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std::ranges::find_first_of - cppreference.com

Call signature

1) Searches the range [first1, last1) for any of the elements in the range [first2, last2), after projecting the ranges with proj1 and proj2 respectively. The projected elements are compared using the binary predicate pred.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Iterator to the first element in the range [first1, last1) that is equal to an element from the range [first2, last2) after projection. If no such element is found, an iterator comparing equal to last1 is returned.

At most S * N applications of the predicate and each projection, where
(1) S = ranges::distance(first2, last2) and N = ranges::distance(first1, last1);
(2) S = ranges::distance(r2) and N = ranges::distance(r1).

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 {};

#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";
}

Output:

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

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