21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.7 Relationships between types [meta.rel]

The templates specified in Table 49 may be used to query relationships between types at compile time.

Each of these templates shall be a Cpp17BinaryTypeTrait ([meta.rqmts]) with a base characteristic of true_type if the corresponding condition is true, otherwise false_type.

Table 49: Type relationship predicates [tab:meta.rel]

🔗	Template	Condition	Comments
🔗	template<class T, class U> struct is_same;	T and U name the same type with the same cv-qualifications
🔗	template<class Base, class Derived> struct is_base_of;	Base is a base class of Derived ([class.derived]) without regard to cv-qualifiers or Base and Derived are not unions and name the same class type without regard to cv-qualifiers	If Base and Derived are non-union class types and are not (possibly cv-qualified versions of) the same type, Derived shall be a complete type. [Note 1: Base classes that are private, protected, or ambiguous are, nonetheless, base classes. — end note]
🔗	template<class From, class To> struct is_convertible;	see below	From and To shall be complete types, cv void, or arrays of unknown bound.
🔗	template<class From, class To> struct is_nothrow_convertible;	is_convertible_v<From, To> is true and the conversion, as defined by is_convertible, is known not to throw any exceptions ([expr.unary.noexcept])	From and To shall be complete types, cv void, or arrays of unknown bound.
🔗	template<class T, class U> struct is_layout_compatible;	T and U are layout-compatible ([basic.types.general])	T and U shall be complete types, cv void, or arrays of unknown bound.
🔗	template<class Base, class Derived> struct is_pointer_interconvertible_base_of;	Derived is unambiguously derived from Base without regard to cv-qualifiers, and each object of type Derived is pointer-interconvertible ([basic.compound]) with its Base subobject, or Base and Derived are not unions and name the same class type without regard to cv-qualifiers.	If Base and Derived are non-union class types and are not (possibly cv-qualified versions of) the same type, Derived shall be a complete type.
🔗	template<class Fn, class... ArgTypes> struct is_invocable;	The expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) ([func.require]) is well-formed when treated as an unevaluated operand ([expr.context])	Fn and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
🔗	template<class R, class Fn, class... ArgTypes> struct is_invocable_r;	The expression INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is well-formed when treated as an unevaluated operand	Fn, R, and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
🔗	template<class Fn, class... ArgTypes> struct is_nothrow_invocable;	is_invocable_v< Fn, ArgTypes...> is true and the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is known not to throw any exceptions ([expr.unary.noexcept])	Fn and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
🔗	template<class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;	is_invocable_r_v< R, Fn, ArgTypes...> is true and the expression INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is known not to throw any exceptions ([expr.unary.noexcept])	Fn, R, and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.

For the purpose of defining the templates in this subclause, a function call expression declval<T>() for any type T is considered to be a trivial ([basic.types.general], [special]) function call that is not an odr-use ([basic.def.odr]) of declval in the context of the corresponding definition notwithstanding the restrictions of [declval].

[Example 1: struct B {}; struct B1 : B {}; struct B2 : B {}; struct D : private B1, private B2 {}; is_base_of_v<B, D> // true is_base_of_v<const B, D> // true is_base_of_v<B, const D> // true is_base_of_v<B, const B> // true is_base_of_v<D, B> // false is_base_of_v<B&, D&> // false is_base_of_v<B[3], D[3]> // false is_base_of_v<int, int> // false — end example]

The predicate condition for a template specialization is_convertible<From, To> shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

To test() { return declval<From>(); }

[Note 2:

This requirement gives well-defined results for reference types, array types, function types, and cv void.

— end note]

Access checking is performed in a context unrelated to To and From.

Only the validity of the immediate context of the expression of the return statement ([stmt.return]) (including initialization of the returned object or reference) is considered.

[Note 3:

The initialization can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.

Such side effects are not in the “immediate context” and can result in the program being ill-formed.

— end note]