std::ranges::search(3)

NAME
std::ranges::search - std::ranges::search

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::forward_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, (since
             class Proj2 = std::identity > (1) C++20)
   requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
   constexpr ranges::subrange<I1>
       search( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
               Proj1 proj1 = {}, Proj2 proj2 = {} );
   template< ranges::forward_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>, Pred, Proj1, C++20)
   Proj2>
   constexpr ranges::borrowed_subrange_t<R1>
       search( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2
   proj2 = {} );
   1) Searches for the first occurrence of the sequence of elements [first2, last2) in
   the range [first1, last1). Elements are compared using binary predicate pred after
   being projected with proj2 and proj1, respectively.
   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 needle)
   r1 - the range of elements to examine (aka haystack)
   r2 - the range of elements to search for (aka needle)
   pred - binary predicate to apply to the projected 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
   1) Returns a ranges::subrange value that is the first occurrence of the sequence
   [first2, last2) (aka needle) in the range [first1, last1) (aka haystack), after
   application of the projections proj1 and proj2 to the elements of both sequences
   respectively with consequencing application of the binary predicate pred to compare
   projected elements.
   If no such occurrence is found, ranges::subrange{last1, last1} is returned.
   If the range to search for (aka needle) is empty, that is first2 == last2, then the
   ranges::subrange{first1, first1} is returned.
   2) Same as (1) but the return type is ranges::borrowed_subrange_t<R1>.

Complexity
   At most S * N applications of the corresponding 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 search_fn {
    template<std::forward_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 ranges::subrange<I1>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (;; ++first1)
        {
            I1 it1 = first1;
            for (I2 it2 = first2;; ++it1, ++it2)
            {
                if (it2 == last2)
                    return {first1, it1};
                if (it1 == last1)
                    return {it1, it1};
                if (!std::invoke(pred, std::invoke(proj1, *it1), std::invoke(proj2, *it2)))
                    break;
            }
        }
    }
    template<ranges::forward_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_subrange_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 search_fn search {};

Example
// Run this code
 #include <algorithm>
 #include <cctype>
 #include <iostream>
 #include <iterator>
 #include <string_view>
 using namespace std::literals;
 void print(int id, const auto& haystack, const auto& needle, const auto& found)
 {
     std::cout << id << ") search(\"" << haystack << "\", \"" << needle << "\"); ";
     const auto first = std::distance(haystack.begin(), found.begin());
     const auto last = std::distance(haystack.begin(), found.end());
     if (found.empty())
         std::cout << "not found;";
     else
     {
         std::cout << "found: \"";
         for (const auto x : found)
             std::cout << x;
         std::cout << "\";";
     }
     std::cout << " subrange: {" << first << ", " << last << "}\n";
 }
 int main()
 {
     constexpr auto haystack {"abcd abcd"sv};
     constexpr auto needle {"bcd"sv};
     // the search uses iterator pairs begin()/end():
     constexpr auto found1 = std::ranges::search(
         haystack.begin(), haystack.end(),
         needle.begin(), needle.end());
     print(1, haystack, needle, found1);
     // the search uses ranges r1, r2:
     constexpr auto found2 = std::ranges::search(haystack, needle);
     print(2, haystack, needle, found2);
     // 'needle' range is empty:
     constexpr auto none {""sv};
     constexpr auto found3 = std::ranges::search(haystack, none);
     print(3, haystack, none, found3);
     // 'needle' will not be found:
     constexpr auto awl {"efg"sv};
     constexpr auto found4 = std::ranges::search(haystack, awl);
     print(4, haystack, awl, found4);
     // the search uses custom comparator and projections:
     constexpr auto bodkin {"234"sv};
     auto found5 = std::ranges::search(haystack, bodkin,
         [](const int x, const int y) { return x == y; }, // pred
         [](const int x) { return std::toupper(x); }, // proj1
         [](const int y) { return y + 'A' - '1'; }); // proj2
     print(5, haystack, bodkin, found5);
 }

Output:
 1) search("abcd abcd", "bcd"); found: "bcd"; subrange: {1, 4}
 2) search("abcd abcd", "bcd"); found: "bcd"; subrange: {1, 4}
 3) search("abcd abcd", ""); not found; subrange: {0, 0}
 4) search("abcd abcd", "efg"); not found; subrange: {9, 9}
 5) search("abcd abcd", "234"); found: "bcd"; subrange: {1, 4}

See also
   ranges::adjacent_find finds the first two adjacent items that are equal (or
   (C++20) satisfy 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::find_first_of searches for any one of a set of elements
   (C++20) (niebloid)
   ranges::contains
   ranges::contains_subrange checks if the range contains the given element or subrange
   (C++23) (niebloid)
   (C++23)
   ranges::includes returns true if one sequence is a subsequence of another
   (C++20) (niebloid)
   ranges::mismatch finds the first position where two ranges differ
   (C++20) (niebloid)
   ranges::search_n searches for a number consecutive copies of an element in
   (C++20) a range
                        (niebloid)
   search searches for a range of elements
                        (function template)