NAME¶

std::ranges::unique - std::ranges::unique

Synopsis¶

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>> (1) (since
C = ranges::equal_to > C++20)
constexpr ranges::subrange<I>

unique( I first, S last, C comp = {}, Proj proj = {} );
template< ranges::forward_range R, class Proj = std::identity,

std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R>,
Proj>> (2) (since
C = ranges::equal_to > C++20)
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, 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 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 creates a copy of some range of elements that contains no
(C++20) consecutive duplicates
(niebloid)
ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy
(C++20) a given predicate)
(niebloid)
ranges::remove
ranges::remove_if removes elements satisfying specific criteria
(C++20) (niebloid)
(C++20)
unique removes consecutive duplicate elements in a range
(function template)
unique removes consecutive duplicate elements
(public member function of std::list<T,Allocator>)
removes consecutive duplicate elements
unique (public member function of std::forward_list<T,Allocator>)