# std::ranges::unique

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
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 = {} ); | (1) | (since C++20)
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 = {} ); | (2) | (since C++20)

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 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 range of elements to process
r | - | the range of elements to process
comp | - | the binary predicate to compare the projected elements
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.

### 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.

### Possible implementation

    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 {};

### Example

Run this code

    #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);
    }

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)

### See also

ranges::unique_copy (C++20) | creates a copy of some range of elements that contains no consecutive duplicates (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)
ranges::removeranges::remove_if (C++20)(C++20) | removes elements satisfying specific criteria (algorithm function object)
unique | removes consecutive duplicate elements in a range (function template)
unique | removes consecutive duplicate elements (public member function of `std::list<T,Allocator>`)
unique | removes consecutive duplicate elements (public member function of `std::forward_list<T,Allocator>`)

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