6 Basic concepts [basic]

If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class ([class.member.lookup]), except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes (Clause [class.derived]). [ Note: A class member can be referred to using a qualified-id at any point in its potential scope ([basic.scope.class]).  — end note ] The exceptions to the name lookup rule above are the following:

• (1.1)

  the lookup for a destructor is as specified in [basic.lookup.qual];

• (1.2)

  a conversion-type-id of a conversion-function-id is looked up in the same manner as a conversion-type-id in a class member access (see [basic.lookup.classref]);

• (1.3)

  the names in a template-argument of a template-id are looked up in the context in which the entire postfix-expression occurs.

• (1.4)

  the lookup for a name specified in a using-declaration also finds class or enumeration names hidden within the same scope.

In a lookup in which function names are not ignored34 and the nested-name-specifier nominates a class C:

• (2.1)

  if the name specified after the nested-name-specifier, when looked up in C, is the injected-class-name of C (Clause [class]), or

• (2.2)

  in a using-declarator of a using-declaration that is a member-declaration, if the name specified after the nested-name-specifier is the same as the identifier or the simple-template-id's template-name in the last component of the nested-name-specifier,

the name is instead considered to name the constructor of class C. [ Note: For example, the constructor is not an acceptable lookup result in an elaborated-type-specifier so the constructor would not be used in place of the injected-class-name.  — end note ] Such a constructor name shall be used only in the declarator-id of a declaration that names a constructor or in a using-declaration. [ Example:

struct A { A(); };
struct B: public A { B(); };

A::A() { }
B::B() { }

B::A ba;            // object of type A
A::A a;             // error, A​::​A is not a type name
struct A::A a2;     // object of type A

 — end example ]

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.

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 $S^{\prime }(X,m)$ be the set of all declarations of m in X and the inline namespace set of X. If $S^{\prime }(X,m)$ is not empty, $S(X,m)$ is $S^{\prime }(X,m)$; otherwise, $S(X,m)$ is the union of $S(N_i,m)$ for all namespaces $N_i$ 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 $\{\text{A::f(int)},\text{B::f(char)}\}$ 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 $\{\text{A::i},\text{B::i}\}$ 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 $\{\text{Y::h(int)},\text{Z::h(double)}\}$ 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 $\{\text{A::a},\text{A::a}\}$
}

namespace D {
  using A::a;
}

namespace BD {
  using namespace B;
  using namespace D;
}

void g()
{
  BD::a++;          // OK: S is $\{\text{A::a},\text{A::a}\}$
}

 — 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 ]