[namespace.def.general]

# 9 Declarations [[dcl]](./#dcl)

## 9.9 Namespaces [[basic.namespace]](basic.namespace#namespace.def.general)

### 9.9.2 Namespace definition [[namespace.def]](namespace.def#general)

#### 9.9.2.1 General [namespace.def.general]

[namespace-name:](#nt:namespace-name "9.9.2.1 General [namespace.def.general]")  
[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")  
[*namespace-alias*](namespace.alias#nt:namespace-alias "9.9.3 Namespace alias [namespace.alias]")

[namespace-definition:](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]")  
[*named-namespace-definition*](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]")  
[*unnamed-namespace-definition*](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]")  
[*nested-namespace-definition*](#nt:nested-namespace-definition "9.9.2.1 General [namespace.def.general]")

[named-namespace-definition:](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]")  
inlineopt namespace [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") { [*namespace-body*](#nt:namespace-body "9.9.2.1 General [namespace.def.general]") }

[unnamed-namespace-definition:](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]")  
inlineopt namespace [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt { [*namespace-body*](#nt:namespace-body "9.9.2.1 General [namespace.def.general]") }

[nested-namespace-definition:](#nt:nested-namespace-definition "9.9.2.1 General [namespace.def.general]")  
namespace [*enclosing-namespace-specifier*](#nt:enclosing-namespace-specifier "9.9.2.1 General [namespace.def.general]") :: inlineopt [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") { [*namespace-body*](#nt:namespace-body "9.9.2.1 General [namespace.def.general]") }

[enclosing-namespace-specifier:](#nt:enclosing-namespace-specifier "9.9.2.1 General [namespace.def.general]")  
[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")  
[*enclosing-namespace-specifier*](#nt:enclosing-namespace-specifier "9.9.2.1 General [namespace.def.general]") :: inlineopt [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")

[namespace-body:](#nt:namespace-body "9.9.2.1 General [namespace.def.general]")  
[*declaration-seq*](dcl.pre#nt:declaration-seq "9.1 Preamble [dcl.pre]")opt

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8270)

Every [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") shall inhabit a namespace scope ([[basic.scope.namespace]](basic.scope.namespace "6.4.6 Namespace scope"))[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8273)

In a [*named-namespace-definition*](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]") D,
the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") is the name of the namespace[.](#2.sentence-1)

The [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") 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[.](#2.sentence-2)

If the lookup finds a [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") for a namespace N,D [*extends*](#def:namespace,extend "9.9.2.1 General [namespace.def.general]") N,
and the target scope of D is the scope to which N belongs[.](#2.sentence-3)

If the lookup finds nothing, the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") is introduced
as a [*namespace-name*](#nt:namespace-name "9.9.2.1 General [namespace.def.general]") into A[.](#2.sentence-4)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8287)

Because a [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") contains[*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]")*s* in its [*namespace-body*](#nt:namespace-body "9.9.2.1 General [namespace.def.general]") and a[*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") is itself a [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]"), it
follows that [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]")*s* can be nested[.](#3.sentence-1)

[*Example [1](#example-1)*: namespace Outer {int i; namespace Inner {void f() { i++; } // Outer​::​iint i; void g() { i++; } // Inner​::​i}} — *end example*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8305)

If the optional initial inline keyword appears in a[*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") for a particular namespace, that namespace is
declared to be an [*inline namespace*](#def:namespace,inline "9.9.2.1 General [namespace.def.general]")[.](#4.sentence-1)

The inline keyword may be
used on a [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") that extends a namespace
only if it was previously used on the [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") that initially declared the [*namespace-name*](#nt:namespace-name "9.9.2.1 General [namespace.def.general]") for that namespace[.](#4.sentence-2)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8313)

The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") in a [*named-namespace-definition*](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]") appertains to the namespace being defined or extended[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8318)

Members of an inline namespace can be used in most respects as though they were members
of the innermost enclosing namespace[.](#6.sentence-1)

Specifically, the inline namespace and its enclosing
namespace are both added to the set of associated namespaces used in[argument-dependent lookup](basic.lookup.argdep "6.5.4 Argument-dependent name lookup [basic.lookup.argdep]") whenever one of them is,
and a [*using-directive*](namespace.udir#nt:using-directive "9.9.4 Using namespace directive [namespace.udir]") ([[namespace.udir]](namespace.udir "9.9.4 Using namespace directive")) that names the inline
namespace is implicitly inserted into the enclosing namespace as for an unnamed
namespace ([[namespace.unnamed]](namespace.unnamed "9.9.2.2 Unnamed namespaces"))[.](#6.sentence-2)

Furthermore, each
member of the inline namespace can subsequently be partially
specialized ([[temp.spec.partial]](temp.spec.partial "13.7.6 Partial specialization")), explicitly
instantiated ([[temp.explicit]](temp.explicit "13.9.3 Explicit instantiation")), or explicitly specialized ([[temp.expl.spec]](temp.expl.spec "13.9.4 Explicit specialization")) as
though it were a member of the enclosing namespace[.](#6.sentence-3)

Finally, looking up a name in the
enclosing namespace via explicit qualification ([[namespace.qual]](namespace.qual "6.5.5.3 Namespace members")) will include
members of the inline namespace even if
there are declarations of that name in the enclosing namespace[.](#6.sentence-4)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8334)

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[.](#7.sentence-1)

The [*inline namespace set*](#def:inline_namespace_set "9.9.2.1 General [namespace.def.general]") of N is the transitive closure of all
inline namespaces in N[.](#7.sentence-2)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8341)

A [*nested-namespace-definition*](#nt:nested-namespace-definition "9.9.2.1 General [namespace.def.general]") with an[*enclosing-namespace-specifier*](#nt:enclosing-namespace-specifier "9.9.2.1 General [namespace.def.general]") E,[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") I and[*namespace-body*](#nt:namespace-body "9.9.2.1 General [namespace.def.general]") B is equivalent tonamespace E { inlineopt namespace I { B } } where the optional inline is present if and only if
the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") I is preceded by inline[.](#8.sentence-1)

[*Example [2](#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*]