std::ranges::find_first_of() algorithm

od C++20

Simplified

// (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 = {});

The type of arguments are generic and have the following constraints:

• I1, I2 - std::input_iterator, std::forward_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• Pred - (none)
• Proj1, Proj2 - (none)
• R1, R2 - std::ranges::input_range, std::ranges::forward_range

The Proj1 and Proj2 template arguments have a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

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

(The std:: namespace was ommitted here for readability)

Detailed

// (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) 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.

Parameters

first1 last1	The range of elements to examine.
first2 last2	The range of elements to search for.
r1	The range of elements to examine.
r2	The range of elements to search for.
pred	Binary predicate to compare the elements with.
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

Given:

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

At most S * N applications of the predicate and each projection.

Exceptions

(none)

Possible implementation

ranges::find_first_of(1) and 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 {};

Examples

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";
}

Output

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