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std:: integer_sequence

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Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence
(C++14)
定义于头文件 <utility>
template < class T, T... Ints >
class integer_sequence ;
(C++14 起)

类模板 std::integer_sequence 表示一个编译期整数序列。当作为 函数模板 的参数使用时，可以推导出 参数包 Ints 并用于包展开。

目录

模板参数

T - 用于序列元素的整数类型
...Ints - 表示序列的常量参数包

成员类型

类型 定义
value_type T

成员函数

size
[static]
返回 Ints 中的元素数量
(公开静态成员函数)

std::integer_sequence:: size

static constexpr std:: size_t size ( ) noexcept ;

返回 Ints 中的元素数量。等价于 sizeof... ( Ints ) 。

返回值

Ints 中的元素数量。

辅助模板

为常见情况定义了一个辅助别名模板 std::index_sequence ，其中 T 为 std::size_t ：

template < std:: size_t ... Ints >
using index_sequence = std :: integer_sequence < std:: size_t , Ints... > ;

辅助别名模板 std::make_integer_sequence 和 std::make_index_sequence 分别用于简化创建以 ​ 0 ​ 、 1 、 2 、 ... 、 N - 1 作为 Ints 的 std::integer_sequence 和 std::index_sequence 类型：

template < class T, T N >
using make_integer_sequence = std :: integer_sequence < T, /* 序列 0, 1, 2, ..., N-1 */ > ;
template < std:: size_t N >
using make_index_sequence = std :: make_integer_sequence < std:: size_t , N > ;

如果 N 为负数，则程序非良构。若 N 为零，则所指示类型为 integer_sequence<T> 。

定义了一个辅助别名模板 std::index_sequence_for ，用于将任意类型参数包转换为相同长度的索引序列：

template < class ... T >
using index_sequence_for = std :: make_index_sequence < sizeof... ( T ) > ;

注释

特性测试 宏 值 标准 功能
__cpp_lib_integer_sequence 201304L (C++14) 编译时整数序列

可能的实现

make_integer_sequence 
namespace detail {
template<class T, T I, T N, T... integers>
struct make_integer_sequence_helper
{
    using type = typename make_integer_sequence_helper<T, I + 1, N, integers..., I>::type;
};
template<class T, T N, T... integers>
struct make_integer_sequence_helper<T, N, N, integers...>
{
    using type = std::integer_sequence<T, integers...>;
};
}
template<class T, T N>
using make_integer_sequence = detail::make_integer_sequence_helper<T, 0, N>::type;

示例

另请参阅 std::apply 的可能实现作为另一个示例。

运行此代码 
#include <array>
#include <cstddef>
#include <iostream>
#include <tuple>
#include <utility>
namespace details {
template <typename Array, std::size_t... I>
constexpr auto array_to_tuple_impl(const Array& a, std::index_sequence<I...>)
{
    return std::make_tuple(a[I]...);
}
template <class Ch, class Tr, class Tuple, std::size_t... Is>
void print_tuple_impl(std::basic_ostream<Ch, Tr>& os,
                      const Tuple& t,
                      std::index_sequence<Is...>)
{
    ((os << (Is ? ", " : "") << std::get<Is>(t)), ...);
}
}
template <typename T, T... ints>
void print_sequence(int id, std::integer_sequence<T, ints...> int_seq)
{
    std::cout << id << ") The sequence of size " << int_seq.size() << ": ";
    ((std::cout << ints << ' '), ...);
    std::cout << '\n';
}
template <typename T, std::size_t N, typename Indx = std::make_index_sequence<N>>
constexpr auto array_to_tuple(const std::array<T, N>& a)
{
    return details::array_to_tuple_impl(a, Indx{});
}
template <class Ch, class Tr, class... Args>
auto& operator<<(std::basic_ostream<Ch, Tr>& os, const std::tuple<Args...>& t)
{
    os << '(';
    details::print_tuple_impl(os, t, std::index_sequence_for<Args...>{});
    return os << ')';
}
int main()
{
    print_sequence(1, std::integer_sequence<unsigned, 9, 2, 5, 1, 9, 1, 6>{});
    print_sequence(2, std::make_integer_sequence<int, 12>{});
    print_sequence(3, std::make_index_sequence<10>{});
    print_sequence(4, std::index_sequence_for<std::ios, float, signed>{});
    constexpr std::array<int, 4> array{1, 2, 3, 4};
    auto tuple1 = array_to_tuple(array);
    static_assert(std::is_same_v<decltype(tuple1),
                                 std::tuple<int, int, int, int>>, "");
    std::cout << "5) tuple1: " << tuple1 << '\n';
    constexpr auto tuple2 = array_to_tuple<int, 4,
        std::integer_sequence<std::size_t, 1, 0, 3, 2>>(array);
    std::cout << "6) tuple2: " << tuple2 << '\n';
}

输出： 

1) The sequence of size 7: 9 2 5 1 9 1 6 
2) The sequence of size 12: 0 1 2 3 4 5 6 7 8 9 10 11 
3) The

参见

(C++20)
从内置数组创建 std::array 对象
(函数模板)
(C++11) (C++17)
具有指定值和指定类型的编译时常量
(类模板)