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Difference between revisions of "cpp/types/extent"

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< cpp‎ | types
m (fmt)
m (→‎Example: static_assert)
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{{cpp/title|extent}}
 
{{cpp/title|extent}}
 
{{cpp/meta/navbar}}
 
{{cpp/meta/navbar}}
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{{dcl begin}}
+
{{ddcl|header=type_traits|since=c++11|1=
−
{{dcl header|type_traits}}
+
−
{{dcl|since=c++11|1=
+
 
template< class T, unsigned N = 0 >
 
template< class T, unsigned N = 0 >
 
struct extent;
 
struct extent;
 
}}
 
}}
−
{{dcl end}}
 
  
−
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 {{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 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}}.
  
 
{{cpp/types/nospec|v}}
 
{{cpp/types/nospec|v}}
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#include <iostream>
 
#include <iostream>
 
#include <type_traits>
 
#include <type_traits>
−
+
 
 
int main()
 
int main()
 
{
 
{
−
     std::cout << std::extent<int[3]>::value << '\n'; //< default dimension is 0
+
     static_assert(
−
    std::cout << std::extent<int[3][4], 0>::value << '\n';
+
        std::extent_v<int[3]> == 3 && //< default dimension is 0
−
    std::cout << std::extent<int[3][4], 1>::value << '\n';
+
        std::extent_v<int[3], 0> == 3 && //< the same as above
−
    std::cout << std::extent<int[3][4], 2>::value << '\n';
+
        std::extent_v<int[3][4], 0> == 3 &&
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    std::cout << std::extent<int[]>::value << '\n';
+
        std::extent_v<int[3][4], 1> == 4 &&
−
+
        std::extent_v<int[3][4], 2> == 0 &&
−
     const auto ext = std::extent<int[9]>{};
+
        std::extent_v<int[]> == 0
 +
    );
 +
 
 +
     const auto ext = std::extent<int['*']>{};
 
     std::cout << ext << '\n'; //< implicit conversion to std::size_t
 
     std::cout << ext << '\n'; //< implicit conversion to std::size_t
−
+
 
−
     const int ints[] = {1, 2, 3, 4};
+
     const int ints[]{1, 2, 3, 4};
−
     std::cout << std::extent<decltype(ints)>::value << '\n'; //< array size  
+
     static_assert(std::extent_v<decltype(ints)> == 4); //< array size
  
 
     [[maybe_unused]] int ary[][3] = {<!---->{1, 2, 3}<!---->};
 
     [[maybe_unused]] int ary[][3] = {<!---->{1, 2, 3}<!---->};
−
   
+
 
−
     // ary[0] is type of reference of 'int[3]', so, extent  
+
     // ary[0] is type of reference of 'int[3]', so, extent
 
     // cannot calculate correctly and return 0
 
     // cannot calculate correctly and return 0
 
     static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
 
     static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
−
     std::cout << std::extent<decltype(ary[0])>::value << '\n';
+
     static_assert(std::extent_v<decltype(ary[0])> == 0);
 
     // removing reference will give correct extent value 3
 
     // removing reference will give correct extent value 3
−
     std::cout << std::extent<std::remove_cvref_t<decltype(ary[0])>>::value << '\n';
+
     static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[0])>> == 3);
 
}
 
}
 
|output=
 
|output=
−
3
+
42
−
3
+
−
4
+
−
0
+
−
0
+
−
9
+
−
4
+
−
0
+
−
3
+
 
}}
 
}}
  

Revision as of 15:42, 4 September 2023

 
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extent
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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

Helper variable template

template< class T, unsigned N = 0 >
inline 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 <iostream>
#include <type_traits>
 
int main()
{
    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
    );
 
    const auto ext = std::extent<int['*']>{};
    std::cout << ext << '\n'; //< 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 type of reference of 'int[3]', so, extent
    // cannot calculate correctly and return 0
    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
    static_assert(std::extent_v<decltype(ary[0])> == 0);
    // removing reference will give correct extent value 3
    static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[0])>> == 3);
}

Output: 

42

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]
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