using keyword

using keyword

Looks up names in alternate classes or namespaces

---

  block-decl  :=  using-decl   using-directive   using-decl  ::= using [typename] [::]  nested-name  ::  unqualified-id  ;       using [::]  unqualified-id  ;  using-directive  ::= using namespace [::] [  nested-name  ::]  namespace-name  ; 

The using keyword starts a using declaration or using directive.

A using declaration imports a name from another namespace into the current namespace. It can also be used to introduce a name into a class scope; this is most often used to promote the access level of an inherited member or bring an inherited member into the derived class for overload resolution.

A using directive tells the compiler to search an additional namespace when looking up unqualified names.

Example

 namespace math {   const long double pi = 3.1415926535897932385L; };  using  math::pi; long double tan(long double x = pi); int main(  ) {  using  namespace std;   cout << "pi=" << math::pi << '\n'; } 

See Also

class , declaration , identifier , namespace , Chapter 2, Chapter 6