Difference between revisions of "cpp/language/injected-class-name"

Revision as of 22:14, 5 May 2022

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 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
};

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;'

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.