21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.8 Transformations between types [meta.trans]

21.3.8.7 Other transformations [meta.trans.other]

Table 56: Other transformations [tab:meta.trans.other]
Template
Comments
template<class T>
struct type_­identity;
The member typedef type denotes T.
template<class T>
struct remove_­cvref;
The member typedef type denotes remove_­cv_­t<remove_­reference_­t<T>>.
template<class T>
struct decay;
Let U be remove_­reference_­t<T>.
If is_­array_­v<U> is true, the member typedef type denotes remove_­extent_­t<U>*.
If is_­function_­v<U> is true, the member typedef type denotes add_­pointer_­t<U>.
Otherwise the member typedef type denotes remove_­cv_­t<U>.
[Note 1:
This behavior is similar to the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) conversions applied when an lvalue is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing.
β€” end note]
template<bool B, class T = void> struct enable_­if;
If B is true, the member typedef type denotes T; otherwise, there shall be no member type.
template<bool B, class T, class F>
struct conditional;
If B is true, the member typedef type denotes T.
If B is false, the member typedef type denotes F.
template<class... T> struct common_­type;
Unless this trait is specialized (as specified in Note B, below), the member type is defined or omitted as specified in Note A, below.
If it is omitted, there shall be no member type.
Each type in the template parameter pack T shall be complete, cv void, or an array of unknown bound.
template<class, class, template<class> class, template<class> class> struct basic_­common_­reference;
Unless this trait is specialized (as specified in Note D, below), there shall be no member type.
template<class... T> struct common_­reference;
The member typedef-name type is defined or omitted as specified in Note C, below.
Each type in the parameter pack T shall be complete or cv void.
template<class T>
struct underlying_­type;
If T is an enumeration type, the member typedef type denotes the underlying type of T ([dcl.enum]); otherwise, there is no member type.

Mandates: T is not an incomplete enumeration type.
template<class Fn,
class... ArgTypes>
struct invoke_­result;
If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well-formed when treated as an unevaluated operand ([expr.context]), the member typedef type denotes the type decltype(INVOKE(declval<Fn>(), declval<ArgTypes>()...)); otherwise, there shall be no member type.
Access checking is performed as if in a context unrelated to Fn and ArgTypes.
Only the validity of the immediate context of the expression is considered.
[Note 2:
The compilation of the expression 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 β€œimmediate context” and can result in the program being ill-formed.
β€” end note]
Preconditions: Fn and all types in the template parameter pack ArgTypes are complete types, cv void, or arrays of unknown bound.
template<class T> struct unwrap_­reference;
If T is a specialization reference_­wrapper<X> for some type X, the member typedef type of unwrap_­reference<T> denotes X&, otherwise type denotes T.
template<class T> unwrap_­ref_­decay;
The member typedef type of unwrap_­ref_­decay<T> denotes the type unwrap_­reference_­t<decay_­t<T>>.