Стандарт C++ 98 (1119566), страница 36
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The identifier in an original-namespacedefinition is the name of the namespace. Subsequently in that declarative region, it is treated as anoriginal-namespace-name.3The original-namespace-name in an extension-namespace-definition shall have previously been defined inan original-namespace-definition in the same declarative region.4Every namespace-definition shall appear in the global scope or in a namespace scope (3.3.5).1125© ISO/IECISO/IEC 14882:1998(E)7 Declarations7.3.1 Namespace definitionBecause a namespace-definition contains declarations in its namespace-body and a namespace-definition isitself a declaration, it follows that namespace-definitions can be nested.
[Example:namespace Outer {int i;namespace Inner {void f() { i++; }int i;void g() { i++; }}}// Outer::i// Inner::i—end example]7.3.1.1 Unnamed namespaces1[namespace.unnamed]An unnamed-namespace-definition behaves as if it were replaced bynamespace unique { /* empty body */ }using namespace unique;namespace unique { namespace-body }where all occurrences of unique in a translation unit are replaced by the same identifier and this identifierdiffers from all other identifiers in the entire program.82) [Example:namespace { int i; }void f() { i++; }namespace A {namespace {int i;int j;}void g() { i++; }}using namespace A;void h() {i++;A::i++;j++;}// unique::i// unique::i++// A::unique::i// A::unique::j// A::unique::i++// error: unique::i or A::unique::i// A::unique::i// A::unique::j—end example]2The use of the static keyword is deprecated when declaring objects in a namespace scope (see annex D);the unnamed-namespace provides a superior alternative.7.3.1.2 Namespace member definitions1[namespace.memdef]Members of a namespace can be defined within that namespace.
[Example:namespace X {void f() { /* ... */ }}—end example]2Members of a named namespace can also be defined outside that namespace by explicit qualification(3.4.3.2) of the name being defined, provided that the entity being defined was already declared in thenamespace and the definition appears after the point of declaration in a namespace that encloses the__________________82) Although entities in an unnamed namespace might have external linkage, they are effectively qualified by a name unique to theirtranslation unit and therefore can never be seen from any other translation unit.113ISO/IEC 14882:1998(E)© ISO/IEC7.3.1.2 Namespace member definitions7 Declarationsdeclaration’s namespace.
[Example:namespace Q {namespace V {void f();}void V::f() { /* ... */ }void V::g() { /* ... */ }namespace V {void g();}}// OK// error: g() is not yet a member of Vnamespace R {void Q::V::g() { /* ... */ }}// error: R doesn’t enclose Q—end example]3Every name first declared in a namespace is a member of that namespace. If a friend declaration in anon-local class first declares a class or function83) the friend class or function is a member of the innermostenclosing namespace.
The name of the friend is not found by simple name lookup until a matching declaration is provided in that namespace scope (either before or after the class declaration granting friendship). Ifa friend function is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments (3.4.2). When looking for a priordeclaration of a class or a function declared as a friend, scopes outside the innermost enclosing namespace scope are not considered. [Example:// Assume f and g have not yet been defined.void h(int);namespace A {class X {friend void f(X);// A::f is a friendclass Y {friend void g();// A::g is a friendfriend void h(int);// A::h is a friend// ::h not considered};};// A::f, A::g and A::h are not visible hereX x;void g() { f(x); }// definition of A::gvoid f(X) { /* ...
*/}// definition of A::fvoid h(int) { /* ... */ }// definition of A::h// A::f, A::g and A::h are visible here and known to be friends}using A::x;void h(){A::f(x);A::X::f(x);A::X::Y::g();}// error: f is not a member of A::X// error: g is not a member of A::X::Y—end example]__________________83) this implies that the name of the class or function is unqualified.114© ISO/IECISO/IEC 14882:1998(E)7 Declarations7.3.2 Namespace alias7.3.2 Namespace alias1[namespace.alias]A namespace-alias-definition declares an alternate name for a namespace according to the following grammar:namespace-alias:identifiernamespace-alias-definition:namespace identifier = qualified-namespace-specifier ;qualified-namespace-specifier:::opt nested-name-specifieropt namespace-name2The identifier in a namespace-alias-definition is a synonym for the name of the namespace denoted by thequalified-namespace-specifier and becomes a namespace-alias.
[Note: when looking up a namespacename in a namespace-alias-definition, only namespace names are considered, see 3.4.6. ]3In a declarative region, a namespace-alias-definition can be used to redefine a namespace-alias declared inthat declarative region to refer only to the namespace to which it already refers. [Example: the followingdeclarations are well-formed:namespacenamespacenamespacenamespaceCompany_with_very_long_name { /* ... */ }CWVLN = Company_with_very_long_name;CWVLN = Company_with_very_long_name;CWVLN = CWVLN;// OK: duplicate—end example]4A namespace-name or namespace-alias shall not be declared as the name of any other entity in the samedeclarative region. A namespace-name defined at global scope shall not be declared as the name of anyother entity in any global scope of the program. No diagnostic is required for a violation of this rule bydeclarations in different translation units.7.3.3 The using declaration1[namespace.udecl]A using-declaration introduces a name into the declarative region in which the using-declaration appears.That name is a synonym for the name of some entity declared elsewhere.using-declaration:using typenameopt ::opt nested-name-specifier unqualified-id ;using :: unqualified-id ;2The member name specified in a using-declaration is declared in the declarative region in which the usingdeclaration appears.
[Note: only the specified name is so declared; specifying an enumeration name in ausing-declaration does not declare its enumerators in the using-declaration’s declarative region. ]3Every using-declaration is a declaration and a member-declaration and so can be used in a class definition.[Example:struct B {void f(char);void g(char);enum E { e };union { int x; };};struct D : B {using B::f;void f(int) { f(’c’); }void g(int) { g(’c’); }};// calls B::f(char)// recursively calls D::g(int)115ISO/IEC 14882:1998(E)© ISO/IEC7.3.3 The using declaration7 Declarations—end example]4A using-declaration used as a member-declaration shall refer to a member of a base class of the class beingdefined, shall refer to a member of an anonymous union that is a member of a base class of the class beingdefined, or shall refer to an enumerator for an enumeration type that is a member of a base class of the classbeing defined.
[Example:class C {int g();};class D2 : public B {using B::f;using B::e;using B::x;using C::g;};// OK: B is a base of D2// OK: e is an enumerator of base B// OK: x is a union member of base B// error: C isn’t a base of D2—end example] [Note: since constructors and destructors do not have names, a using-declaration cannotrefer to a constructor or a destructor for a base class. Since specializations of member templates for conversion functions are not found by name lookup, they are not considered when a using-declaration specifies aconversion function (14.5.2). ] If an assignment operator brought from a base class into a derived classscope has the signature of a copy-assignment operator for the derived class (12.8), the using-declarationdoes not by itself suppress the implicit declaration of the derived class copy-assignment operator; thecopy-assignment operator from the base class is hidden or overridden by the implicitly-declared copyassignment operator of the derived class, as described below.5A using-declaration shall not name a template-id.
[Example:class A {public:template <class T> void f(T);template <class T> struct X { };};class B : public A {public:using A::f<double>;// ill-formedusing A::X<int>;// ill-formed};—end example]6A using-declaration for a class member shall be a member-declaration. [Example:struct X {int i;static int s;};void f(){using X::i;using X::s;// error: X::i is a class member// and this is not a member declaration.// error: X::s is a class member// and this is not a member declaration.}—end example]7Members declared by a using-declaration can be referred to by explicit qualification just like other membernames (3.4.3.2). In a using-declaration, a prefix :: refers to the global namespace.
[Example:116© ISO/IECISO/IEC 14882:1998(E)7 Declarations7.3.3 The using declarationvoid f();namespace A {void g();}namespace X {using ::f;using A::g;}void h(){X::f();X::g();}// global f// A’s g// calls ::f// calls A::g—end example]8A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multipledeclarations are allowed.
[Example:namespace A {int i;}namespace A1 {using A::i;using A::i;}void f(){using A::i;using A::i;}// OK: double declaration// error: double declarationclass B {public:int i;};class X : public B {using B::i;using B::i;};// error: double member declaration—end example]9The entity declared by a using-declaration shall be known in the context using it according to its definitionat the point of the using-declaration. Definitions added to the namespace after the using-declaration arenot considered when a use of the name is made. [Example:namespace A {void f(int);}using A::f;// f is a synonym for A::f;// that is, for A::f(int).namespace A {void f(char);}117ISO/IEC 14882:1998(E)© ISO/IEC7.3.3 The using declarationvoid foo(){f(’a’);}void bar(){using A::f;f(’a’);7 Declarations// calls f(int),// even though f(char) exists.// f is a synonym for A::f;// that is, for A::f(int) and A::f(char).// calls f(char)}—end example] [Note: partial specializations of class templates are found by looking up the primary classtemplate and then considering all partial specializations of that template.
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