Injected-class-name
The injected-class-name is the unqualified name of a class within the scope of said class.
In a class template, the injected-class-name can be used either as a template name that refers to the current template, or as a class name that refers to the current instantiation.
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[edit] Explanation
In a class scope, the class name of the current class or the template name of the current class template is treated as if it were a public member name; this is called injected-class-name. The point of declaration of the name is immediately following the opening brace of the class (template) definition.
int X; struct X { void f() { X* p; // OK, X is an injected-class-name ::X* q; // Error: name lookup finds a variable name, which hides the struct name } }; template<class T> struct Y { void g() { Y* p; // OK, Y is an injected-class-name Y<T>* q; // OK, Y is an injected-class-name, but Y<T> is not } };
Like other members, injected-class-names are inherited. In the presence of private or protected inheritance, the injected-class-name of an indirect base class might end up being inaccessible in a derived class.
struct A {}; struct B : private A {}; struct C : public B { A* p; // Error: injected-class-name A is inaccessible ::A* q; // OK, does not use the injected-class-name };
[edit] In class template
The injected-class-name of a class template can be used as a template-name or a type-name.
In the following cases, the injected-class-name is treated as a template-name of the class template itself:
- It is followed by
<. - It is used as a template template argument.
- It is the final identifier in the elaborated class specifier of a friend class template declaration.
Otherwise, it is treated as a type-name, and is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.
template<template<class, class> class> struct A; template<class T1, class T2> struct X { X<T1, T2>* p; // OK, X is treated as a template-name using a = A<X>; // OK, X is treated as a template-name template<class U1, class U2> friend class X; // OK, X is treated as a template-name X* q; // OK, X is treated as a type-name, equivalent to X<T1, T2> };
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 <>.
template<> struct X<void, void> { X* p; // OK, X is treated as a type-name, equivalent to X<void, void> template<class, class> friend class X; // OK, X is treated as a template-name (same as in primary template) X<void, void>* q; // OK, X is treated as a template-name }; template<class T> struct X<char, T> { X* p, q; // OK, X is treated as a type-name, equivalent to X<char, T> using r = X<int, int>; // OK, can be used to name another specialization };
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.
template<> class X<int, char> { class B { X a; // meaning X<int, char> template<class, class> friend class X; // meaning ::X }; }; template<class T> struct Base { Base* p; // OK: Base means Base<T> }; template<class T> struct Derived : public Base<T*> { typename Derived::Base* p; // OK: Derived::Base means Derived<T>::Base, // which is Base<T*> }; template<class T, template<class> class U = T::template Base> struct Third {}; Third<Derived<int>> t; // OK: default argument uses injected-class-name as a template
A lookup that finds an injected-class-name 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.
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 };
[edit] injected-class-name and constructors
Constructors do not have names, but the injected-class-name of the enclosing class is considered to name a constructor in constructor declarations and definitions.
In a qualified name C::D, if
- name lookup does not ignore function names, and
- lookup of
Din the scope of the classCfinds its injected-class-name
the qualified name is always considered to name C's constructor. Such a name can only be used in the declaration of a constructor (e.g. in a friend constructor declaration, a constructor template specialization, constructor template instantiation, or constructor definition) or be used to inherit constructors(since C++11).
struct A { A(); A(int); template<class T> A(T) {} }; using A_alias = A; A::A() {} A_alias::A(int) {} template A::A(double); struct B : A { using A_alias::A; }; A::A a; // Error: A::A is considered to name a constructor, not a type struct A::A a2; // OK, same as 'A a2;' B::A b; // OK, same as 'A b;'
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 1004 | C++98 | an injected-class-name could not be a template template argument |
allowed, it refers to the class template itself in this case |
| CWG 2637 | C++98 | the entire template-id could be an injected-class-name | only the template name can |
