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Difference between revisions of "cpp/types/extent"

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{{cpp/title|extent}}
 
{{cpp/title|extent}}
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{{cpp/types/sidebar}}
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{{cpp/meta/navbar}}
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{{ddcl list begin}}
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{{ddcl|header=type_traits|since=c++11|1=
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{{ddcl list header | type_traits}}
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template< class T, unsigned N = 0 >
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{{ddcl list item | notes={{mark c++11 feature}} | 1=
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template< class T, unsigned N = 0>
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struct extent;
 
struct extent;
 
}}
 
}}
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{{ddcl list end}}
 
  
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If {{tt|T}} is an array type, provides the member constant {{tt|value}} equal to the number of elements along the {{tt|N}}th dimension of the array, if {{tt|N}} is in {{tt|[0, std::rank<T>::value)}}. For any other type, or if {{tt|T}} is array of unknown bound along its first dimension and {{tt|N}} is 0, {{tt|value}} is 0.
+
If {{tt|T}} is an array type, provides the member constant {{tt|value}} equal to the number of elements along the {{petty|{{tt|N}}{{sup|th}}}} dimension of the array, if {{tt|N}} is in {{range|0|std::rank<T>::value}}. For any other type, or if {{tt|T}} is an array of unknown bound along its first dimension and {{tt|N}} is {{c|0}}, {{tt|value}} is {{c|0}}.
  
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===Member objects===
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{{cpp/types/nospec|v}}
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{{tdcl list begin}}
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{{tdcl list hitem | Name | Definition}}
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{{tdcl list item | {{tt|value}} | the number of elements along the {{tt|N}}th dimension of {{tt|T}}}}
+
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{{tdcl list end}}
+
  
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===Equivalent definition===
+
===Helper variable template===
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{{eq fun cpp
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{{ddcl|since=c++17|1=
−
| 1=
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template< class T, unsigned N = 0 >
 
template< class T, unsigned N = 0 >
 +
constexpr std::size_t extent_v = extent<T, N>::value;
 +
}}
 +
 +
{{cpp/types/integral_constant/inherit2|the number of elements along the {{tt|N}}th dimension of {{tt|T}}|std::size_t}}
 +
 +
===Possible implementation===
 +
{{eq fun
 +
|1=
 +
template<class T, unsigned N = 0>
 
struct extent : std::integral_constant<std::size_t, 0> {};
 
struct extent : std::integral_constant<std::size_t, 0> {};
  
−
template< class T >
+
template<class T>
 
struct extent<T[], 0> : std::integral_constant<std::size_t, 0> {};
 
struct extent<T[], 0> : std::integral_constant<std::size_t, 0> {};
  
−
template< class T, unsigned N >
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template<class T, unsigned N>
−
struct extent<T[], N> : std::integral_constant<std::size_t, std::extent<T, N-1>::value> {};
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struct extent<T[], N> : std::extent<T, N - 1> {};
  
−
template< class T, std::size_t N >
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template<class T, std::size_t I>
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struct extent<T[N], 0> : std::integral_constant<std::size_t, N> {};
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struct extent<T[I], 0> : std::integral_constant<std::size_t, I> {};
  
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template <class T, std::size_t I, unsigned N >
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template<class T, std::size_t I, unsigned N>
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struct extent<T[I], N> : std::integral_constant<std::size_t, std::extent<T, N-1>::value> {};
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struct extent<T[I], N> : std::extent<T, N - 1> {};
 
}}
 
}}
  
 
===Example===
 
===Example===
−
{{example cpp
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{{example
−
|
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|code=
−
| code=
+
−
#include <iostream>
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#include <type_traits>
 
#include <type_traits>
  
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template<typename A>
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static_assert(
−
typename std::enable_if< std::rank<A>::value == 1 >::type print_1d(const A& a)
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    std::extent_v<int[3]> == 3 && // default dimension is 0
−
{
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    std::extent_v<int[3], 0> == 3 && // the same as above
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     for(std::size_t i = 0; i<std::extent<A>::value; ++i)
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     std::extent_v<int[3][4], 0> == 3 &&
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        std::cout << a[i] << ' ';
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    std::extent_v<int[3][4], 1> == 4 &&
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     std::cout << '\n';
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    std::extent_v<int[3][4], 2> == 0 &&
−
}
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     std::extent_v<int[]> == 0
 +
);
  
 
int main()
 
int main()
 
{
 
{
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     int a[][3] = {{1,2,3},{4,5,6}};
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     const auto ext = std::extent<int['*']>{};
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// print_1d(a); // compile-time error
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    static_assert(ext == 42); // with implicit conversion to std::size_t
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     print_1d(a[1]);
+
 
−
     int* p = a[1];
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    const int ints[]{1, 2, 3, 4};
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// print_1d(p); // compile-time error
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    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);
 
}
 
}
−
| output=
 
−
4 5 6
 
 
}}
 
}}
  
 
===See also===
 
===See also===
−
{{dcl list begin}}
+
{{dsc begin}}
−
{{dcl list template | cpp/types/dcl list is_array}}
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{{dsc inc|cpp/types/dsc is_array}}
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{{dcl list template | cpp/types/dcl list rank}}
+
{{dsc inc|cpp/types/dsc rank}}
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{{dcl list template | cpp/types/dcl list remove_extent}}
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{{dsc inc|cpp/types/dsc remove_extent}}
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{{dcl list template | cpp/types/dcl list remove_all_extents}}
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{{dsc inc|cpp/types/dsc remove_all_extents}}
−
{{dcl list end}}
+
{{dsc inc|cpp/container/mdspan/dsc extents}}
 +
{{dsc end}}
 +
 
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 05:15, 24 September 2024

 
C++
 
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Relationships and property queries
(C++11)
(C++26)
(C++11)
(C++11)
extent
(C++11)
(C++17)(C++17)(C++17)(C++17)
Type modifications
(C++11)(C++11)(C++11)
(C++11)(C++11)(C++11)
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(C++11)(deprecated in C++23)
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(C++11)
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(C++11)
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(C++11)
(C++11)(until C++20*)(C++17)
(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
(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 Nth 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.

Contents

[edit] 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>

[edit] 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> {};

[edit] 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);
}

[edit] See also

(C++11)
 checks if a type is an array type
(class template) [edit]
(C++11)
 obtains the number of dimensions of an array type
(class template) [edit]
(C++11)
 removes one extent from the given array type
(class template) [edit]
(C++11)
 removes all extents from the given array type
(class template) [edit]
(C++23)
 a descriptor of a multidimensional index space of some rank
(class template) [edit]
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