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Used to prevent name collisions when using multiple libraries, a namespace is a declarative prefix for functions, classes, types, etc.


  • namespace identifier(opt) { declaration-seq }
  • inline namespace identifier(opt) { declaration-seq } /* since C++11 */
  • inline(opt) namespace attribute-specifier-seq identifier(opt) { declaration-seq } /* since C++17 */
  • namespace enclosing-namespace-specifier :: identifier { declaration-seq } /* since C++17 */
  • namespace identifier = qualified-namespace-specifier;
  • using namespace nested-name-specifier(opt) namespace-name;
  • attribute-specifier-seq using namespace nested-name-specifier(opt) namespace-name; /* since C++11 */


The keyword namespace has three different meanings depending on context:

  1. When followed by an optional name and a brace-enclosed sequence of declarations, it defines a new namespace or extends an existing namespace with those declarations. If the name is omitted, the namespace is an unnamed namespace.

  2. When followed by a name and an equal sign, it declares a namespace alias.

  3. When preceded by using and followed by a namespace name, it forms a using directive, which allows names in the given namespace to be found by unqualified name lookup (but does not redeclare those names in the current scope). A using-directive cannot occur at class scope.

using namespace std; is discouraged. Why? Because namespace std is huge! This means that there is a high chance that names will collide:

//Really bad!
using namespace std;

//Calculates p^e and outputs it to std::cout
void pow(double p, double e) { /*...*/ }

//Calls pow
pow(5.0, 2.0); //Error! There is already a pow function in namespace std with the same signature,
               //so the call is ambiguous

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