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Difference between revisions of "cpp/types/rank"

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< cpp‎ | types
m (Text replace - "===Equivalent definition===" to "===Possible implementation===")
m (-`inline`: CWG2387; fmt.)
 
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{{cpp/title|rank}}
 
{{cpp/title|rank}}
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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|
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{{ddcl list header | type_traits}}
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{{ddcl list item | notes={{mark since c++11}} | 1=
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template< class T >
 
template< class T >
 
struct rank;
 
struct rank;
 
}}
 
}}
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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 dimensions of the array. For any other type, {{tt|value}} is 0.
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If {{tt|T}} is an array type, provides the member constant {{c|value}} equal to the number of dimensions of the array. For any other type, {{c|value}} is {{c|0}}.
  
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{{cpp/types/integral_constant/inherit2 |the number of dimensions of {{tt|T}} or zero|std::size_t}}
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{{cpp/types/nospec|v}}
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===Helper variable template===
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{{ddcl|since=c++17|1=
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template< class T >
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constexpr std::size_t rank_v = rank<T>::value;
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}}
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{{cpp/types/integral_constant/inherit2|the number of dimensions of {{tt|T}} or zero|std::size_t}}
  
 
===Possible implementation===
 
===Possible implementation===
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{{eq fun cpp
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{{eq fun
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| 1=
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|1=
 
template<class T>
 
template<class T>
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struct rank : std::integral_constant<std::size_t, 0> {};
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struct rank : public std::integral_constant<std::size_t, 0> {};
  
 
template<class T>
 
template<class T>
 
struct rank<T[]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};
 
struct rank<T[]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};
  
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template<class T, size_t N>
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template<class T, std::size_t N>
 
struct rank<T[N]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};
 
struct rank<T[N]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};
 
}}
 
}}
  
 
===Example===
 
===Example===
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{{example cpp
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{{example
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|
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|code=
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| code=
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#include <iostream>
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#include <type_traits>
 
#include <type_traits>
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 +
static_assert(std::rank<int>{} == 0);
 +
static_assert(std::rank<int[5]>{} == 1);
 +
static_assert(std::rank<int[5][5]>{} == 2);
 +
static_assert(std::rank<int[][5][5]>{} == 3);
  
 
int main()
 
int main()
 
{
 
{
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     std::cout << std::rank<int[1][2][3]>::value << '\n';
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     [[maybe_unused]] int ary[][3] = {<!---->{1, 2, 3}<!---->};
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     std::cout << std::rank<int[][2][3][4]>::value << '\n';
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     std::cout << std::rank<int>::value << '\n';
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     // The rank of reference type, e.g., ary[0], that is int(&)[3], is 0:
 +
    static_assert(std::rank_v<decltype(ary[0])> == 0);
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    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
 +
 
 +
     // The solution is to remove the reference type.
 +
    static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);
 
}
 
}
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| output=
 
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3
 
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4
 
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0
 
 
}}
 
}}
  
 
===See also===
 
===See also===
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{{dcl list begin}}
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{{dsc begin}}
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{{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 extent}}
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{{dsc inc|cpp/types/dsc extent}}
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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}}
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{{dcl list end}}
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{{dsc end}}
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 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 09:19, 24 September 2024

 
C++
 
Metaprogramming library
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(C++14)
 
Defined in header <type_traits>
template< class T >
struct rank;
 (since C++11)

If T is an array type, provides the member constant value equal to the number of dimensions of the array. For any other type, value is ​0​.

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

Contents

[edit] Helper variable template

template< class T >
constexpr std::size_t rank_v = rank<T>::value;
 (since C++17)

Inherited from std::integral_constant

Member constants

value
[static]
 the number of dimensions of T or zero
(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>
struct rank : public std::integral_constant<std::size_t, 0> {};
 
template<class T>
struct rank<T[]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};
 
template<class T, std::size_t N>
struct rank<T[N]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};

[edit] Example

Run this code
#include <type_traits>
 
static_assert(std::rank<int>{} == 0);
static_assert(std::rank<int[5]>{} == 1);
static_assert(std::rank<int[5][5]>{} == 2);
static_assert(std::rank<int[][5][5]>{} == 3);
 
int main()
{
    [[maybe_unused]] int ary[][3] = {{1, 2, 3}};
 
    // The rank of reference type, e.g., ary[0], that is int(&)[3], is 0:
    static_assert(std::rank_v<decltype(ary[0])> == 0);
    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
 
    // The solution is to remove the reference type.
    static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);
}

[edit] See also

(C++11)
 checks if a type is an array type
(class template) [edit]
(C++11)
 obtains the size of an array type along a specified dimension
(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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