20 General utilities library [utilities]

20.9 Metaprogramming and type traits [meta]

20.9.6 Relationships between types [meta.rel]

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 51 — Type relationship predicates
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 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.


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

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

 — end example ]

Given the following function prototype:

template <class T>
  typename add_rvalue_reference<T>::type create();

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 ]