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

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< cpp‎ | algorithm‎ | ranges

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> ||
( 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 = {} );

(1)

(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 = {} );

(2)

(since C++20)

Helper types |  |
template< class I, class O > using unique_copy_result = ranges::in_out_result <I, O>; | (3) | (since 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 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.

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

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