std::ranges...copy_result(3)

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

okay bestie, welcome to the C++ Standard Library manual page (3) for std::ranges::unique_copy and std::ranges::unique_copy_result. It’s giving algorithm royalty, no cap. 💖

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🎀 NAME 🎀

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

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✨ Synopsis ✨

Defined in header <algorithm>

🌸 Call signature 🌸

(1) since C++20

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 unique_copy_result<I, O>
    unique_copy( I first, S last, O result, C comp = {}, Proj proj = {} );

(2) since C++20

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 unique_copy_result<ranges::borrowed_iterator_t<R>, O>
    unique_copy( R&& r, O result, C comp = {}, Proj proj = {} );

🧚 Helper types 🧚

(3) since C++20

template< class I, class O >
using unique_copy_result = ranges::in_out_result<I, O>;

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).

🚨 bestie, this is important: the ranges [first, last) and [result, result + N) must not overlap. N = ranges::distance(first, last). Do not play with that. 💀

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.

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💜 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

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🫶 Return value 🫶

{last, result + N}

that’s the whole return vibe, bestie ✨

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🔥 Complexity 🔥

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

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👑 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 {};

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🍓 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

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🌈 See also 🌈

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

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okay that’s the whole page, bestie 💅 source: http://cppreference.com, version/date: 2024.06.10, and the manual title on the tin is std::ranges...copy_result(3). mwah 🫶