The name of a class or namespace member or enumerator can be referred to after the :: scope resolution operator ([expr.prim]) applied to a nested-name-specifier that denotes its class, namespace, or enumeration. If a :: scope resolution operator in a nested-name-specifier is not preceded by a decltype-specifier, lookup of the name preceding that :: considers only namespaces, types, and templates whose specializations are types. If the name found does not designate a namespace or a class, enumeration, or dependent type, the program is ill-formed.[ Example:
class A {
public:
static int n;
};
int main() {
int A;
A::n = 42; A b; }
— end example ]
In a declaration in which the declarator-id is a qualified-id, names used before the qualified-id being declared are looked up in the defining namespace scope; names following the qualified-id are looked up in the scope of the member's class or namespace. [ Example:
class X { };
class C {
class X { };
static const int number = 50;
static X arr[number];
};
X C::arr[number];
— end example ]
3.4.3.1 Class members [class.qual]A class member name hidden by a name in a nested declarative region or by the name of a derived class member can still be found if qualified by the name of its class followed by the :: operator.
3.4.3.2 Namespace members [namespace.qual]For a namespace X and name m, the namespace-qualified lookup set S(X, m) is defined as follows: Let S'(X, m) be the set of all declarations of m in X and the inline namespace set of X ([namespace.def]). If S'(X, m) is not empty, S(X, m) is S'(X, m); otherwise, S(X, m) is the union of S(Ni, m) for all namespaces Ni nominated by using-directives in X and its inline namespace set.
Given X::m (where X is a user-declared namespace), or given ::m (where X is the global namespace), if S(X, m) is the empty set, the program is ill-formed. Otherwise, if S(X, m) has exactly one member, or if the context of the reference is a using-declaration ([namespace.udecl]), S(X, m) is the required set of declarations of m. Otherwise if the use of m is not one that allows a unique declaration to be chosen from S(X, m), the program is ill-formed. [ Example:
int x;
namespace Y {
void f(float);
void h(int);
}
namespace Z {
void h(double);
}
namespace A {
using namespace Y;
void f(int);
void g(int);
int i;
}
namespace B {
using namespace Z;
void f(char);
int i;
}
namespace AB {
using namespace A;
using namespace B;
void g();
}
void h(){
AB::g(); AB::f(1); AB::f('c');
AB::x++; AB::i++; AB::h(16.8); }
The same declaration found more than once is not an ambiguity (because it is still a unique declaration). For example:
namespace A {
int a;
}
namespace B {
using namespace A;
}
namespace C {
using namespace A;
}
namespace BC {
using namespace B;
using namespace C;
}
void f(){
BC::a++; }
namespace D {
using A::a;
}
namespace BD {
using namespace B;
using namespace D;
}
void g(){
BD::a++; }
Because each referenced namespace is searched at most once, the following is well-defined:
namespace B {
int b;
}
namespace A {
using namespace B;
int a;
}
namespace B {
using namespace A;
}
void f(){
A::a++; B::a++; A::b++; B::b++; }
— end example ]
During the lookup of a qualified namespace member name, if the lookup finds more than one declaration of the member, and if one declaration introduces a class name or enumeration name and the other declarations either introduce the same variable, the same enumerator or a set of functions, the non-type name hides the class or enumeration name if and only if the declarations are from the same namespace; otherwise (the declarations are from different namespaces), the program is ill-formed. [ Example:
namespace A {
struct x { };
int x;
int y;
}
namespace B {
struct y { };
}
namespace C {
using namespace A;
using namespace B;
int i = C::x; int j = C::y; }
— end example ]
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