9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

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

Every namespace-definition shall appear at namespace scope ([basic.scope.namespace]).
In a named-namespace-definition, the identifier is the name of the namespace.
If the identifier, when looked up, refers to a namespace-name (but not a namespace-alias) that was introduced in the namespace in which the named-namespace-definition appears or that was introduced in a member of the inline namespace set of that namespace, the namespace-definition extends the previously-declared namespace.
Otherwise, the identifier is introduced as a namespace-name into the declarative region in which the named-namespace-definition appears.
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]
The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as specified in [namespace.memdef]).
Such a redeclaration has the same enclosing namespaces as the original declaration.
[Example 2: namespace Q { namespace V { void f(); // enclosing namespaces are the global namespace, Q, and Q​::​V class C { void m(); }; } void V::f() { // enclosing namespaces are the global namespace, Q, and Q​::​V extern void h(); // ... so this declares Q​::​V​::​h } void V::C::m() { // enclosing namespaces are the global namespace, Q, and Q​::​V } } — 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 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.
Furthermore, each member of the inline namespace can subsequently be partially specialized, explicitly instantiated, or explicitly specialized 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 brought in by the using-directive 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.
The enclosing namespace set of O is the set of namespaces consisting of the innermost non-inline namespace enclosing an inline namespace O, together with any intervening inline namespaces.
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 3: 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]