std::extent

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定义于头文件 <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

继承自 std::integral_constant

成员常量

value
[静态]
the number of elements along the Nth dimension of T
(公开静态成员常量)

成员函数

operator std::size_t
转换对象为std::size_t,返回value
(公开成员函数)
operator()
(C++14)
返回value
(公开成员函数)

成员类型

类型 定义
value_type std::size_t
type std::integral_constant<std::size_t, value>

[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] 示例

#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';
}

输出:

3
3
4
0
0

[编辑] 另请参阅

(C++11)
检查类型是否是数组类型
(类模板) [edit]
(C++11)
获取数组类型的维度
(类模板) [edit]
删除数组类型的一个给定维度
(类模板) [edit]
移除数组类型所有维度
(类模板) [edit]