If the nested-name-specifier of a qualified-id nominates a namespace (including the case where the nested-name-specifier is ::, i.e., nominating the global namespace), the name specified after the nested-name-specifier is looked up in the scope of the namespace. The names in a template-argument of a template-id are looked up in the context in which the entire postfix-expression occurs.
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. 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, 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(); // g is declared directly in AB, therefore S is { AB::g() } and AB::g() is chosen
AB::f(1); // f is not declared directly in AB so the rules are applied recursively to A and B;
// namespace Y is not searched and Y::f(float) is not considered;
// S is {A::f(int),B::f(char)} and overload resolution chooses A::f(int)
AB::f('c'); // as above but resolution chooses B::f(char)
AB::x++; // x is not declared directly in AB, and is not declared in A or B, so the rules
// are applied recursively to Y and Z, S is { } so the program is ill-formed
AB::i++; // i is not declared directly in AB so the rules are applied recursively to A and B,
// S is {A::i,B::i} so the use is ambiguous and the program is ill-formed
AB::h(16.8); // h is not declared directly in AB and not declared directly in A or B so the rules
// are applied recursively to Y and Z, S is {Y::h(int),Z::h(double)} and
// overload resolution chooses Z::h(double)
}— end example ]
[ Note: The same declaration found more than once is not an ambiguity (because it is still a unique declaration). [ 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++; // OK: S is {A::a,A::a}
}
namespace D {
using A::a;
}
namespace BD {
using namespace B;
using namespace D;
}
void g()
{
BD::a++; // OK: S is {A::a,A::a}
}— end example ] — end note ]
[ Example: 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++; // OK: a declared directly in A, S is { A::a }
B::a++; // OK: both A and B searched (once), S is { A::a }
A::b++; // OK: both A and B searched (once), S is { B::b }
B::b++; // OK: b declared directly in B, S is { 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; // OK, A::x (of type int)
int j = C::y; // ambiguous, A::y or B::y
}— end example ]
In a declaration for a namespace member in which the declarator-id is a qualified-id, given that the qualified-id for the namespace member has the form
nested-name-specifier unqualified-id
the unqualified-id shall name a member of the namespace designated by the nested-name-specifier or of an element of the inline namespace set of that namespace. [ Example:
namespace A {
namespace B {
void f1(int);
}
using namespace B;
}
void A::f1(int){ } // ill-formed, f1 is not a member of A
— end example ] However, in such namespace member declarations, the nested-name-specifier may rely on using-directives to implicitly provide the initial part of the nested-name-specifier. [ Example:
namespace A {
namespace B {
void f1(int);
}
}
namespace C {
namespace D {
void f1(int);
}
}
using namespace A;
using namespace C::D;
void B::f1(int){ } // OK, defines A::B::f1(int)
— end example ]