151 lines
11 KiB
Markdown
151 lines
11 KiB
Markdown
[namespace.def]
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# 9 Declarations [[dcl]](./#dcl)
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## 9.9 Namespaces [[basic.namespace]](basic.namespace#namespace.def)
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### 9.9.2 Namespace definition [namespace.def]
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#### [9.9.2.1](#general) General [[namespace.def.general]](namespace.def.general)
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[namespace-name:](#nt:namespace-name "9.9.2.1 General [namespace.def.general]")
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[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")
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[*namespace-alias*](namespace.alias#nt:namespace-alias "9.9.3 Namespace alias [namespace.alias]")
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[namespace-definition:](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]")
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[*named-namespace-definition*](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]")
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[*unnamed-namespace-definition*](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]")
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[*nested-namespace-definition*](#nt:nested-namespace-definition "9.9.2.1 General [namespace.def.general]")
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[named-namespace-definition:](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]")
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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]") }
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[unnamed-namespace-definition:](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]")
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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]") }
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[nested-namespace-definition:](#nt:nested-namespace-definition "9.9.2.1 General [namespace.def.general]")
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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]") }
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[enclosing-namespace-specifier:](#nt:enclosing-namespace-specifier "9.9.2.1 General [namespace.def.general]")
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[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")
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[*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]")
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[namespace-body:](#nt:namespace-body "9.9.2.1 General [namespace.def.general]")
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[*declaration-seq*](dcl.pre#nt:declaration-seq "9.1 Preamble [dcl.pre]")opt
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[1](#general-1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8270)
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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"))[.](#general-1.sentence-1)
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[2](#general-2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8273)
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In a [*named-namespace-definition*](#nt:named-namespace-definition "9.9.2.1 General [namespace.def.general]") D,
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the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") is the name of the namespace[.](#general-2.sentence-1)
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The [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") is looked up
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by searching for it in the scopes of the namespace A in which D appears
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and of every element of the inline namespace set of A[.](#general-2.sentence-2)
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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,
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and the target scope of D is the scope to which N belongs[.](#general-2.sentence-3)
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If the lookup finds nothing, the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") is introduced
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as a [*namespace-name*](#nt:namespace-name "9.9.2.1 General [namespace.def.general]") into A[.](#general-2.sentence-4)
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[3](#general-3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8287)
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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
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follows that [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]")*s* can be nested[.](#general-3.sentence-1)
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[*Example [1](#general-example-1)*: namespace Outer {int i; namespace Inner {void f() { i++; } // Outer::iint i; void g() { i++; } // Inner::i}} â *end example*]
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[4](#general-4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8305)
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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
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declared to be an [*inline namespace*](#def:namespace,inline "9.9.2.1 General [namespace.def.general]")[.](#general-4.sentence-1)
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The inline keyword may be
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used on a [*namespace-definition*](#nt:namespace-definition "9.9.2.1 General [namespace.def.general]") that extends a namespace
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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[.](#general-4.sentence-2)
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[5](#general-5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8313)
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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[.](#general-5.sentence-1)
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[6](#general-6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8318)
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Members of an inline namespace can be used in most respects as though they were members
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of the innermost enclosing namespace[.](#general-6.sentence-1)
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Specifically, the inline namespace and its enclosing
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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,
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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
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namespace is implicitly inserted into the enclosing namespace as for an unnamed
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namespace ([[namespace.unnamed]](#namespace.unnamed "9.9.2.2 Unnamed namespaces"))[.](#general-6.sentence-2)
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Furthermore, each
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member of the inline namespace can subsequently be partially
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specialized ([[temp.spec.partial]](temp.spec.partial "13.7.6 Partial specialization")), explicitly
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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
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though it were a member of the enclosing namespace[.](#general-6.sentence-3)
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Finally, looking up a name in the
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enclosing namespace via explicit qualification ([[namespace.qual]](namespace.qual "6.5.5.3 Namespace members")) will include
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members of the inline namespace even if
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there are declarations of that name in the enclosing namespace[.](#general-6.sentence-4)
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[7](#general-7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8334)
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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[.](#general-7.sentence-1)
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The [*inline namespace set*](#def:inline_namespace_set "9.9.2.1 General [namespace.def.general]") of N is the transitive closure of all
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inline namespaces in N[.](#general-7.sentence-2)
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[8](#general-8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8341)
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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
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the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") I is preceded by inline[.](#general-8.sentence-1)
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[*Example [2](#general-example-2)*: namespace A::inline B::C {int i;}
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The above has the same effect as:namespace A {inline namespace B {namespace C {int i; }}}
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â *end example*]
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#### [9.9.2.2](#namespace.unnamed) Unnamed namespaces [[namespace.unnamed]](namespace.unnamed)
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[1](#namespace.unnamed-1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L8373)
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An [*unnamed-namespace-definition*](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]") behaves as if it were
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replaced by
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inlineopt namespace *unique* { /* empty body */ }
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using namespace *unique* ;
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namespace *unique* { [*namespace-body*](#nt:namespace-body "9.9.2.1 General [namespace.def.general]") }
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whereinline appears if and only if it appears in the[*unnamed-namespace-definition*](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]") and all occurrences of *unique* in a translation unit are replaced by
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the same identifier, and this identifier differs from all other
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identifiers in the program[.](#namespace.unnamed-1.sentence-1)
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The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") in the [*unnamed-namespace-definition*](#nt:unnamed-namespace-definition "9.9.2.1 General [namespace.def.general]") appertains to *unique*[.](#namespace.unnamed-1.sentence-2)
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[*Example [1](#namespace.unnamed-example-1)*: namespace { int i; } // *unique*::ivoid f() { i++; } // *unique*::i++namespace A {namespace {int i; // A::*unique*::iint 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} â *end example*]
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