[namespace.def.general] (original) (raw)

9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.2 Namespace definition [namespace.def]

9.8.2.1 General [namespace.def.general]

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 containsdeclarations in its namespace-body and anamespace-definition is itself a declaration, it follows that namespace-definitions can be nested.

[Example 1: namespace Outer { int i;namespace Inner { void f() { i++; } int i;void g() { 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(); class C { void m(); };} void V::f() { extern void h(); } void V::C::m() { } } — _end example_]

If the optional initial inline keyword appears in anamespace-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-definitionthat initially declared the namespace-name for that namespace.

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 inargument-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.

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 namespaceM, which in turn contains an inline namespace O, then the members ofO 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 anenclosing-namespace-specifier E,identifier I andnamespace-body Bis equivalent tonamespace 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_]