9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.1 General [basic.namespace.general]

A namespace is an optionally-named entity whose scope can contain declarations of any kind of entity.
The name of a namespace can be used to access entities that belong to that namespace; that is, the members of the namespace.
Unlike other entities, the definition of a namespace can be split over several parts of one or more translation units and modules.
[Note 1: 
A namespace-definition is exported if it contains any export-declarations ([module.interface]).
A namespace is never attached to a named module and never has a name with module linkage.
— end note]
[Example 1: export module M; namespace N1 {} // N1 is not exported export namespace N2 {} // N2 is exported namespace N3 { export int n; } // N3 is exported — end example]
There is a global namespace with no declaration; see [basic.scope.namespace].
The global namespace belongs to the global scope; it is not an unnamed namespace ([namespace.unnamed]).
[Note 2: 
Lacking a declaration, it cannot be found by name lookup.
— end note]

9.8.2 Namespace definition [namespace.def] General [namespace.def.general]

Every namespace-definition shall inhabit a namespace scope ([basic.scope.namespace]).
In a named-namespace-definition D, the identifier is the name of the namespace.
The identifier is looked up by searching for it in the scopes of the namespace A in which D appears and of every element of the inline namespace set of A.
If the lookup finds a namespace-definition for a namespace N, D extends N, and the target scope of D is the scope to which N belongs.
If the lookup finds nothing, the identifier is introduced as a namespace-name into A.
Because a namespace-definition contains declarations in its namespace-body and a namespace-definition is itself a declaration, it follows that namespace-definitions can be nested.
[Example 1: namespace Outer { int i; namespace Inner { void f() { i++; } // Outer​::​i int i; void g() { i++; } // Inner​::​i } } — end example]
If the optional initial inline keyword appears in a namespace-definition for a particular namespace, that namespace is declared to be an inline namespace.
The inline keyword may be used on a namespace-definition that extends a namespace only if it was previously used on the namespace-definition that initially declared the namespace-name for that namespace.
The optional attribute-specifier-seq in a named-namespace-definition appertains to the namespace being defined or extended.
Members of an inline namespace can be used in most respects as though they were members of the innermost enclosing namespace.
Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup whenever one of them is, and a using-directive ([namespace.udir]) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace ([namespace.unnamed]).
Furthermore, each member of the inline namespace can subsequently be partially specialized ([temp.spec.partial]), explicitly instantiated ([temp.explicit]), or explicitly specialized ([temp.expl.spec]) as though it were a member of the enclosing namespace.
Finally, looking up a name in the enclosing namespace via explicit qualification ([namespace.qual]) will include members of the inline namespace even if there are declarations of that name in the enclosing namespace.
These properties are transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O, then the members of O can be used as though they were members of M or N.
The inline namespace set of N is the transitive closure of all inline namespaces in N.
A nested-namespace-definition with an enclosing-namespace-specifier E, identifier I and namespace-body B is equivalent to namespace E { inline namespace I { B } } where the optional inline is present if and only if the identifier I is preceded by inline.
[Example 2: namespace A::inline B::C { int i; }
The above has the same effect as: namespace A { inline namespace B { namespace C { int i; } } }
— end example] Unnamed namespaces [namespace.unnamed]

An unnamed-namespace-definition behaves as if it were replaced by
inline namespace unique { /* empty body */ }
using namespace unique ;
namespace unique { namespace-body }
where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit.
The optional attribute-specifier-seq in the unnamed-namespace-definition appertains to unique.
[Example 1: namespace { int i; } // unique​::​i void f() { i++; } // unique​::​i++ namespace A { namespace { int i; // A​::​unique​::​i int j; // A​::​unique​::​j } void g() { i++; } // A​::​unique​::​i++ } using namespace A; void h() { i++; // error: unique​::​i or A​::​unique​::​i A::i++; // A​::​unique​::​i j++; // A​::​unique​::​j } — end example]

9.8.3 Namespace alias [namespace.alias]

The identifier in a namespace-alias-definition becomes a namespace-alias and denotes the namespace denoted by the qualified-namespace-specifier.
[Note 1: 
When looking up a namespace-name in a namespace-alias-definition, only namespace names are considered, see [basic.lookup.udir].
— end note]

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.