10 Declarations [dcl.dcl]

A using-directive shall not appear in class scope, but may appear in namespace scope or in block scope. [ Note: When looking up a namespace-name in a using-directive, only namespace names are considered, see [basic.lookup.udir].  — end note ] The optional attribute-specifier-seq appertains to the using-directive.

A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive. During unqualified name lookup, the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace. [ Note: In this context, “contains” means “contains directly or indirectly”.  — end note ]

A using-directive does not add any members to the declarative region in which it appears. [ Example:

namespace A {
  int i;
  namespace B {
    namespace C {
      int i;
    }
    using namespace A::B::C;
    void f1() {
      i = 5;        // OK, C​::​i visible in B and hides A​::​i
    }
  }
  namespace D {
    using namespace B;
    using namespace C;
    void f2() {
      i = 5;        // ambiguous, B​::​C​::​i or A​::​i?
    }
  }
  void f3() {
    i = 5;          // uses A​::​i
  }
}
void f4() {
  i = 5;            // ill-formed; neither i is visible
}

 — end example ]

For unqualified lookup, the using-directive is transitive: if a scope contains a using-directive that nominates a second namespace that itself contains using-directives, the effect is as if the using-directives from the second namespace also appeared in the first. [ Note: For qualified lookup, see [namespace.qual].  — end note ] [ Example:

namespace M {
  int i;
}

namespace N {
  int i;
  using namespace M;
}

void f() {
  using namespace N;
  i = 7;            // error: both M​::​i and N​::​i are visible
}

For another example,

namespace A {
  int i;
}
namespace B {
  int i;
  int j;
  namespace C {
    namespace D {
      using namespace A;
      int j;
      int k;
      int a = i;    // B​::​i hides A​::​i
    }
    using namespace D;
    int k = 89;     // no problem yet
    int l = k;      // ambiguous: C​::​k or D​::​k
    int m = i;      // B​::​i hides A​::​i
    int n = j;      // D​::​j hides B​::​j
  }
}

 — end example ]

If a namespace is extended after a using-directive for that namespace is given, the additional members of the extended namespace and the members of namespaces nominated by using-directives in the extending namespace-definition can be used after the extending namespace-definition.

If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed. [ Note: In particular, the name of a variable, function or enumerator does not hide the name of a class or enumeration declared in a different namespace. For example,

namespace A {
  class X { };
  extern "C"   int g();
  extern "C++" int h();
}
namespace B {
  void X(int);
  extern "C"   int g();
  extern "C++" int h(int);
}
using namespace A;
using namespace B;

void f() {
  X(1);             // error: name X found in two namespaces
  g();              // OK: name g refers to the same entity
  h();              // OK: overload resolution selects A​::​h
}

 — end note ]

During overload resolution, all functions from the transitive search are considered for argument matching. The set of declarations found by the transitive search is unordered. [ Note: In particular, the order in which namespaces were considered and the relationships among the namespaces implied by the using-directives do not cause preference to be given to any of the declarations found by the search.  — end note ] An ambiguity exists if the best match finds two functions with the same signature, even if one is in a namespace reachable through using-directives in the namespace of the other.99 [ Example:

namespace D {
  int d1;
  void f(char);
}
using namespace D;

int d1;             // OK: no conflict with D​::​d1

namespace E {
  int e;
  void f(int);
}

namespace D {       // namespace extension
  int d2;
  using namespace E;
  void f(int);
}

void f() {
  d1++;             // error: ambiguous ​::​d1 or D​::​d1?
  ::d1++;           // OK
  D::d1++;          // OK
  d2++;             // OK: D​::​d2
  e++;              // OK: E​::​e
  f(1);             // error: ambiguous: D​::​f(int) or E​::​f(int)?
  f('a');           // OK: D​::​f(char)
}

 — end example ]