std::ranges::unique() algorithm

since C++20

Simplified

// (1)
constexpr ranges::subrange<I>
    unique( I first, S last, C comp = {}, Proj proj = {} );

// (2)
constexpr ranges::borrowed_subrange_t<R>
    unique( R&& r, C comp = {}, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::permutable
• S - std::sentinel_for<I>
• R - std::ranges::forward_range
• C:
  • (1) - std::indirect_equivalence_relation<std::projected<I, Proj>>
  • (2) - std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R>, Proj>>
• Proj - (none)

The Proj and C template argument have the following default types std::identity, ranges::equal_to for all overloads.

Additionally, each overload has the following constraints:

• (2) - std::permutable<ranges::iterator_t<R>>

Detailed

// (1)
template<
  std::permutable I,
  std::sentinel_for<I> S,
  class Proj = std::identity,
  std::indirect_equivalence_relation<std::projected<I, Proj>> C = ranges::equal_to
>
constexpr ranges::subrange<I>
    unique( I first, S last, C comp = {}, Proj proj = {} );

// (2)
template<
  ranges::forward_range R,
  class Proj = std::identity,
  std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R>, Proj>> C = ranges::equal_to >
  requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>
    unique( R&& r, C comp = {}, Proj proj = {} );

• (1) Eliminates all except the first element from every consecutive group of equivalent elements from the range [first; last) and returns a subrange [ret; last), where ret is a past-the-end iterator for the new end of the range.

  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.

Parameters

first last	The range of elements to process.
r	The range of elements to process.
comp	The binary predicate to compare the projected elements with.
proj	The projection to apply to the elements.

Return value

Returns

{
  ret,
  last
}

Where ret is a past-the-end iterator for the new end of the range.

Complexity

For nonempty ranges, exactly ranges::distance(first, last) - 1 applications of the corresponding predicate comp and no more that twice as many applications of any projection proj.

Exceptions

(none)

Possible implementation

unique(1) and unique(2)

struct unique_fn
{
    template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_equivalence_relation<std::projected<I, Proj>>
                 C = ranges::equal_to>
    constexpr ranges::subrange<I>
        operator()(I first, S last, C comp = {}, Proj proj = {}) const
    {
        first = ranges::adjacent_find(first, last, comp, proj);
        if (first == last)
            return {first, first};
        auto i {first};
        ++first;
        while (++first != last)
            if (!std::invoke(comp, std::invoke(proj, *i), std::invoke(proj, *first)))
                *++i = ranges::iter_move(first);
        return {++i, first};
    }

    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R>, Proj>>
                 C = ranges::equal_to>
    requires std::permutable<ranges::iterator_t<R>>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, C comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       std::move(comp), std::move(proj));
    }
};

inline constexpr unique_fn unique {};

Notes

Removing is done by shifting (by means of move assignment) the elements in the range in such a way that the elements that are not to be removed appear in the beginning of the range. Relative order of the elements that remain is preserved and the physical size of the container is unchanged.

Iterators in [ret; last) (if any) are still dereferenceable, but the elements themselves have unspecified values (as per MoveAssignable post-condition).

A call to ranges::unique is sometimes followed by a call to a container’s erase member function, which erases the unspecified values and reduces the physical size of the container to match its new logical size. These two invocations together model the Erase–remove idiom.

Examples

Main.cpp

#include <algorithm>
#include <cmath>
#include <complex>
#include <iostream>
#include <vector>

struct id {
    int i;
    explicit id(int i) : i {i} {}
};

void print(id i, const auto& v)
{
    std::cout << i.i << ") ";
    std::ranges::for_each(v, [](auto const& e) { std::cout << e << ' '; });
    std::cout << '\n';
}

int main()
{
    // a vector containing several duplicated elements
    std::vector<int> v {1, 2, 1, 1, 3, 3, 3, 4, 5, 4};

    print(id {1}, v);

    // remove consecutive (adjacent) duplicates
    const auto ret = std::ranges::unique(v);
    // v now holds {1 2 1 3 4 5 4 x x x}, where 'x' is indeterminate
    v.erase(ret.begin(), ret.end());
    print(id {2}, v);

    // sort followed by unique, to remove all duplicates
    std::ranges::sort(v); // {1 1 2 3 4 4 5}
    print(id {3}, v);

    const auto [first, last] = std::ranges::unique(v.begin(), v.end());
    // v now holds {1 2 3 4 5 x x}, where 'x' is indeterminate
    v.erase(first, last);
    print(id {4}, v);

    // unique with custom comparison and projection
    std::vector<std::complex<int>> vc { {1, 1}, {-1, 2}, {-2, 3}, {2, 4}, {-3, 5} };
    print(id {5}, vc);

    const auto ret2 = std::ranges::unique(vc,
        // consider two complex nums equal if their real parts are equal by module:
        [](int x, int y) { return std::abs(x) == std::abs(y); }, // comp
        [](std::complex<int> z) { return z.real(); }             // proj
    );
    vc.erase(ret2.begin(), ret2.end());
    print(id {6}, vc);
}

Possible Output

1) 1 2 1 1 3 3 3 4 5 4
2) 1 2 1 3 4 5 4
3) 1 1 2 3 4 4 5
4) 1 2 3 4 5
5) (1,1) (-1,2) (-2,3) (2,4) (-3,5)
6) (1,1) (-2,3) (-3,5)