

std::string my_string;

This declares my_string as an object of the STL string type.