std::extent

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

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 array of unknown bound along its first dimension and N is 0, value is 0.

目录

Inherited from std::integral_constant

Member constants

value
[静态的] </ SPAN></div></div>
the number of elements along the Nth dimension of T
（公共静态成员常量）

Member functions

operator std::size_t
转换的对象std::size_t，返回value
Original:
converts the object to std::size_t, returns value
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（公共成员函数）

Member types

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

</div>

[编辑] 可能的实现

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

[编辑] 为例

#include <iostream>
#include <type_traits>
 
int main()
{
    std::cout << std::extent<int[3]>::value << '\n';
    std::cout << std::extent<int[3][4]>::value << '\n';
    std::cout << std::extent<int[3][4],1>::value << '\n';
    std::cout << std::extent<int[3][4],2>::value << '\n';
    std::cout << std::extent<int[]>::value << '\n';
}

Output: 

3
3
4
0
0

[编辑] 另请参阅

（C++11）
检查如果一个类型是数组类型
Original:
checks if a type is an array type
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（类模板） [edit]
（C++11）
获得一个数组类型的尺寸
Original:
obtains the number of dimensions of an array type
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（类模板） [edit]
（C++11）
删除一个程度上给定的数组类型
Original:
removes one extent from the given array type
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（类模板） [edit]
（C++11）
 removes all extents from the given array type
（类模板） [edit]
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/types/extent&oldid=27621”