std:: lexicographical_compare_three_way

使用三路比较按字典序比较两个范围 [ first1 , last1 ) 和 [ first2 , last2 ) ，并生成适用比较类别类型中最强类型的结果。

如果返回类型不是三种比较类别类型之一，则程序非良构：

• std::strong_ordering
• std::weak_ordering
• std::partial_ordering

参数

返回值

上述比较类别类型的值。

复杂度

给定 \(\scriptsize N_1\) N 1 为 std:: distance ( first1, last1 ) 且 \(\scriptsize N_2\) N 2 为 std:: distance ( first2, last2 ) ：

可能的实现

template<class I1, class I2, class Cmp>
constexpr auto lexicographical_compare_three_way(I1 f1, I1 l1, I2 f2, I2 l2, Cmp comp)
    -> decltype(comp(*f1, *f2))
{
    using ret_t = decltype(comp(*f1, *f2));
    static_assert(std::disjunction_v<
                      std::is_same<ret_t, std::strong_ordering>,
                      std::is_same<ret_t, std::weak_ordering>,
                      std::is_same<ret_t, std::partial_ordering>>,
                  "返回值类型必须是比较类别类型。");
    bool exhaust1 = (f1 == l1);
    bool exhaust2 = (f2 == l2);
    for (; !exhaust1 && !exhaust2; exhaust1 = (++f1 == l1), exhaust2 = (++f2 == l2))
        if (auto c = comp(*f1, *f2); c != 0)
            return c;
    return !exhaust1 ? std::strong_ordering::greater:
           !exhaust2 ? std::strong_ordering::less:
                       std::strong_ordering::equal;
}

示例

#include <algorithm>
#include <cctype>
#include <compare>
#include <iomanip>
#include <iostream>
#include <string_view>
#include <utility>
using namespace std::literals;
void show_result(std::string_view s1, std::string_view s2, std::strong_ordering o)
{
    std::cout << std::quoted(s1) << " 是 ";
    std::is_lt(o) ? std::cout << "小于 ":
    std::is_gt(o) ? std::cout << "大于 ":
                    std::cout << "等于 ";
    std::cout << std::quoted(s2) << '\n';
}
std::strong_ordering cmp_icase(unsigned char x, unsigned char y)
{
    return std::toupper(x) <=> std::toupper(y);
};
int main()
{
    for (const auto& [s1, s2] :
    {
        std::pair{"one"sv, "ONE"sv}, {"two"sv, "four"sv}, {"three"sv, "two"sv}
    })
    {
        const auto res = std::lexicographical_compare_three_way(
            s1.cbegin(), s1.cend(), s2.cbegin(), s2.cend(), cmp_icase);
        show_result(s1, s2, res);
    }
}

"one" 等于 "ONE"
"two" 大于 "four"
"three" 小于 "two"

缺陷报告

下列行为变更缺陷报告被追溯应用于先前发布的C++标准。

参见