# Variable templates for type traits (library fundamentals TS)

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< cpp‎ | experimental

Version 1 of the C++ Extensions for Library Fundamentals provides a number of `constexpr` variable templates for type traits classes, as well as other similar class templates with public `::value` members.

Unless otherwise noted, the following variable templates are defined in the `std::experimental` namespace.

### Type traits

##### Primary type categories

Variable template | Value
Defined in header `<experimental/type_traits>`
template <class T> constexpr bool is_void_v = | std::is_void<T>::value;
template <class T> constexpr bool is_null_pointer_v = | std::is_null_pointer<T>::value;
template <class T> constexpr bool is_integral_v = | std::is_integral<T>::value;
template <class T> constexpr bool is_floating_point_v = | std::is_floating_point<T>::value;
template <class T> constexpr bool is_array_v = | std::is_array<T>::value;
template <class T> constexpr bool is_pointer_v = | std::is_pointer<T>::value;
template <class T> constexpr bool is_lvalue_reference_v = | std::is_lvalue_reference<T>::value;
template <class T> constexpr bool is_rvalue_reference_v = | std::is_rvalue_reference<T>::value;
template <class T> constexpr bool is_member_object_pointer_v = | std::is_member_object_pointer<T>::value;
template <class T> constexpr bool is_member_function_pointer_v = | std::is_member_function_pointer<T>::value;
template <class T> constexpr bool is_enum_v = | std::is_enum<T>::value;
template <class T> constexpr bool is_union_v = | std::is_union<T>::value;
template <class T> constexpr bool is_class_v = | std::is_class<T>::value;
template <class T> constexpr bool is_function_v = | std::is_function<T>::value;

##### Composite type categories

Variable template | Value
Defined in header `<experimental/type_traits>`
template <class T> constexpr bool is_reference_v = | std::is_reference<T>::value;
template <class T> constexpr bool is_arithmetic_v = | std::is_arithmetic<T>::value;
template <class T> constexpr bool is_fundamental_v = | std::is_fundamental<T>::value;
template <class T> constexpr bool is_object_v = | std::is_object<T>::value;
template <class T> constexpr bool is_scalar_v = | std::is_scalar<T>::value;
template <class T> constexpr bool is_compound_v = | std::is_compound<T>::value;
template <class T> constexpr bool is_member_pointer_v = | std::is_member_pointer<T>::value;

##### Type properties

Variable template | Value
Defined in header `<experimental/type_traits>`
template <class T> constexpr bool is_const_v = | std::is_const<T>::value;
template <class T> constexpr bool is_volatile_v = | std::is_volatile<T>::value;
template <class T> constexpr bool is_trivial_v = | std::is_trivial<T>::value;
template <class T> constexpr bool is_trivially_copyable_v = | std::is_trivially_copyable<T>::value;
template <class T> constexpr bool is_standard_layout_v = | std::is_standard_layout<T>::value;
template <class T> constexpr bool is_pod_v = | std::is_pod<T>::value;
template <class T> constexpr bool is_literal_type_v = | std::is_literal_type<T>::value;
template <class T> constexpr bool is_empty_v = | std::is_empty<T>::value;
template <class T> constexpr bool is_polymorphic_v = | std::is_polymorphic<T>::value;
template <class T> constexpr bool is_abstract_v = | std::is_abstract<T>::value;
template <class T> constexpr bool is_final_v = | std::is_final<T>::value;
template <class T> constexpr bool is_signed_v = | std::is_signed<T>::value;
template <class T> constexpr bool is_unsigned_v = | std::is_unsigned<T>::value;

##### Supported operations

Variable template | Value
Defined in header `<experimental/type_traits>`
template <class T, class... Args> constexpr bool is_constructible_v = | std::is_constructible<T, Args...>::value;
template <class T, class... Args> constexpr bool is_trivially_constructible_v = | std::is_trivially_constructible<T, Args...>::value;
template <class T, class... Args> constexpr bool is_nothrow_constructible_v = | std::is_nothrow_constructible<T, Args...>::value;
template <class T> constexpr bool is_default_constructible_v = | std::is_default_constructible<T>::value;
template <class T> constexpr bool is_trivially_default_constructible_v = | std::is_trivially_default_constructible<T>::value;
template <class T> constexpr bool is_nothrow_default_constructible_v = | std::is_nothrow_default_constructible<T>::value;
template <class T> constexpr bool is_copy_constructible_v = | std::is_copy_constructible<T>::value;
template <class T> constexpr bool is_trivially_copy_constructible_v = | std::is_trivially_copy_constructible<T>::value;
template <class T> constexpr bool is_nothrow_copy_constructible_v = | std::is_nothrow_copy_constructible<T>::value;
template <class T> constexpr bool is_move_constructible_v = | std::is_move_constructible<T>::value;
template <class T> constexpr bool is_trivially_move_constructible_v = | std::is_trivially_move_constructible<T>::value;
template <class T> constexpr bool is_nothrow_move_constructible_v = | std::is_nothrow_move_constructible<T>::value;
template <class T, class U> constexpr bool is_assignable_v = | std::is_assignable<T, U>::value;
template <class T, class U> constexpr bool is_trivially_assignable_v = | std::is_trivially_assignable<T, U>::value;
template <class T, class U> constexpr bool is_nothrow_assignable_v = | std::is_nothrow_assignable<T, U>::value;
template <class T> constexpr bool is_copy_assignable_v = | std::is_copy_assignable<T>::value;
template <class T> constexpr bool is_trivially_copy_assignable_v = | std::is_trivially_copy_assignable<T>::value;
template <class T> constexpr bool is_nothrow_copy_assignable_v = | std::is_nothrow_copy_assignable<T>::value;
template <class T> constexpr bool is_move_assignable_v = | std::is_move_assignable<T>::value;
template <class T> constexpr bool is_trivially_move_assignable_v = | std::is_trivially_move_assignable<T>::value;
template <class T> constexpr bool is_nothrow_move_assignable_v = | std::is_nothrow_move_assignable<T>::value;
template <class T> constexpr bool is_destructible_v = | std::is_destructible<T>::value;
template <class T> constexpr bool is_trivially_destructible_v = | std::is_trivially_destructible<T>::value;
template <class T> constexpr bool is_nothrow_destructible_v = | std::is_nothrow_destructible<T>::value;
template <class T> constexpr bool has_virtual_destructor_v = | std::has_virtual_destructor<T>::value;

##### Property queries

Variable template | Value
Defined in header `<experimental/type_traits>`
template <class T> constexpr std::size_t alignment_of_v = | std::alignment_of<T>::value;
template <class T> constexpr std::size_t rank_v = | std::rank<T>::value;
template <class T, unsigned I = 0> constexpr std::size_t extent_v = | std::extent<T, I>::value;

##### Type relationships

Variable template | Value
Defined in header `<experimental/type_traits>`
template <class T, class U> constexpr bool is_same_v = | std::is_same<T, U>::value;
template <class Base, class Derived> constexpr bool is_base_of_v = | std::is_base_of<Base, Derived>::value;
template <class From, class To> constexpr bool is_convertible_v = | std::is_convertible<From, To>::value;

### Other variable templates

##### std::ratio comparison

Variable template | Value
Defined in header `<experimental/ratio>`
template <class R1, class R2> constexpr bool ratio_equal_v = | std::ratio_equal<R1, R2>::value;
template <class R1, class R2> constexpr bool ratio_not_equal_v = | std::ratio_not_equal<R1, R2>::value;
template <class R1, class R2> constexpr bool ratio_less_v = | std::ratio_less<R1, R2>::value;
template <class R1, class R2> constexpr bool ratio_less_equal_v = | std::ratio_less_equal<R1, R2>::value;
template <class R1, class R2> constexpr bool ratio_greater_v = | std::ratio_greater<R1, R2>::value;
template <class R1, class R2> constexpr bool ratio_greater_equal_v = | std::ratio_greater_equal<R1, R2>::value;

##### Miscellaneous

Variable template | Value
Defined in header `<experimental/tuple>`
template <class T> constexpr std::size_t tuple_size_v = | std::tuple_size<T>::value;
Defined in header `<experimental/chrono>`
template <class Rep> constexpr bool treat_as_floating_point_v = | std::chrono::treat_as_floating_point<Rep>::value;[1]
Defined in header `<experimental/system_error>`
template <class T> constexpr bool is_error_code_enum_v = | std::is_error_code_enum<T>::value;
template <class T> constexpr bool is_error_condition_enum_v = | std::is_error_condition_enum<T>::value;
Defined in header `<experimental/functional>`
template <class T> constexpr bool is_bind_expression_v = | std::is_bind_expression<T>::value;
template <class T> constexpr bool is_placeholder_v = | std::is_placeholder<T>::value;
Defined in header `<experimental/memory>`
template <class T, class Alloc> constexpr bool uses_allocator_v = | std::uses_allocator<T, Alloc>::value;

### Notes

[1]: Defined in the `std::chrono::experimental` namespace.

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