std::ranges::find_first_of(3)

NAME
std::ranges::find_first_of - std::ranges::find_first_of

Synopsis
   Defined in header <algorithm>
   Call signature
   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 > (1) (since
   requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2> C++20)
   constexpr I1
       find_first_of( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred =
   {},
                      Proj1 proj1 = {}, Proj2 proj2 = {} );
   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>, (2) (since
                        ranges::iterator_t<R2>, C++20)
                        Pred, Proj1, Proj2>
   constexpr ranges::borrowed_iterator_t<R1>
       find_first_of( R1&& r1, R2&& r2, Pred pred = {},
                      Proj1 proj1 = {}, Proj2 proj2 = {} );
   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.
   2) Same as (1), but uses r1 as the first source range and r2 as the second source
   range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1,
   ranges::begin(r2) as first2, and ranges::end(r2) as last2.
   The function-like entities described on this page are 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.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first1, last1 - the range of elements to examine (aka haystack)
   first2, last2 - the range of elements to search for (aka needles)
   r1 - the range of elements to examine (aka haystack)
   r2 - the range of elements to search for (aka needles)
   pred - binary predicate to compare the elements
   proj1 - projection to apply to the elements in the first range
   proj2 - projection to apply to the elements in the second range

Return value
   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.

Complexity
   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).

Possible implementation
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 {};

Example
// Run this code
 #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.

See also
   find_first_of searches for any one of a set of elements
                        (function template)
   ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy
   (C++20) a given predicate)
                        (niebloid)
   ranges::find
   ranges::find_if
   ranges::find_if_not finds the first element satisfying specific criteria
   (C++20) (niebloid)
   (C++20)
   (C++20)
   ranges::find_end finds the last sequence of elements in a certain range
   (C++20) (niebloid)
   ranges::search searches for a range of elements
   (C++20) (niebloid)
   ranges::search_n searches for a number consecutive copies of an element in a
   (C++20) range
                        (niebloid)