std::ranges::find_first_of() 算法

自 C++20 起

简化

// (1)
constexpr I1 find_first_of( I1 first1, S1 last1, I2 first2, S2 last2,
                            Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R1>
    find_first_of(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

参数类型是泛型的，并具有以下约束

• I1, I2 - std::input_iterator, std::forward_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• Pred - (无)
• Proj1, Proj2 - (无)
• R1, R2 - std::ranges::input_range, std::ranges::forward_range

Proj1 和 Proj2 模板参数对于所有重载都具有默认类型 std::identity。

此外，每个重载都有以下约束

• (1) - indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
• (2) - indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>

（为方便阅读，此处省略了 std:: 命名空间）

详细

// (1)
template<
  std::input_iterator I1,
  std::sentinel_for<I1> S1,
  std::forward_iterator I2,
  std::sentinel_for<I2> S2,
  class Pred = ranges::equal_to,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr I1 find_first_of( I1 first1, S1 last1, I2 first2, S2 last2,
                            Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
template<
  ranges::input_range R1,
  ranges::forward_range R2,
  class Pred = ranges::equal_to,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::indirectly_comparable< ranges::iterator_t<R1>,
                                       ranges::iterator_t<R2>,
                                       Pred, Proj1, Proj2 >
constexpr ranges::borrowed_iterator_t<R1>
    find_first_of(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

• (1) 在投影范围 proj1 和 proj2 后，在范围 [first1; last1) 中搜索范围 [first2; last2) 中的任何元素。投影后的元素使用二元谓词 pred 进行比较。

• (2) 与 (1) 相同，但使用 r1 作为第一个源范围，r2 作为第二个源范围，如同使用 ranges::begin(r1) 作为 first1，ranges::end(r1) 作为 last1，ranges::begin(r2) 作为 first2 和 ranges::end(r2) 作为 last2。

本页描述的函数类实体是niebloids。

参数

first1 last1	要检查的元素范围。
first2 last2	要搜索的元素范围。
r1	要检查的元素范围。
r2	要搜索的元素范围。
pred	用于比较元素的二元谓词。
proj1	应用于第一个范围中元素的投影。
proj2	应用于第二个范围中元素的投影。

返回值

在投影后，范围 [first1; last1) 中等于范围 [first2; last2) 中元素的第一个元素的迭代器。如果未找到此类元素，则返回与 last1 相等的迭代器。

复杂度

给定

• (1) S 为 ranges::distance(first2, last2)，N 为 ranges::distance(first1, last1)
• (2) S 为 ranges::distance(r2)，N 为 ranges::distance(r1)

谓词和每个投影最多应用 S * N 次。

异常

(无)

可能的实现

ranges::find_first_of(1) 和 ranges::find_first_of(2)

struct find_first_of_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::forward_iterator I2, std::sentinel_for<I2> S2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
    constexpr I1 operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                            Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; first1 != last1; ++first1)
            for (auto i = first2; i != last2; ++i)
                if (std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *i)))
                    return first1;
        return first1;
    }

    template<ranges::input_range R1, ranges::forward_range R2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R1>,
                                        ranges::iterator_t<R2>,
                                        Pred, Proj1, Proj2>
    constexpr ranges::borrowed_iterator_t<R1>
        operator()(R1&& r1, R2&& r2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(pred), std::move(proj1), std::move(proj2));
    }
;

inline constexpr find_first_of_fn find_first_of {};

示例

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>

int main()
{
    namespace rng = std::ranges;

    constexpr static auto haystack = {1, 2, 3, 4};
    constexpr static auto needles  = {0, 3, 4, 3};

    constexpr auto found1 = rng::find_first_of(haystack.begin(), haystack.end(),
                                               needles.begin(), needles.end());
    static_assert(std::distance(haystack.begin(), found1) == 2);

    constexpr auto found2 = rng::find_first_of(haystack, needles);
    static_assert(std::distance(haystack.begin(), found2) == 2);

    constexpr static auto negatives = {-6, -3, -4, -3};
    constexpr auto not_found = rng::find_first_of(haystack, negatives);
    static_assert(not_found == haystack.end());

    constexpr auto found3 = rng::find_first_of(haystack, negatives,
        [](int x, int y) { return x == -y; }); // uses a binary comparator
    static_assert(std::distance(haystack.begin(), found3) == 2);

    struct P { int x, y; };
    constexpr static auto p1 = { P{1, -1}, P{2, -2}, P{3, -3}, P{4, -4} };
    constexpr static auto p2 = { P{5, -5}, P{6, -3}, P{7, -5}, P{8, -3} };

    // Compare only P::y data members by projecting them:
    const auto found4 = rng::find_first_of(p1, p2, {}, &P::y, &P::y);
    std::cout << "First equivalent element {" << found4->x << ", " << found4->y
              << "} was found at position " << std::distance(p1.begin(), found4)
              << ".\n";
}

输出

First equivalent element {3, -3} was found at position 2.