9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.4 Using namespace directive [namespace.udir]

A using-directive shall not appear in class scope, but may appear in namespace scope or in block scope.
[Note 1: 
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.
[Note 2: 
A using-directive makes the names in the nominated namespace usable in the scope in which the using-directive appears after the using-directive ([basic.lookup.unqual], [namespace.qual]).
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.
— end note]
[Note 3: 
A using-directive does not introduce any names.
— end note]
[Example 1: 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; // error: neither i is visible } — end example]
[Note 4: 
A using-directive is transitive: if a scope contains a using-directive that nominates a namespace that itself contains using-directives, the namespaces nominated by those using-directives are also eligible to be considered.
— end note]
[Example 2: 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]
[Note 5: 
Declarations in a namespace that appear after a using-directive for that namespace can be found through that using-directive after they appear.
— end note]
[Note 6: 
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 or function templates, the use of the name is ill-formed ([basic.lookup]).
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]
[Note 7: 
The order in which namespaces are considered and the relationships among the namespaces implied by the using-directives do not affect overload resolution.
Neither is any function excluded because another has the same signature, even if one is in a namespace reachable through using-directives in the namespace of the other.85
— end note]
[Example 3: 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]
During name lookup in a class hierarchy, some ambiguities can be resolved by considering whether one member hides the other along some paths ([class.member.lookup]).
There is no such disambiguation when considering the set of names found as a result of following using-directives.