17 Templates [temp]

17.6 Name resolution [temp.res]

17.6.2 Dependent names [temp.dep]

17.6.2.1 Dependent types [temp.dep.type]

1
#

A name refers to the current instantiation if it is

2
#

The template argument list of a primary template is a template argument list in which the nth template argument has the value of the nth template parameter of the class template. If the nth template parameter is a template parameter pack, the nth template argument is a pack expansion whose pattern is the name of the template parameter pack.

3
#

A template argument that is equivalent to a template parameter (i.e., has the same constant value or the same type as the template parameter) can be used in place of that template parameter in a reference to the current instantiation. In the case of a non-type template argument, the argument must have been given the value of the template parameter and not an expression in which the template parameter appears as a subexpression. [ Example: 

template <class T> class A {
  A* p1;                        // A is the current instantiation
  A<T>* p2;                     // A<T> is the current instantiation
  A<T*> p3;                     // A<T*> is not the current instantiation
  ::A<T>* p4;                   // ​::​A<T> is the current instantiation
  class B {
    B* p1;                      // B is the current instantiation
    A<T>::B* p2;                // A<T>​::​B is the current instantiation
    typename A<T*>::B* p3;      // A<T*>​::​B is not the current instantiation
  };
};

template <class T> class A<T*> {
  A<T*>* p1;                    // A<T*> is the current instantiation
  A<T>* p2;                     // A<T> is not the current instantiation
};

template <class T1, class T2, int I> struct B {
  B<T1, T2, I>* b1;             // refers to the current instantiation
  B<T2, T1, I>* b2;             // not the current instantiation
  typedef T1 my_T1;
  static const int my_I = I;
  static const int my_I2 = I+0;
  static const int my_I3 = my_I;
  B<my_T1, T2, my_I>* b3;       // refers to the current instantiation
  B<my_T1, T2, my_I2>* b4;      // not the current instantiation
  B<my_T1, T2, my_I3>* b5;      // refers to the current instantiation
};

 — end example ]

4
#

A dependent base class is a base class that is a dependent type and is not the current instantiation. [ Note: A base class can be the current instantiation in the case of a nested class naming an enclosing class as a base. [ Example: 

template<class T> struct A {
  typedef int M;
  struct B {
    typedef void M;
    struct C;
  };
};

template<class T> struct A<T>::B::C : A<T> {
  M m;                          // OK, A<T>​::​M
};

 — end example ]  — end note ]

5
#

A name is a member of the current instantiation if it is

[ Example: 

template <class T> class A {
  static const int i = 5;
  int n1[i];                    // i refers to a member of the current instantiation
  int n2[A::i];                 // A​::​i refers to a member of the current instantiation
  int n3[A<T>::i];              // A<T>​::​i refers to a member of the current instantiation
  int f();
};

template <class T> int A<T>::f() {
  return i;                     // i refers to a member of the current instantiation
}

 — end example ]

A name is a dependent member of the current instantiation if it is a member of the current instantiation that, when looked up, refers to at least one member of a class that is the current instantiation.

6
#

A name is a member of an unknown specialization if it is

7
#

If a qualified-id in which the nested-name-specifier refers to the current instantiation is not a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the qualified-id is not instantiated; no diagnostic required. Similarly, if the id-expression in a class member access expression for which the type of the object expression is the current instantiation does not refer to a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the member access expression is not instantiated; no diagnostic required. [ Example: 

template<class T> class A {
  typedef int type;
  void f() {
    A<T>::type i;               // OK: refers to a member of the current instantiation
    typename A<T>::other j;     // error: neither a member of the current instantiation nor
                                // a member of an unknown specialization
  }
};

 — end example ]

8
#

If, for a given set of template arguments, a specialization of a template is instantiated that refers to a member of the current instantiation with a qualified-id or class member access expression, the name in the qualified-id or class member access expression is looked up in the template instantiation context. If the result of this lookup differs from the result of name lookup in the template definition context, name lookup is ambiguous. [ Example: 

struct A {
  int m;
};

struct B {
  int m;
};

template<typename T>
struct C : A, T {
  int f() { return this->m; }   // finds A​::​m in the template definition context
  int g() { return m; }         // finds A​::​m in the template definition context
};

template int C<B>::f();     // error: finds both A​::​m and B​::​m
template int C<B>::g();     // OK: transformation to class member access syntax
                            // does not occur in the template definition context; see [class.mfct.non-static]

 — end example ]

9
#

A type is dependent if it is

10
#

[ Note: Because typedefs do not introduce new types, but instead simply refer to other types, a name that refers to a typedef that is a member of the current instantiation is dependent only if the type referred to is dependent.  — end note ]