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      Namespaces

      From cppreference.com
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      Namespaces provide a method for preventing name conflicts in large projects.

      Entities declared inside a namespace block are placed in a namespace scope, which prevents them from being mistaken for identically-named entities in other scopes.

      Entities declared outside all namespace blocks belong to theglobal namespace. The global namespace belongs to theglobal scope, and can be referred to explicitly with a leading::. While it has no declaration, the global namespace is not anunnamed namespace.

      Multiple namespace blocks with the same name are allowed. All declarations within these blocks are declared in the same namespace scope.

      Contents

      [edit]Syntax

      namespacens-name{declarations} (1)
      inlinenamespacens-name{declarations} (2)(since C++11)
      namespace{declarations} (3)
      ns-name::member-name (4)
      usingnamespacens-name; (5)
      usingns-name::member-name; (6)
      namespacename=qualified-namespace; (7)
      namespacens-name::member-name{declarations} (8)(since C++17)
      namespacens-name::inlinemember-name{declarations} (9)(since C++20)
      1)Named namespace definition for the namespacens-name.
      2)Inline namespace definition for the namespacens-name. Declarations insidens-name will be visible in its enclosing namespace.
      3)Unnamed namespace definition. Its members have potential scope from their point of declaration to the end of the translation unit, and haveinternal linkage.
      4) Namespace names (along with class names) can appear on the left hand side of the scope resolution operator, as part ofqualified name lookup.
      5)using-directive: From the point of view of unqualifiedname lookup of any name after a using-directive and until the end of the scope in which it appears, every name fromns-name is visible as if it were declared in the nearest enclosing namespace which contains both the using-directive andns-name.
      6)using-declaration: makes the symbolmember-name from the namespacens-name accessible forunqualified lookup as if declared in the same class scope, block scope, or namespace as where this using-declaration appears.
      7)namespace-alias-definition: makesname a synonym for another namespace: seenamespace alias
      8) nested namespace definition:namespace A::B::C{ ...} is equivalent tonamespace A{namespace B{namespace C{ ...}}}.
      9) nested inline namespace definition:namespace A::B::inline C{ ...} is equivalent tonamespace A::B{inlinenamespace C{ ...}}.inline may appear in front of every namespace name except the first:namespace A::inline B::C{} is equivalent tonamespace A{inlinenamespace B{namespace C{}}}.

      [edit]Explanation

      [edit]Namespaces

      inline(optional)namespaceattr (optional)identifier{namespace-body}
      inline -(since C++11) if present, makes this an inline namespace (see below). Cannot appear on theextension-namespace-definition if theoriginal-namespace-definition did not useinline
      attr -(since C++17) optional sequence of any number ofattributes
      identifier - either
      • a previously unused identifier, in which case this isoriginal-namespace-definition;
      • the name of a namespace, in which case this isextension-namespace-definition;
      • a sequence of enclosing namespace specifiers separated by::, ending withidentifier, in which case this is anested-namespace-definition
      (since C++17)
      namespace-body - possibly empty sequence ofdeclarations of any kind (including class and function definitions as well as nested namespaces)

      Namespace definitions are only allowed at namespace scope, including the global scope.

      To reopen an existing namespace (formally, to be anextension-namespace-definition), the lookup for theidentifier used in the namespace definition must resolve to a namespace name (not a namespace alias), that was declared as a member of the enclosing namespace or of an inline namespace within an enclosing namespace.

      Thenamespace-body defines anamespace scope, which affectsname lookup.

      All names introduced by the declarations that appear withinnamespace-body (including nested namespace definitions) become members of the namespaceidentifier, whether this namespace definition is the original namespace definition (which introducedidentifier), or an extension namespace definition (which "reopened" the already defined namespace)

      A namespace member that was declared within a namespace body may be defined or redeclared outside of it using explicit qualification

      namespace Q{namespace V// V is a member of Q, and is fully defined within Q{// namespace Q::V { // C++17 alternative to the lines aboveclass C{void m();};// C is a member of V and is fully defined within V// C::m is only declaredvoid f();// f is a member of V, but is only declared here} void V::f()// definition of V's member f outside of V// f's enclosing namespaces are still the global namespace, Q, and Q::V{externvoid h();// This declares ::Q::V::h} void V::C::m()// definition of V::C::m outside of the namespace (and the class body)// enclosing namespaces are the global namespace, Q, and Q::V{}}

      Out-of-namespace definitions and redeclarations are only allowed

      • after the point of declaration,
      • at namespace scope, and
      • in namespaces that enclose the original namespace (including the global namespace).

      Also, they must use qualified-id syntax.

      namespace Q{namespace V// original-namespace-definition for V{void f();// declaration of Q::V::f} void V::f(){}// OKvoid V::g(){}// Error: g() is not yet a member of V namespace V// extension-namespace-definition for V{void g();// declaration of Q::V::g}} namespace R// not an enclosing namespace for Q{void Q::V::g(){}// Error: cannot define Q::V::g inside R} void Q::V::g(){}// OK: global namespace encloses Q

      Names introduced byfriend declarations within a non-local class X become members of the innermost enclosing namespace of X, but they do not become visible to ordinaryname lookup (neitherunqualified norqualified) unless a matching declaration is provided at namespace scope, either before or after the class definition. Such name may be found throughADL which considers both namespaces and classes.

      Only the innermost enclosing namespace is considered by such friend declaration when deciding whether the name would conflict with a previously declared name.

      void h(int);namespace A{class X{friendvoid f(X);// A::f is a friend class Y{friendvoid g();// A::g is a friendfriendvoid h(int);// A::h is a friend, no conflict with ::h};};// A::f, A::g and A::h are not visible at namespace scope// even though they are members of the namespace A     X x;void g()// definition of A::g{        f(x);// A::X::f is found through ADL} void f(X){}// definition of A::fvoid h(int){}// definition of A::h// A::f, A::g and A::h are now visible at namespace scope// and they are also friends of A::X and A::X::Y}

      Inline namespaces

      An inline namespace is a namespace that uses the optional keywordinline in itsoriginal-namespace-definition.

      Members of an inline namespace are treated as if they are members of the enclosing namespace in many situations (listed below). This property is 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.

      • Ausing-directive that names the inline namespace is implicitly inserted in the enclosing namespace (similar to the implicit using-directive for the unnamed namespace)
      • Inargument-dependent lookup, when a namespace is added to the set of associated namespaces, its inline namespaces are added as well, and if an inline namespace is added to the list of associated namespaces, its enclosing namespace is added as well.
      • Each member of an inline namespace can be partially specialized, explicitly instantiated, or explicitly specialized as if it were a member of the enclosing namespace.
      • Qualifiedname lookup that examines the enclosing namespace will include the names from the inline namespaces even if the same name is present in the enclosing namespace.
      // in C++14, std::literals and its member namespaces are inline{usingnamespace std::string_literals;// makes visible operator""s// from std::literals::string_literalsauto str="abc"s;} {usingnamespace std::literals;// makes visible both// std::literals::string_literals::operator""s// and std::literals::chrono_literals::operator""sauto str="abc"s;auto min= 60s;} {using std::operator""s;// makes both std::literals::string_literals::operator""s// and std::literals::chrono_literals::operator""s visibleauto str="abc"s;auto min= 60s;}

      Note: the rule about specializations allows library versioning: different implementations of a library template may be defined in different inline namespaces, while still allowing the user to extend the parent namespace with an explicit specialization of the primary template:

      Run this code
      namespace Lib{inlinenamespace Lib_1{template<typename T>class A;} template<typename T>void g(T){/* ... */}}/* ... */struct MyClass{/* ... */};namespace Lib{template<>class A<MyClass>{/* ... */};} int main(){    Lib::A<MyClass> a;    g(a);// ok, Lib is an associated namespace of A}
      (since C++11)

      [edit]Unnamed namespaces

      Theunnamed-namespace-definition is a namespace definition of the form

      inline(optional)namespaceattr (optional){namespace-body}
      inline -(since C++11) if present, makes this an inline namespace
      attr -(since C++17) optional sequence of any number ofattributes

      This definition is treated as a definition of a namespace with unique name and ausing-directive in the current scope that nominates this unnamed namespace (Note: implicitly added using directive makes namespace available for thequalified name lookup andunqualified name lookup, but not for theargument-dependent lookup).The unique name is unique over the entire program, but within a translation unit each unnamed namespace definition maps to the same unique name: multiple unnamed namespace definitions in the same scope denote the same unnamed namespace.

      namespace{int i;// defines ::(unique)::i} void f(){    i++;// increments ::(unique)::i} namespace A{namespace{int i;// A::(unique)::iint j;// A::(unique)::j} void g(){ i++;}// A::(unique)::i++} usingnamespace A;// introduces all names from A into global namespace void h(){    i++;// error: ::(unique)::i and ::A::(unique)::i are both in scope    A::i++;// ok, increments ::A::(unique)::i    j++;// ok, increments ::A::(unique)::j}

      Even though names in an unnamed namespace may be declared with external linkage, they are never accessible from other translation units because their namespace name is unique.

      (until C++11)

      Unnamed namespaces as well as all namespaces declared directly or indirectly within an unnamed namespace haveinternal linkage, which means that any name that is declared within an unnamed namespace has internal linkage.

      (since C++11)

      [edit]Using-declarations

      Introduces a name that is defined elsewhere into the declarative region where this using-declaration appears.

      usingtypename(optional)nested-name-specifierunqualified-id;(until C++17)
      usingdeclarator-list;(since C++17)
      typename - the keywordtypename may be used as necessary to resolvedependent names, when the using-declaration introduces a member type from a base class into a class template
      nested-name-specifier - a sequence of names and scope resolution operators::, ending with a scope resolution operator. A single:: refers to the global namespace.
      unqualified-id - anid-expression
      declarator-list - comma-separated list of one or more declarators of the formtypename(optional)nested-name-specifierunqualified-id. A declarator may be followed by an ellipsis to indicatepack expansion, although that form is only meaningful inderived class definitions

      Using-declarations can be used to introduce namespace members into other namespaces and block scopes, or to introduce base class members into derived class definitions, or to introduceenumerators into namespaces, block, and class scopes(since C++20).

      A using-declaration with more than one using-declarator is equivalent to a corresponding sequence of using-declarations with one using-declarator.

      (since C++17)

      For the use in derived class definitions, seeusing declaration.

      Names introduced into a namespace scope by a using-declaration can be used just like any other names, including qualified lookup from other scopes:

      void f();namespace A{void g();} namespace X{using::f;// global f is now visible as ::X::fusing A::g;// A::g is now visible as ::X::gusing A::g, A::g;// (C++17) OK: double declaration allowed at namespace scope} void h(){    X::f();// calls ::f    X::g();// calls A::g}

      If, after the using-declaration was used to take a member from a namespace, the namespace is extended and additional declarations for the same name are introduced, those additional declarations do not become visible through the using-declaration (in contrast with using-directive). One exception is when a using-declaration names a class template: partial specializations introduced later are effectively visible, because theirlookup proceeds through the primary template.

      namespace A{void f(int);}using A::f;// ::f is now a synonym for A::f(int) namespace A// namespace extension{void f(char);// does not change what ::f means} void foo(){    f('a');// calls f(int), even though f(char) exists.} void bar(){using A::f;// this f is a synonym for both A::f(int) and A::f(char)    f('a');// calls f(char)}

      Using-declarations cannot nametemplate-id, or namespace, or a scoped enumerator(until C++20). Each declarator in a using-declaration introduces one and only one name, for example using-declaration for anenumeration does not introduce any of its enumerators.

      All restrictions on regular declarations of the same names, hiding, and overloading rules apply to using-declarations:

      namespace A{int x;} namespace B{int i;struct g{};struct x{}; void f(int);void f(double);void g(char);// OK: function name g hides struct g} void func(){int i;using B::i;// error: i declared twice void f(char);using B::f;// OK: f(char), f(int), f(double) are overloads    f(3.5);// calls B::f(double) using B::g;    g('a');// calls B::g(char)struct g g1;// declares g1 to have type struct B::g using B::x;using A::x;// OK: hides struct B::x    x=99;// assigns to A::xstruct x x1;// declares x1 to have type struct B::x}

      If a function was introduced by a using-declaration, declaring a function with the same name and parameter list is ill-formed (unless the declaration is for the same function). If a function template was introduced by a using-declaration, declaring a function template with the same name, parameter type list, return type, and template parameter list is ill-formed.Two using-declarations can introduce functions with the same name and parameter list, but if a call to that function is attempted, the program is ill-formed.

      namespace B{void f(int);void f(double);} namespace C{void f(int);void f(double);void f(char);} void h(){using B::f;// introduces B::f(int), B::f(double)using C::f;// introduces C::f(int), C::f(double), and C::f(char)    f('h');// calls C::f(char)    f(1);// error: B::f(int) or C::f(int)?void f(int);// error: f(int) conflicts with C::f(int) and B::f(int)}

      If an entity is declared, but not defined in some inner namespace, and then declared through using-declaration in the outer namespace, and then a definition appears in the outer namespace with the same unqualified name, that definition is a member of the outer namespace and conflicts with the using-declaration:

      namespace X{namespace M{void g();// declares, but doesn't define X::M::g()}using M::g; void g();// Error: attempt to declare X::g which conflicts with X::M::g()}

      More generally, a declaration that appears in any namespace scope and introduces a name using an unqualified identifier always introduces a member into the namespace it's in and not to any other namespace. The exceptions are explicit instantiations and explicit specializations of a primary template that is defined in an inline namespace: because they do not introduce a new name, they may use unqualified-id in an enclosing namespace.

      [edit]Using-directives

      Ausing-directive is ablock-declaration with the following syntax:

      attr (optional)usingnamespacenested-name-specifier (optional)namespace-name; (1)
      attr -(since C++11) any number ofattributes that apply to this using-directive
      nested-name-specifier - a sequence of names and scope resolution operators::, ending with a scope resolution operator. A single:: refers to the global namespace. When looking up the names in this sequence,lookup considers namespace declarations only
      namespace-name - a name of a namespace. When looking up this name,lookup considers namespace declarations only

      Using-directives are allowed only in namespacescope and in block scope. From the point of view ofunqualified name lookup of any name after a using-directive and until the end of the scope in which it appears, every name fromnamespace-name is visible as if it were declared in the nearest enclosing namespace which contains both the using-directive andnamespace-name.

      Using-directive does not add any names to the declarative region in which it appears (unlike the using-declaration), and thus does not prevent identical names from being declared.

      Using-directives are transitive for the purposes ofunqualified lookup: if a scope contains a using-directive that nominates anamespace-name, which itself contains using-directive for somenamespace-name-2, the effect is as if the using directives from the second namespace appear within the first. The order in which these transitive namespaces occur does not influence name lookup.

      namespace A{int i;} namespace B{int i;int j; namespace C{namespace D{usingnamespace A;// Names from A are "injected" into D.// Unqualified lookup within D considers these names to have the same// scope as the global scope (e.g. for the purposes of name hiding).// Qualified lookup referring to D (D::name for some name)// will find the same name as unqualified lookup within D. int j;int k;int a= i;// i is B::i, because A::i is hidden by B::iint b=::i;// error: there is still no i in the global namespace} usingnamespace D;// names from D and A are injected into C int k=89;// OK to declare name identical to one introduced by a usingint l= k;// ambiguous: C::k or D::kint m= i;// ok: B::i hides A::iint n= j;// ok: D::j hides B::j}} // These are all equivalent definitions:int t0= B::i;int t1= B::C::a;int t2= B::C::D::a;

      If, after a using-directive was used to nominate some namespace, the namespace is extended and additional members and/or using-directives are added to it, those additional members and the additional namespaces are visible through the using-directive (in contrast with using-declaration)

      namespace D{int d1;void f(char);}usingnamespace D;// introduces D::d1, D::f, D::d2, D::f,// E::e, and E::f into global namespace! int d1;// OK: no conflict with D::d1 when declaring namespace E{int e;void f(int);} namespace D// namespace extension{int d2;usingnamespace E;// transitive using-directivevoid f(int);} void f(){    d1++;// error: ambiguous ::d1 or D::d1?::d1++;// OK    D::d1++;// OK    d2++;// OK, d2 is D::d2     e++;// OK: e is E::e due to transitive using     f(1);// error: ambiguous: D::f(int) or E::f(int)?    f('a');// OK: the only f(char) is D::f(char)}

      [edit]Notes

      The using-directiveusingnamespace std; at any namespace scope introduces every name from the namespacestd into the global namespace (since the global namespace is the nearest namespace that contains bothstd and any user-declared namespace), which may lead to undesirable name collisions. This, and other using directives are generally considered bad practice at file scope of a header file (SF.7: Don’t writeusingnamespace at global scope in a header file).

      Feature-test macroValueStdFeature
      __cpp_namespace_attributes201411L(C++17)Attributes for namespaces

      [edit]Keywords

      namespace,using,inline

      [edit]Example

      This example shows how to use a namespace to create a class that already has been named in thestd namespace.

      Run this code
      #include <vector> namespace vec{template<typename T>class vector{// ...};}// of vec int main(){std::vector<int> v1;// Standard vector.    vec::vector<int> v2;// User defined vector. // v1 = v2;          // Error: v1 and v2 are different object's type. {usingnamespace std;        vector<int> v3;// Same as std::vector        v1= v3;// OK} {using vec::vector;        vector<int> v4;// Same as vec::vector        v2= v4;// OK}}

      [edit]Defect reports

      The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

      DRApplied toBehavior as publishedCorrect behavior
      CWG 101C++98the program is ill-formed if a function declaration in namespace
      scope or block scope and a function introduced by a
      using-declaration declare the same function (no ambiguity)
      allowed
      CWG 373C++98lookup only considered namespace declarations only for
      the last name in the operand of a using-directive (which is
      sub-optimal, because classes cannot contain namespaces)
      the lookup restriction
      applies to all names in the
      operands of using-directives
      CWG 460C++98a using-declaration could name a namespaceprohibited
      CWG 565C++98a using-declaration cannot introduce a function
      identical to another function in the same scope, but
      the restriction was not applied to function templates
      apply the same restriction
      to function templates as well
      CWG 986C++98using-directive was transitive for qualified lookuponly transitive for unqualified lookup
      CWG 987C++98entities declared in a nested namespace was
      also members of the enclosing namespace
      nested scopes excluded
      CWG 1021C++98it was unclear whether an entity whose definition
      is introduced to a namespace via using-declaration
      is considered to be defined in that namespace
      not defined in that namespace
      CWG 1838C++98unqualified definition in an outer namespace
      could define an entity declared, but not defined in
      another namespace and pulled in by a using
      unqualified definition
      always refers to
      its namespace
      CWG 2155C++98the resolution ofCWG issue 1838 was not
      applied to class and enumeration declarations
      applied

      [edit]See also

      namespace alias creates an alias of an existing namespace[edit]
      Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/language/namespace&oldid=175087"

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