# Standard library header <type_traits>

From cppreference.com
< cpp‎ | header

This header is part of the type support library.

### Classes

##### Helper Classes

integral_constant bool_constant (C++11)(C++17) | compile-time constant of specified type with specified value (class template)
`true_type` | std::integral_constant<bool, true>
`false_type` | std::integral_constant<bool, false>

##### Primary type categories

is_void (C++11) | checks if a type is void (class template)
is_null_pointer (C++14) | checks if a type is std::nullptr_t (class template)
is_integral (C++11) | checks if a type is an integral type (class template)
is_floating_point (C++11) | checks if a type is a floating-point type (class template)
is_array (C++11) | checks if a type is an array type (class template)
is_enum (C++11) | checks if a type is an enumeration type (class template)
is_union (C++11) | checks if a type is an union type (class template)
is_class (C++11) | checks if a type is a non-union class type (class template)
is_function (C++11) | checks if a type is a function type (class template)
is_pointer (C++11) | checks if a type is a pointer type (class template)
is_lvalue_reference (C++11) | checks if a type is a lvalue reference (class template)
is_rvalue_reference (C++11) | checks if a type is a rvalue reference (class template)
is_member_object_pointer (C++11) | checks if a type is a pointer to a non-static member object (class template)
is_member_function_pointer (C++11) | checks if a type is a pointer to a non-static member function (class template)

##### Composite type categories

is_fundamental (C++11) | checks if a type is a fundamental type (class template)
is_arithmetic (C++11) | checks if a type is an arithmetic type (class template)
is_scalar (C++11) | checks if a type is a scalar type (class template)
is_object (C++11) | checks if a type is an object type (class template)
is_compound (C++11) | checks if a type is a compound type (class template)
is_reference (C++11) | checks if a type is either a lvalue reference or rvalue reference (class template)
is_member_pointer (C++11) | checks if a type is a pointer to an non-static member function or object (class template)

##### Type properties

is_const (C++11) | checks if a type is const-qualified (class template)
is_volatile (C++11) | checks if a type is volatile-qualified (class template)
is_trivial (C++11) | checks if a type is trivial (class template)
is_trivially_copyable (C++11) | checks if a type is trivially copyable (class template)
is_standard_layout (C++11) | checks if a type is a standard-layout type (class template)
is_pod (C++11)(deprecated in C++20) | checks if a type is a plain-old data (POD) type (class template)
is_literal_type (C++11)(deprecated in C++17)(removed in C++20) | checks if a type is a literal type (class template)
has_unique_object_representations (C++17) | checks if every bit in the type's object representation contributes to its value (class template)
is_empty (C++11) | checks if a type is a class (but not union) type and has no non-static data members (class template)
is_polymorphic (C++11) | checks if a type is a polymorphic class type (class template)
is_abstract (C++11) | checks if a type is an abstract class type (class template)
is_final (C++14) | checks if a type is a final class type (class template)
is_aggregate (C++17) | checks if a type is an aggregate type (class template)
is_signed (C++11) | checks if a type is a signed arithmetic type (class template)
is_unsigned (C++11) | checks if a type is an unsigned arithmetic type (class template)
is_bounded_array (C++20) | checks if a type is an array type of known bound (class template)
is_unbounded_array (C++20) | checks if a type is an array type of unknown bound (class template)

##### Supported operations

is_constructible is_trivially_constructible is_nothrow_constructible (C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template)
is_default_constructible is_trivially_default_constructible is_nothrow_default_constructible (C++11)(C++11)(C++11) | checks if a type has a default constructor (class template)
is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11)(C++11)(C++11) | checks if a type has a copy constructor (class template)
is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11)(C++11)(C++11) | checks if a type can be constructed from an rvalue reference (class template)
is_assignable is_trivially_assignable is_nothrow_assignable (C++11)(C++11)(C++11) | checks if a type has a assignment operator for a specific argument (class template)
is_copy_assignable is_trivially_copy_assignable is_nothrow_copy_assignable (C++11)(C++11)(C++11) | checks if a type has a copy assignment operator (class template)
is_move_assignable is_trivially_move_assignable is_nothrow_move_assignable (C++11)(C++11)(C++11) | checks if a type has a move assignment operator (class template)
is_destructible is_trivially_destructible is_nothrow_destructible (C++11)(C++11)(C++11) | checks if a type has a non-deleted destructor (class template)
has_virtual_destructor (C++11) | checks if a type has a virtual destructor (class template)
is_swappable_with is_swappable is_nothrow_swappable_with is_nothrow_swappable (C++17)(C++17)(C++17)(C++17) | checks if objects of a type can be swapped with objects of same or different type (class template)

##### Property queries

alignment_of (C++11) | obtains the type's alignment requirements (class template)
rank (C++11) | obtains the number of dimensions of an array type (class template)
extent (C++11) | obtains the size of an array type along a specified dimension (class template)

##### Type relationships

is_same (C++11) | checks if two types are the same (class template)
is_base_of (C++11) | checks if a type is derived from the other type (class template)
is_convertible is_nothrow_convertible (C++11)(C++20) | checks if a type can be converted to the other type (class template)
is_invocable is_invocable_r is_nothrow_invocable is_nothrow_invocable_r (C++17) | checks if a type can be invoked (as if by std::invoke) with the given argument types (class template)

##### Const-volatility specifiers

remove_cv remove_const remove_volatile (C++11)(C++11)(C++11) | removes const or/and volatile specifiers from the given type (class template)
add_cv add_const add_volatile (C++11)(C++11)(C++11) | adds const or/and volatile specifiers to the given type (class template)

##### References

remove_reference (C++11) | removes a reference from the given type (class template)
add_lvalue_reference add_rvalue_reference (C++11)(C++11) | adds a lvalue or rvalue reference to the given type (class template)

##### Pointers

remove_pointer (C++11) | removes a pointer from the given type (class template)
add_pointer (C++11) | adds a pointer to the given type (class template)

##### Sign modifiers

make_signed (C++11) | makes the given integral type signed (class template)
make_unsigned (C++11) | makes the given integral type unsigned (class template)

##### Arrays

remove_extent (C++11) | removes one extent from the given array type (class template)
remove_all_extents (C++11) | removes all extents from the given array type (class template)

##### Miscellaneous transformations

aligned_storage (C++11) | defines the type suitable for use as uninitialized storage for types of given size (class template)
aligned_union (C++11) | defines the type suitable for use as uninitialized storage for all given types (class template)
decay (C++11) | applies type transformations as when passing a function argument by value (class template)
remove_cvref (C++20) | combines std::remove_cv and std::remove_reference (class template)
enable_if (C++11) | hides a function overload or template specialization based on compile-time boolean (class template)
conditional (C++11) | chooses one type or another based on compile-time boolean (class template)
common_type (C++11) | determines the common type of a group of types (class template)
underlying_type (C++11) | obtains the underlying integer type for a given enumeration type (class template)
result_of invoke_result (C++11)(removed in C++20)(C++17) | deduces the result type of invoking a callable object with a set of arguments (class template)
void_t (C++17) | void variadic alias template (alias template)

##### Operations on traits

conjunction (C++17) | variadic logical AND metafunction (class template)
disjunction (C++17) | variadic logical OR metafunction (class template)
negation (C++17) | logical NOT metafunction (class template)

##### Endian

endian (C++20) | indicates the endianness of scalar types (enum)

##### Constant evaluation context

is_constant_evaluated (C++20) | detects whether the call occurs within a constant-evaluated context (function)

### Synopsis

    namespace std {
      // 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>;

      // 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;

      // 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;

      // 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> struct is_bounded_array;
      template <class T> struct is_unbounded_array;
      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;

      // type property queries:
      template <class T> struct alignment_of;
      template <class T> struct rank;
      template <class T, unsigned I = 0> struct extent;

      // 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;

      // 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;

      // 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;

      // 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;

      // 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;

      // 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;

      // other transformations:
      template <size_t Len,
      size_t Align = /*default-alignment*/ >
      struct aligned_storage;
      template <size_t Len, class... Types> struct aligned_union;
      template <class T> struct decay;
      template <class T> struct remove_cvref;
      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> class result_of; // not defined
      template <class F, class... ArgTypes> class result_of<F(ArgTypes...)>;
      template <class F, class... ArgTypes> class invoke_result;
      template <size_t Len,
      size_t Align = /*default-alignment*/ >
      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 <class T>
      using remove_cvref_t = typename remove_cvref<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 T>
      using result_of_t = typename result_of<T>::type;
      template <class F, class... ArgTypes>
      using invoke_result_t = typename invoke_result<F, ArgTypes...>::type;
      template <class...>
      using void_t = void;

      // logical operator traits:
      template<class... B> struct conjunction;
      template<class... B> struct disjunction;
      template<class B> struct negation;

      // endian
      enum class endian {
        little = /*implementation-defined*/,
        big    = /*implementation-defined*/,
        native = /*implementation-defined*/
      };

      // 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;

      // 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;

      // 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> inline constexpr bool is_bounded_array_v
      = is_bounded_array<T>::value;
      template <class T> inline constexpr bool is_unbounded_array_v
      = is_unbounded_array<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;

      // 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;

      // 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;

      // 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;

      // constant evaluation context
      constexpr bool is_constant_evaluated() noexcept;
    } // namespace std

#### Class std::integral_constant

    namespace std {
      template <class T, T v>
      struct integral_constant {
        static constexpr T value = v;
        using value_type = T;
        using type = integral_constant<T, v>;
        constexpr operator value_type() const noexcept { return value; }
        constexpr value_type operator()() const noexcept { return value; }
      };
    }

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