std::ranges::unique_copy,std::ranges::unique_copy_result(3)

NAME
std::ranges::unique_copy,std::ranges::unique_copy_result - std::ranges::unique_copy,std::ranges::unique_copy_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::sentinel_for<I> S,
   std::weakly_incrementable O,
             class Proj = std::identity,
             std::indirect_equivalence_relation<std::projected<I, Proj>> C =
   ranges::equal_to >
   requires std::indirectly_copyable<I, O> && (std::forward_iterator<I> || (since
                (std::input_iterator<O> && (1) C++20)
   std::same_as<std::iter_value_t<I>,
                    std::iter_value_t<O>>) ||
   std::indirectly_copyable_storable<I, O>)
   constexpr unique_copy_result<I, O>
       unique_copy( I first, S last, O result, C comp = {}, Proj proj = {}
   );
   template< ranges::input_range R, std::weakly_incrementable O, class Proj
   = std::identity,
   std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R>,
                 Proj>> C = ranges::equal_to >
   requires std::indirectly_copyable<ranges::iterator_t<R>, O> && (since
                (std::forward_iterator<ranges::iterator_t<R>> || (2) C++20)
                (std::input_iterator<O> &&
   std::same_as<ranges::range_value_t<R>,
                    std::iter_value_t<O>>) ||
                std::indirectly_copyable_storable<ranges::iterator_t<R>, O>)
   constexpr unique_copy_result<ranges::borrowed_iterator_t<R>, O>
       unique_copy( R&& r, O result, C comp = {}, Proj proj = {} );

Helper types
   template< class I, class O > (3) (since
   using unique_copy_result = ranges::in_out_result<I, O>; C++20)
   1) Copies the elements from the source range [first, last), to the destination range
   beginning at result in such a way that there are no consecutive equal elements. Only
   the first element of each group of equal elements is copied.
   The ranges [first, last) and [result, result + N) must not overlap. N =
   ranges::distance(first, last).
   Two consecutive elements *(i - 1) and *i are considered equivalent if
   std::invoke(comp, std::invoke(proj, *(i - 1)), std::invoke(proj, *i)) == true, where
   i is an iterator in the range [first + 1, last).
   2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and
   ranges::end(r) as last.
   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
   first, last - the source range of elements
   r - the source range of elements
   result - the destination range of elements
   comp - the binary predicate to compare the projected elements
   proj - the projection to apply to the elements

Return value
   {last, result + N}

Complexity
   Exactly N - 1 applications of the corresponding predicate comp and no more than
   twice as many applications of any projection proj.

Possible implementation
   See also the implementations in libstdc++ and MSVC STL (and third-party libraries:
   cmcstl2, NanoRange, and range-v3). struct unique_copy_fn {
    template<std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O,
             class Proj = std::identity,
             std::indirect_equivalence_relation<std::projected<I,
                 Proj>> C = ranges::equal_to>
    requires std::indirectly_copyable<I, O> && (std::forward_iterator<I> ||
                 (std::input_iterator<O> && std::same_as<std::iter_value_t<I>,
                     std::iter_value_t<O>>) || std::indirectly_copyable_storable<I, O>)
    constexpr ranges::unique_copy_result<I, O>
        operator()(I first, S last, O result, C comp = {}, Proj proj = {}) const
    {
        if (!(first == last))
        {
            std::iter_value_t<I> value = *first;
            *result = value;
            ++result;
            while (!(++first == last))
            {
                auto&& value2 = *first;
                if (!std::invoke(comp, std::invoke(proj, value2),
                        std::invoke(proj, value)))
                {
                    value = std::forward<decltype(value2)>(value2);
                    *result = value;
                    ++result;
                }
            }
        }
        return {std::move(first), std::move(result)};
    }
    template<ranges::input_range R, std::weakly_incrementable O, class Proj = std::identity,
             std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R>,
                 Proj>> C = ranges::equal_to>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
                 (std::forward_iterator<ranges::iterator_t<R>> ||
                 (std::input_iterator<O> && std::same_as<ranges::range_value_t<R>,
                     std::iter_value_t<O>>) ||
                 std::indirectly_copyable_storable<ranges::iterator_t<R>, O>)
    constexpr ranges::unique_copy_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, C comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                       std::move(comp), std::move(proj));
    } }; inline constexpr unique_copy_fn unique_copy {};

Example
// Run this code
 #include <algorithm>
 #include <cmath>
 #include <iostream>
 #include <iterator>
 #include <list>
 #include <string>
 #include <type_traits>
 void print(const auto& rem, const auto& v)
 {
     using V = std::remove_cvref_t<decltype(v)>;
     constexpr bool sep{std::is_same_v<typename V::value_type, int>};
     std::cout << rem << std::showpos;
     for (const auto& e : v)
         std::cout << e << (sep ? " " : "");
     std::cout << '\n';
 }
 int main()
 {
     std::string s1{"The string with many spaces!"};
     print("s1: ", s1);
     std::string s2;
     std::ranges::unique_copy(
         s1.begin(), s1.end(), std::back_inserter(s2),
         [](char c1, char c2) { return c1 == ' ' && c2 == ' '; }
     );
     print("s2: ", s2);
     const auto v1 = {-1, +1, +2, -2, -3, +3, -3};
     print("v1: ", v1);
     std::list<int> v2;
     std::ranges::unique_copy(
         v1, std::back_inserter(v2),
         {}, // default comparator std::ranges::equal_to
         [](int x) { return std::abs(x); } // projection
     );
     print("v2: ", v2);
 }

Output:
 s1: The string with many spaces!
 s2: The string with many spaces!
 v1: -1 +1 +2 -2 -3 +3 -3
 v2: -1 +2 -3

See also
   ranges::unique removes consecutive duplicate elements in a range
   (C++20) (niebloid)
   ranges::copy
   ranges::copy_if copies a range of elements to a new location
   (C++20) (niebloid)
   (C++20)
   ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy
   (C++20) a given predicate)
                        (niebloid)
                        creates a copy of some range of elements that contains no
   unique_copy consecutive duplicates
                        (function template)