17 Templates [temp]

Like normal (non-template) classes, class templates have an injected-class-name. The injected-class-name can be used as a template-name or a type-name. When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it refers to the class template itself. Otherwise, it is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.

Within the scope of a class template specialization or partial specialization, when the injected-class-name is used as a type-name, it is equivalent to the template-name followed by the template-arguments of the class template specialization or partial specialization enclosed in <>. [ Example:

template<template<class> class T> class A { };
template<class T> class Y;
template<> class Y<int> {
  Y* p;                               // meaning Y<int>
  Y<char>* q;                         // meaning Y<char>
  A<Y>* a;                            // meaning A<​::​Y>
  class B {
    template<class> friend class Y;   // meaning ​::​Y
  };
};

 — end example ]

The injected-class-name of a class template or class template specialization can be used either as a template-name or a type-name wherever it is in scope. [ Example:

template <class T> struct Base {
  Base* p;
};

template <class T> struct Derived: public Base<T> {
  typename Derived::Base* p;    // meaning Derived​::​Base<T>
};

template<class T, template<class> class U = T::template Base> struct Third { };
Third<Base<int> > t;            // OK: default argument uses injected-class-name as a template

 — end example ]

A lookup that finds an injected-class-name ([class.member.lookup]) can result in an ambiguity in certain cases (for example, if it is found in more than one base class). If all of the injected-class-names that are found refer to specializations of the same class template, and if the name is used as a template-name, the reference refers to the class template itself and not a specialization thereof, and is not ambiguous. [ Example:

template <class T> struct Base { };
template <class T> struct Derived: Base<int>, Base<char> {
  typename Derived::Base b;             // error: ambiguous
  typename Derived::Base<double> d;     // OK
};

 — end example ]

When the normal name of the template (i.e., the name from the enclosing scope, not the injected-class-name) is used, it always refers to the class template itself and not a specialization of the template. [ Example:

template<class T> class X {
  X* p;             // meaning X<T>
  X<T>* p2;
  X<int>* p3;
  ::X* p4;          // error: missing template argument list
                    // ​::​X does not refer to the injected-class-name
};

 — end example ]

A template-parameter shall not be redeclared within its scope (including nested scopes). A template-parameter shall not have the same name as the template name. [ Example:

template<class T, int i> class Y {
  int T;            // error: template-parameter redeclared
  void f() {
    char T;         // error: template-parameter redeclared
  }
};

template<class X> class X;      // error: template-parameter redeclared

 — end example ]

In the definition of a member of a class template that appears outside of the class template definition, the name of a member of the class template hides the name of a template-parameter of any enclosing class templates (but not a template-parameter of the member if the member is a class or function template). [ Example:

template<class T> struct A {
  struct B { /* ... */ };
  typedef void C;
  void f();
  template<class U> void g(U);
};

template<class B> void A<B>::f() {
  B b;              // A's B, not the template parameter
}

template<class B> template<class C> void A<B>::g(C) {
  B b;              // A's B, not the template parameter
  C c;              // the template parameter C, not A's C
}

 — end example ]

In the definition of a member of a class template that appears outside of the namespace containing the class template definition, the name of a template-parameter hides the name of a member of this namespace. [ Example:

namespace N {
  class C { };
  template<class T> class B {
    void f(T);
  };
}
template<class C> void N::B<C>::f(C) {
  C b;              // C is the template parameter, not N​::​C
}

 — end example ]

In the definition of a class template or in the definition of a member of such a template that appears outside of the template definition, for each non-dependent base class ([temp.dep.type]), if the name of the base class or the name of a member of the base class is the same as the name of a template-parameter, the base class name or member name hides the template-parameter name. [ Example:

struct A {
  struct B { /* ... */ };
  int a;
  int Y;
};

template<class B, class a> struct X : A {
  B b;              // A's B
  a b;              // error: A's a isn't a type name
};

 — end example ]