# injected-class-name

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The injected-class-name is the 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.

### Explanation

In a class scope, the name of the current class 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 definition.

    int X;
    struct X {
        void f() {
            X* p; // OK. X refers to the injected-class-name
            ::X* q; // Error: name lookup finds a variable name, which hides the struct name
        }
    };

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
    };

### In class template

Like other classes, class templates have an injected-class-name. The injected-class-name 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 argument that corresponds to a template template parameter
- 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;
    };
    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<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
    };

### 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 `D` in the scope of the class `C` finds 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;'

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