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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/ranges/search.html below:

std::ranges::search - cppreference.com

Call signature

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.

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.

Returns a

value that is the first occurrence of the sequence

(aka

) in the range

(aka

), after application of the projections

and

to the elements of both sequences respectively with consequencing application of the binary predicate

to compare projected elements.

If no such occurrence is found, ranges::subrange{last1, last1} is returned.

If the range to search for (aka

) is empty, that is

, then the

is returned.

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).

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 {};

#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}

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