std::ranges::min_element

template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
          std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less >
constexpr I
    min_element( I first, S last, Comp comp = {}, Proj proj = {} );

template< ranges::forward_range R, class Proj = std::identity,
          std::indirect_strict_weak_order<
              std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
constexpr ranges::borrowed_iterator_t<R>
    min_element( R&& r, Comp comp = {}, Proj proj = {} );

此页面上描述的函数式实体是 算法函数对象（非正式称为 niebloid），即：

• 在调用它们中的任何一个时不能指定显式模板实参列表。
• 它们中的任何一个都对 实参依赖查找不可见。
• 当其中任何一个通过 普通无限定查找 被找到，作为函数调用运算符左侧的名称时，实参依赖查找 被抑制。

参数

返回值

指向范围 [ first , last ) 中最小元素的迭代器。若范围内存在多个等价的最小元素，则返回首个此类元素的迭代器。若范围为空（即 first == last ），则返回与 last 相等的迭代器。

复杂度

恰好需要 max(N - 1, 0) 次比较，其中 N = ranges:: distance ( first, last ) 。

可能的实现

struct min_element_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
    constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == last)
            return last;
        auto smallest = first;
        while (++first != last)
            if (std::invoke(comp, std::invoke(proj, *first), std::invoke(proj, *smallest)))
                smallest = first;
        return smallest;
    }
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
    }
};
inline constexpr min_element_fn min_element;

示例

#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>
int main()
{
    namespace ranges = std::ranges;
    std::array v{3, 1, -13, 1, 3, 7, -13};
    auto iterator = ranges::min_element(v.begin(), v.end());
    auto position = ranges::distance(v.begin(), iterator);
    std::cout << "min element is v[" << position << "] == " << *iterator << '\n';
    auto abs_compare = [](int a, int b) { return (std::abs(a) < std::abs(b)); };
    iterator = ranges::min_element(v, abs_compare);
    position = ranges::distance(v.begin(), iterator);
    std::cout << "|min| element is v[" << position << "] == " << *iterator << '\n';
}

min element is v[2] == -13
|min| element is v[1] == 1

参见