std::extent

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Metaprogramming library

Type traits
Type categories is_void (C++11) is_null_pointer (C++14)   is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11) is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11)   is_fundamental (C++11) is_arithmetic (C++11) is_reference (C++11) is_lvalue_reference (C++11) is_rvalue_reference (C++11) is_member_pointer (C++11) is_member_object_pointer (C++11) is_member_function_pointer (C++11) Type properties is_const (C++11) is_volatile (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11) is_aggregate (C++17) is_implicit_lifetime (C++23) is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(until C++20*) is_pod (C++11)(deprecated in C++20) is_signed (C++11) is_unsigned (C++11) is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_representations (C++17) Type trait constants integral_constantbool_constanttrue_typefalse_type (C++11)(C++17)(C++11)(C++11) Metafunctions conjunction (C++17) disjunction (C++17) negation (C++17)	Supported operations is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11) is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11) is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11) is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11) has_virtual_destructor (C++11) is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) Relationships and property queries is_same (C++11) is_convertibleis_nothrow_convertible (C++11)(C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20)   is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) reference_converts_from_temporary (C++23) is_base_of (C++11) is_virtual_base_of (C++26) alignment_of (C++11) rank (C++11) extent (C++11) is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) Type modifications remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11) remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11) Type transformations aligned_storage (C++11)(deprecated in C++23) aligned_union (C++11)(deprecated in C++23) decay (C++11) remove_cvref (C++20) enable_if (C++11) void_t (C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) result_ofinvoke_result (C++11)(until C++20*)(C++17) type_identity (C++20)
Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence (C++14)

 

Defined in header <type_traits>
template< class T, unsigned N = 0 > struct extent;		(since C++11)

If T is an array type, provides the member constant value equal to the number of elements along the N^th dimension of the array, if N is in [​0​, std::rank<T>::value). For any other type, or if T is an array of unknown bound along its first dimension and N is ​0​, value is ​0​.

If the program adds specializations for std::extent or std::extent_v(since C++17), the behavior is undefined.

Helper variable template

template< class T, unsigned N = 0 > constexpr std::size_t extent_v = extent<T, N>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

the number of elements along the Nth dimension of T (public static member constant)

Member functions

operator std::size_t	converts the object to std::size_t, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	std::size_t
type	std::integral_constant<std::size_t, value>

Possible implementation

template<class T, unsigned N = 0> struct extent : std::integral_constant<std::size_t, 0> {};   template<class T> struct extent<T[], 0> : std::integral_constant<std::size_t, 0> {};   template<class T, unsigned N> struct extent<T[], N> : std::extent<T, N - 1> {};   template<class T, std::size_t I> struct extent<T[I], 0> : std::integral_constant<std::size_t, I> {};   template<class T, std::size_t I, unsigned N> struct extent<T[I], N> : std::extent<T, N - 1> {};

Example

Run this code

#include <type_traits>
 
static_assert(
    std::extent_v<int[3]> == 3 && // default dimension is 0
    std::extent_v<int[3], 0> == 3 && // the same as above
    std::extent_v<int[3][4], 0> == 3 &&
    std::extent_v<int[3][4], 1> == 4 &&
    std::extent_v<int[3][4], 2> == 0 &&
    std::extent_v<int[]> == 0
);
 
int main()
{
    const auto ext = std::extent<int['*']>{};
    static_assert(ext == 42); // with implicit conversion to std::size_t
 
    const int ints[]{1, 2, 3, 4};
    static_assert(std::extent_v<decltype(ints)> == 4); // array size
 
    [[maybe_unused]] int ary[][3] = {{1, 2, 3}};
 
    // ary[0] is of type reference to 'int[3]', so, the extent
    // cannot be calculated correctly and it returns 0
    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
    static_assert(std::extent_v<decltype(ary[0])> == 0);
 
    // removing reference gives correct extent value 3
    static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[0])>> == 3);
}

See also

is_array (C++11)	checks if a type is an array type (class template)
rank (C++11)	obtains the number of dimensions of an array type (class template)
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)
extents (C++23)	a descriptor of a multidimensional index space of some rank (class template)