21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.9 Transformations between types [meta.trans]

21.3.9.7 Other transformations [meta.trans.other]

Table 62 — Other transformations [tab:meta.trans.other]
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Template
Comments
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template<class T>
struct type_identity;
The member typedef type denotes T.
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template<class T>
struct remove_cvref;
The member typedef type denotes remove_cv_t<remove_reference_t<T>>.
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template<class T>
struct decay;
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.
— end note]
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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.
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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.
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template<class... T> struct common_type;
Unless this trait is specialized, the member type is defined or omitted as specified 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.
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template<class, class, template<class> class, template<class> class> struct basic_common_reference;
Unless this trait is specialized, there shall be no member type.
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template<class... T> struct common_reference;
The member typedef-name type is defined or omitted as specified below.
Each type in the parameter pack T shall be complete or cv void.
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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.
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template<class Fn,
class... ArgTypes>
struct invoke_result;
If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) ([func.require]) 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 “immediate context” and can result in the program being ill-formed.
— end note]
Preconditions: Fn and all types in the template parameter pack ArgTypes are complete types, cv void, or arrays of unknown bound.
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template<class Fn, class Tuple>
struct apply_result;
If tuple-like<Tuple> is true and the expression INVOKE(declval<Fn>(), ELEMS-OF(Tuple)...) ([func.require]) is well-formed when treated as an unevaluated operand ([expr.context]), the member typedef type denotes the type decltype(INVOKE(declval<Fn>(), ELEMS-OF(Tuple)...)); otherwise, there shall be no member type.
Access checking is performed as if in a context unrelated to Fn and Tuple.
Only the validity of the immediate context of the expression is considered.
[Note 3: 
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 “immediate context” and can result in the program being ill-formed.
— end note]
Preconditions: Fn and Tuple are complete types, cv void, or arrays of unknown bound.
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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.
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template<class T> unwrap_ref_decay;
The member typedef type of unwrap_ref_decay<T> denotes the type unwrap_reference_t<decay_t<T>>.