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Difference between revisions of "cpp/types/rank"

From cppreference.com
< cpp‎ | types
m (→‎Example: ⇒ static_assert)
m (-`inline`: CWG2387; fmt.)
 
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{{cpp/title|rank}}
 
{{cpp/title|rank}}
 
{{cpp/meta/navbar}}
 
{{cpp/meta/navbar}}
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{{dcl begin}}
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{{ddcl|header=type_traits|since=c++11|
−
{{dcl header|type_traits}}
+
−
{{dcl|since=c++11|1=
+
 
template< class T >
 
template< class T >
 
struct rank;
 
struct rank;
 
}}
 
}}
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{{dcl end}}
 
  
−
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 {{c|0}}.
+
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}}.
  
 
{{cpp/types/nospec|v}}
 
{{cpp/types/nospec|v}}
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{{ddcl|since=c++17|1=
 
{{ddcl|since=c++17|1=
 
template< class T >
 
template< class T >
−
inline constexpr std::size_t rank_v = rank<T>::value;
+
constexpr std::size_t rank_v = rank<T>::value;
 
}}
 
}}
  
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|code=
 
|code=
 
#include <type_traits>
 
#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()
 
int main()
 
{
 
{
−
    static_assert(
 
−
            std::rank<int>{} == 0
 
−
        &&  std::rank<int[5]>{} == 1
 
−
        &&  std::rank<int[5][5]>{} == 2
 
−
        &&  std::rank<int[][5][5]>{} == 3 );
 
−
 
 
     [[maybe_unused]] int ary[][3] = {<!---->{1, 2, 3}<!---->};
 
     [[maybe_unused]] int ary[][3] = {<!---->{1, 2, 3}<!---->};
−
     // The reason of rank of "ary[0]" is calculated as 0
+
 
 +
     // 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::rank_v<decltype(ary[0])> == 0);
−
    // is that rank cannot deal with reference type. i.e. int(&)[3]
 
 
     static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
 
     static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
−
     // The solution is to remove reference type
+
 
 +
     // The solution is to remove the reference type.
 
     static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);
 
     static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);
 
}
 
}
−
 
 
}}
 
}}
  

Latest revision as of 09:19, 24 September 2024

 
C++
 
Metaprogramming library
Type traits
Type categories
(C++11)
(C++14)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
(C++17)
(C++23)
(C++11)
(C++11)
(C++11)
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
(C++11)
(C++11)
(C++20)
(C++20)
(C++23)
(C++17)
Type trait constants
(C++11)(C++17)(C++11)(C++11)
Metafunctions
(C++17)
(C++17)
(C++17)
Supported operations
Relationships and property queries
Type modifications
(C++11)(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)
(C++11)
(C++20)
(C++11)
(C++17)

(C++11)
(C++11)
(C++20)
(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 >
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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