std::ranges::find_last_if() 算法

自 C++20 起

简化

// (1)
constexpr ranges::subrange<I>
    find_last_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
constexpr ranges::borrowed_subrange_t<R>
    find_last_if( R&& r, Pred pred, Proj proj = {} );

参数类型是泛型的，并具有以下约束

• I - std::input_iterator
• S - std::sentinel_for<I>
• Proj - (无)
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
• (2) - R - std::ranges::input_range

对于所有重载，Proj 模板参数的默认类型为 std::identity。

详细

// (1)
template<
  std::forward_iterator I,
  std::sentinel_for<I> S,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<I, Proj>> Pred
>
constexpr ranges::subrange<I>
    find_last_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
template<
  ranges::forward_range R,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred
>
constexpr ranges::borrowed_subrange_t<R>
    find_last_if( R&& r, Pred pred, Proj proj = {} );

返回范围 [first; last) 中满足特定条件的最后一个元素

• (1) 搜索谓词 pred 返回 true 的元素。

• (2) 与 (1) 相同，但使用 r 作为源范围，如同使用 ranges::begin(r) 作为 first 和 ranges::end(r) 作为 last。

本页描述的函数类实体是niebloids。

参数

first last	要检查的元素范围。
r	要检查的元素范围。
pred	应用于投影元素的谓词。
proj	应用于元素的投影。

返回值

• (1) 令 i 为范围 [first; last) 中最后一个满足 *i == value 为 true 的迭代器。返回一个类型为 ranges::subrange<I> 的值，初始化如下

  {
    i,
    last
  }

  如果没有找到这样的迭代器，则返回 { last, last }。

• (2) 与 (1) 相同，但返回类型为 ranges::borrowed_subrange_t<I>。

复杂度

谓词和投影最多应用 last - first 次。

异常

(无)

可能的实现

find_last_if(1)

struct find_last_if_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    constexpr ranges::subrange<I>
        operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        // Note: if I is mere forward_iterator, we may only go from begin to end.
        I found {};
        for (; first != last; ++first)
            if (std::invoke(pred, std::invoke(proj, *first)))
                found = first;

        if (found == I {})
            return {first, first};

        return {found, std::ranges::next(found, last)};
    }

    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
                 Pred>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return this->operator()(ranges::begin(r), ranges::end(r),
                                std::ref(pred), std::ref(proj));
    }
};

inline constexpr find_last_if_fn find_last_if;

示例

Main.cpp

#include <algorithm>
#include <forward_list>
#include <iomanip>
#include <iostream>
#include <string_view>

int main()
{
    constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};

    {
        constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), 3);
        constexpr auto i2 = std::ranges::find_last(v, 3);
        static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), -3);
        constexpr auto i2 = std::ranges::find_last(v, -3);
        static_assert(i1.begin() == v.end());
        static_assert(i2.begin() == v.end());
    }

    auto abs = [](int x) { return x < 0 ? -x : x; };

    {
        auto pred = [](int x) { return x == 3; };
        constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
        static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == -3; };
        constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
        static_assert(i1.begin() == v.end());
        static_assert(i2.begin() == v.end());
    }

    {
        auto pred = [](int x) { return x == 1 or x == 2; };
        constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
        static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == 1 or x == 2 or x == 3; };
        constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
        static_assert(i1.begin() == v.end());
        static_assert(i2.begin() == v.end());
    }

    using P = std::pair<std::string_view, int>;
    std::forward_list<P> list
    {
        {"one", 1}, {"two", 2}, {"three", 3},
        {"one", 4}, {"two", 5}, {"three", 6},
    };
    auto cmp_one = [](const std::string_view &s) { return s == "one"; };

    // find latest element that satisfy the comparator, and projecting pair::first
    const auto subrange = std::ranges::find_last_if(list, cmp_one, &P::first);

    // print the found element and the "tail" after it
    for (P const& e : subrange)
        std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
    std::cout << '\n';
}

输出

{"one", 4} {"two", 5} {"three", 6}