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

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 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

#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

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

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