23 General utilities library [utilities]

23.15 Metaprogramming and type traits [meta]

23.15.2 Header <type_­traits> synopsis [meta.type.synop]

namespace std {
  // [meta.help], helper class
  template <class T, T v> struct integral_constant;

  template <bool B>
    using bool_constant = integral_constant<bool, B>;
  using true_type  = bool_constant<true>;
  using false_type = bool_constant<false>;

  // [meta.unary.cat], primary type categories
  template <class T> struct is_void;
  template <class T> struct is_null_pointer;
  template <class T> struct is_integral;
  template <class T> struct is_floating_point;
  template <class T> struct is_array;
  template <class T> struct is_pointer;
  template <class T> struct is_lvalue_reference;
  template <class T> struct is_rvalue_reference;
  template <class T> struct is_member_object_pointer;
  template <class T> struct is_member_function_pointer;
  template <class T> struct is_enum;
  template <class T> struct is_union;
  template <class T> struct is_class;
  template <class T> struct is_function;

  // [meta.unary.comp], composite type categories
  template <class T> struct is_reference;
  template <class T> struct is_arithmetic;
  template <class T> struct is_fundamental;
  template <class T> struct is_object;
  template <class T> struct is_scalar;
  template <class T> struct is_compound;
  template <class T> struct is_member_pointer;

  // [meta.unary.prop], type properties
  template <class T> struct is_const;
  template <class T> struct is_volatile;
  template <class T> struct is_trivial;
  template <class T> struct is_trivially_copyable;
  template <class T> struct is_standard_layout;
  template <class T> struct is_pod;
  template <class T> struct is_empty;
  template <class T> struct is_polymorphic;
  template <class T> struct is_abstract;
  template <class T> struct is_final;
  template <class T> struct is_aggregate;

  template <class T> struct is_signed;
  template <class T> struct is_unsigned;

  template <class T, class... Args> struct is_constructible;
  template <class T> struct is_default_constructible;
  template <class T> struct is_copy_constructible;
  template <class T> struct is_move_constructible;

  template <class T, class U> struct is_assignable;
  template <class T> struct is_copy_assignable;
  template <class T> struct is_move_assignable;

  template <class T, class U> struct is_swappable_with;
  template <class T> struct is_swappable;

  template <class T> struct is_destructible;

  template <class T, class... Args> struct is_trivially_constructible;
  template <class T> struct is_trivially_default_constructible;
  template <class T> struct is_trivially_copy_constructible;
  template <class T> struct is_trivially_move_constructible;

  template <class T, class U> struct is_trivially_assignable;
  template <class T> struct is_trivially_copy_assignable;
  template <class T> struct is_trivially_move_assignable;
  template <class T> struct is_trivially_destructible;

  template <class T, class... Args> struct is_nothrow_constructible;
  template <class T> struct is_nothrow_default_constructible;
  template <class T> struct is_nothrow_copy_constructible;
  template <class T> struct is_nothrow_move_constructible;

  template <class T, class U> struct is_nothrow_assignable;
  template <class T> struct is_nothrow_copy_assignable;
  template <class T> struct is_nothrow_move_assignable;

  template <class T, class U> struct is_nothrow_swappable_with;
  template <class T> struct is_nothrow_swappable;

  template <class T> struct is_nothrow_destructible;

  template <class T> struct has_virtual_destructor;

  template <class T> struct has_unique_object_representations;

  // [meta.unary.prop.query], type property queries
  template <class T> struct alignment_of;
  template <class T> struct rank;
  template <class T, unsigned I = 0> struct extent;

  // [meta.rel], type relations
  template <class T, class U> struct is_same;
  template <class Base, class Derived> struct is_base_of;
  template <class From, class To> struct is_convertible;

  template <class Fn, class... ArgTypes> struct is_invocable;
  template <class R, class Fn, class... ArgTypes> struct is_invocable_r;

  template <class Fn, class... ArgTypes> struct is_nothrow_invocable;
  template <class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;

  // [meta.trans.cv], const-volatile modifications
  template <class T> struct remove_const;
  template <class T> struct remove_volatile;
  template <class T> struct remove_cv;
  template <class T> struct add_const;
  template <class T> struct add_volatile;
  template <class T> struct add_cv;

  template <class T>
    using remove_const_t    = typename remove_const<T>::type;
  template <class T>
    using remove_volatile_t = typename remove_volatile<T>::type;
  template <class T>
    using remove_cv_t       = typename remove_cv<T>::type;
  template <class T>
    using add_const_t       = typename add_const<T>::type;
  template <class T>
    using add_volatile_t    = typename add_volatile<T>::type;
  template <class T>
    using add_cv_t          = typename add_cv<T>::type;

  // [meta.trans.ref], reference modifications
  template <class T> struct remove_reference;
  template <class T> struct add_lvalue_reference;
  template <class T> struct add_rvalue_reference;

  template <class T>
    using remove_reference_t     = typename remove_reference<T>::type;
  template <class T>
    using add_lvalue_reference_t = typename add_lvalue_reference<T>::type;
  template <class T>
    using add_rvalue_reference_t = typename add_rvalue_reference<T>::type;

  // [meta.trans.sign], sign modifications
  template <class T> struct make_signed;
  template <class T> struct make_unsigned;

  template <class T>
    using make_signed_t   = typename make_signed<T>::type;
  template <class T>
    using make_unsigned_t = typename make_unsigned<T>::type;

  // [meta.trans.arr], array modifications
  template <class T> struct remove_extent;
  template <class T> struct remove_all_extents;

  template <class T>
    using remove_extent_t      = typename remove_extent<T>::type;
  template <class T>
    using remove_all_extents_t = typename remove_all_extents<T>::type;

  // [meta.trans.ptr], pointer modifications
  template <class T> struct remove_pointer;
  template <class T> struct add_pointer;

  template <class T>
    using remove_pointer_t = typename remove_pointer<T>::type;
  template <class T>
    using add_pointer_t    = typename add_pointer<T>::type;

  // [meta.trans.other], other transformations
  template <size_t Len,
            size_t Align = default-alignment> // see [meta.trans.other]
    struct aligned_storage;
  template <size_t Len, class... Types> struct aligned_union;
  template <class T> struct decay;
  template <bool, class T = void> struct enable_if;
  template <bool, class T, class F> struct conditional;
  template <class... T> struct common_type;
  template <class T> struct underlying_type;
  template <class Fn, class... ArgTypes> struct invoke_result;

  template <size_t Len,
            size_t Align = default-alignment> // see [meta.trans.other]
    using aligned_storage_t = typename aligned_storage<Len, Align>::type;
  template <size_t Len, class... Types>
    using aligned_union_t   = typename aligned_union<Len, Types...>::type;
  template <class T>
    using decay_t           = typename decay<T>::type;
  template <bool b, class T = void>
    using enable_if_t       = typename enable_if<b, T>::type;
  template <bool b, class T, class F>
    using conditional_t     = typename conditional<b, T, F>::type;
  template <class... T>
    using common_type_t     = typename common_type<T...>::type;
  template <class T>
    using underlying_type_t = typename underlying_type<T>::type;
  template <class Fn, class... ArgTypes>
    using invoke_result_t   = typename invoke_result<Fn, ArgTypes...>::type;
  template <class...>
    using void_t            = void;

  // [meta.logical], logical operator traits
  template<class... B> struct conjunction;
  template<class... B> struct disjunction;
  template<class B> struct negation;

  // [meta.endian], endian
  enum class endian {
    little = see below,
    big    = see below,
    native = see below
  };

  // [meta.unary.cat], primary type categories
  template <class T> inline constexpr bool is_void_v
    = is_void<T>::value;
  template <class T> inline constexpr bool is_null_pointer_v
    = is_null_pointer<T>::value;
  template <class T> inline constexpr bool is_integral_v
    = is_integral<T>::value;
  template <class T> inline constexpr bool is_floating_point_v
    = is_floating_point<T>::value;
  template <class T> inline constexpr bool is_array_v
    = is_array<T>::value;
  template <class T> inline constexpr bool is_pointer_v
    = is_pointer<T>::value;
  template <class T> inline constexpr bool is_lvalue_reference_v
    = is_lvalue_reference<T>::value;
  template <class T> inline constexpr bool is_rvalue_reference_v
    = is_rvalue_reference<T>::value;
  template <class T> inline constexpr bool is_member_object_pointer_v
    = is_member_object_pointer<T>::value;
  template <class T> inline constexpr bool is_member_function_pointer_v
    = is_member_function_pointer<T>::value;
  template <class T> inline constexpr bool is_enum_v
    = is_enum<T>::value;
  template <class T> inline constexpr bool is_union_v
    = is_union<T>::value;
  template <class T> inline constexpr bool is_class_v
    = is_class<T>::value;
  template <class T> inline constexpr bool is_function_v
    = is_function<T>::value;

  // [meta.unary.comp], composite type categories
  template <class T> inline constexpr bool is_reference_v
    = is_reference<T>::value;
  template <class T> inline constexpr bool is_arithmetic_v
    = is_arithmetic<T>::value;
  template <class T> inline constexpr bool is_fundamental_v
    = is_fundamental<T>::value;
  template <class T> inline constexpr bool is_object_v
    = is_object<T>::value;
  template <class T> inline constexpr bool is_scalar_v
    = is_scalar<T>::value;
  template <class T> inline constexpr bool is_compound_v
    = is_compound<T>::value;
  template <class T> inline constexpr bool is_member_pointer_v
    = is_member_pointer<T>::value;

  // [meta.unary.prop], type properties
  template <class T> inline constexpr bool is_const_v
    = is_const<T>::value;
  template <class T> inline constexpr bool is_volatile_v
    = is_volatile<T>::value;
  template <class T> inline constexpr bool is_trivial_v
    = is_trivial<T>::value;
  template <class T> inline constexpr bool is_trivially_copyable_v
    = is_trivially_copyable<T>::value;
  template <class T> inline constexpr bool is_standard_layout_v
    = is_standard_layout<T>::value;
  template <class T> inline constexpr bool is_pod_v
    = is_pod<T>::value;
  template <class T> inline constexpr bool is_empty_v
    = is_empty<T>::value;
  template <class T> inline constexpr bool is_polymorphic_v
    = is_polymorphic<T>::value;
  template <class T> inline constexpr bool is_abstract_v
    = is_abstract<T>::value;
  template <class T> inline constexpr bool is_final_v
    = is_final<T>::value;
  template <class T> inline constexpr bool is_aggregate_v
    = is_aggregate<T>::value;
  template <class T> inline constexpr bool is_signed_v
    = is_signed<T>::value;
  template <class T> inline constexpr bool is_unsigned_v
    = is_unsigned<T>::value;
  template <class T, class... Args> inline constexpr bool is_constructible_v
    = is_constructible<T, Args...>::value;
  template <class T> inline constexpr bool is_default_constructible_v
    = is_default_constructible<T>::value;
  template <class T> inline constexpr bool is_copy_constructible_v
    = is_copy_constructible<T>::value;
  template <class T> inline constexpr bool is_move_constructible_v
    = is_move_constructible<T>::value;
  template <class T, class U> inline constexpr bool is_assignable_v
    = is_assignable<T, U>::value;
  template <class T> inline constexpr bool is_copy_assignable_v
    = is_copy_assignable<T>::value;
  template <class T> inline constexpr bool is_move_assignable_v
    = is_move_assignable<T>::value;
  template <class T, class U> inline constexpr bool is_swappable_with_v
    = is_swappable_with<T, U>::value;
  template <class T> inline constexpr bool is_swappable_v
    = is_swappable<T>::value;
  template <class T> inline constexpr bool is_destructible_v
    = is_destructible<T>::value;
  template <class T, class... Args> inline constexpr bool is_trivially_constructible_v
    = is_trivially_constructible<T, Args...>::value;
  template <class T> inline constexpr bool is_trivially_default_constructible_v
    = is_trivially_default_constructible<T>::value;
  template <class T> inline constexpr bool is_trivially_copy_constructible_v
    = is_trivially_copy_constructible<T>::value;
  template <class T> inline constexpr bool is_trivially_move_constructible_v
    = is_trivially_move_constructible<T>::value;
  template <class T, class U> inline constexpr bool is_trivially_assignable_v
    = is_trivially_assignable<T, U>::value;
  template <class T> inline constexpr bool is_trivially_copy_assignable_v
    = is_trivially_copy_assignable<T>::value;
  template <class T> inline constexpr bool is_trivially_move_assignable_v
    = is_trivially_move_assignable<T>::value;
  template <class T> inline constexpr bool is_trivially_destructible_v
    = is_trivially_destructible<T>::value;
  template <class T, class... Args> inline constexpr bool is_nothrow_constructible_v
    = is_nothrow_constructible<T, Args...>::value;
  template <class T> inline constexpr bool is_nothrow_default_constructible_v
    = is_nothrow_default_constructible<T>::value;
  template <class T> inline constexpr bool is_nothrow_copy_constructible_v
    = is_nothrow_copy_constructible<T>::value;
  template <class T> inline constexpr bool is_nothrow_move_constructible_v
    = is_nothrow_move_constructible<T>::value;
  template <class T, class U> inline constexpr bool is_nothrow_assignable_v
    = is_nothrow_assignable<T, U>::value;
  template <class T> inline constexpr bool is_nothrow_copy_assignable_v
    = is_nothrow_copy_assignable<T>::value;
  template <class T> inline constexpr bool is_nothrow_move_assignable_v
    = is_nothrow_move_assignable<T>::value;
  template <class T, class U> inline constexpr bool is_nothrow_swappable_with_v
    = is_nothrow_swappable_with<T, U>::value;
  template <class T> inline constexpr bool is_nothrow_swappable_v
    = is_nothrow_swappable<T>::value;
  template <class T> inline constexpr bool is_nothrow_destructible_v
    = is_nothrow_destructible<T>::value;
  template <class T> inline constexpr bool has_virtual_destructor_v
    = has_virtual_destructor<T>::value;
  template <class T> inline constexpr bool has_unique_object_representations_v
    = has_unique_object_representations<T>::value;

  // [meta.unary.prop.query], type property queries
  template <class T> inline constexpr size_t alignment_of_v
    = alignment_of<T>::value;
  template <class T> inline constexpr size_t rank_v
    = rank<T>::value;
  template <class T, unsigned I = 0> inline constexpr size_t extent_v
    = extent<T, I>::value;

  // [meta.rel], type relations
  template <class T, class U> inline constexpr bool is_same_v
    = is_same<T, U>::value;
  template <class Base, class Derived> inline constexpr bool is_base_of_v
    = is_base_of<Base, Derived>::value;
  template <class From, class To> inline constexpr bool is_convertible_v
    = is_convertible<From, To>::value;
  template <class Fn, class... ArgTypes> inline constexpr bool is_invocable_v
    = is_invocable<Fn, ArgTypes...>::value;
  template <class R, class Fn, class... ArgTypes> inline constexpr bool is_invocable_r_v
    = is_invocable_r<R, Fn, ArgTypes...>::value;
  template <class Fn, class... ArgTypes> inline constexpr bool is_nothrow_invocable_v
    = is_nothrow_invocable<Fn, ArgTypes...>::value;
  template <class R, class Fn, class... ArgTypes> inline constexpr bool is_nothrow_invocable_r_v
    = is_nothrow_invocable_r<R, Fn, ArgTypes...>::value;

  // [meta.logical], logical operator traits
  template<class... B> inline constexpr bool conjunction_v = conjunction<B...>::value;
  template<class... B> inline constexpr bool disjunction_v = disjunction<B...>::value;
  template<class B> inline constexpr bool negation_v = negation<B>::value;
}
The behavior of a program that adds specializations for any of the templates defined in this subclause is undefined unless otherwise specified.
Unless otherwise specified, an incomplete type may be used to instantiate a template in this subclause.