std::ranges::unique_copy, std::ranges::unique_copy_result

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 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.

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 {};

[edit] 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

[edit] See also

ranges::unique

(C++20)

removes consecutive duplicate elements in a range
(algorithm function object)[edit]

ranges::copyranges::copy_if

(C++20)(C++20)

copies a range of elements to a new location
(algorithm function object)[edit]

ranges::adjacent_find

(C++20)

finds the first two adjacent items that are equal (or satisfy a given predicate)
(algorithm function object)[edit]

unique_copy

creates a copy of some range of elements that contains no consecutive duplicates
(function template) [edit]

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std::ranges::unique_copy, std::ranges::unique_copy_result

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