This subclause 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.
| 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 not possibly cv-qualified versions of the same type, 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. |
| template <class Fn, class... ArgTypes, class R> struct is_callable< Fn(ArgTypes...), R>; | The expression INVOKE(declval<Fn>(), declval<ArgTypes>()..., R) is well formed when treated as an unevaluated operand | Fn, R, and all types in the parameter pack ArgTypes shall be complete types, (possibly cv-qualified) void, or arrays of unknown bound. |
| template <class Fn, class... ArgTypes, class R> struct is_nothrow_callable< Fn(ArgTypes...), R>; |
is_callable_v< Fn(ArgTypes...), R> is true and the expression INVOKE(declval<Fn>(), declval<ArgTypes>()..., R) is known not to throw any exceptions | Fn, R, and all types in the parameter pack ArgTypes shall be complete types, (possibly cv-qualified) 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], [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:
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: 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 ]