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20 General utilities library [utilities]

This sub-clause contains templates that may be used to query relationships between types at compile time.

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

Table

— Type relationship predicates

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 (Clause [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 different types (ignoring possible cv-qualifiers) then Derived shall be a complete type. [ Note: 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, arrays of unknown bound, or (possibly cv-qualified) void types.

[ Example:

struct B {};
struct B1 : B {};
struct B2 : B {};
struct D : private B1, private B2 {};

is_base_of<B, D>::value         is_base_of<const B, D>::value   is_base_of<B, const D>::value   is_base_of<B, const B>::value   is_base_of<D, B>::value         is_base_of<B&, D&>::value       is_base_of<B[3], D[3]>::value   is_base_of<int, int>::value

— end example ]

Given the following function prototype:

template <class T>
  add_rvalue_reference_t<T> create() noexcept;

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 create<From>();
}

[ Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note ] Access checking is performed as if in a context unrelated to To and From. Only the validity of the immediate context of the expression of the return-statement (including conversions to the return type) is considered. [ Note: The evaluation of the conversion 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 ]

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