std::unique - cppreference.com

template< class ForwardIt >
ForwardIt unique( ForwardIt first, ForwardIt last );

template< class ExecutionPolicy, class ForwardIt >
ForwardIt unique( ExecutionPolicy&& policy,
                  ForwardIt first, ForwardIt last );

template< class ForwardIt, class BinaryPred >
ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPred p );

template< class ExecutionPolicy,
          class ForwardIt, class BinaryPred >
ForwardIt unique( ExecutionPolicy&& policy,
                  ForwardIt first, ForwardIt last, BinaryPred p );

Removes all except the first element from every consecutive group of equivalent elements from the range [first, last) and returns a past-the-end iterator for the new end of the range.

1) Elements are compared using operator==.

If operator== does not establish an equivalence relation, the behavior is undefined.

3) Elements are compared using the given binary predicate p.

If p does not establish an equivalence relation, the behavior is undefined.

2,4) Same as (1,3), but executed according to policy.

Explanation

Removing is done by shifting 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.

• Shifting is done by copy assignment(until C++11)move assignment(since C++11).
• The removing operation is stable: the relative order of the elements not to be removed stays the same.
• The underlying sequence of [first, last) is not shortened by the removing operation. Given result as the returned iterator:

• All iterators in [result, last) are still dereferenceable.

Parameters

Return value

A ForwardIt to the new end of the range.

Complexity

Given N as std::distance(first, last):

1,2) Exactly max(0,N-1) comparisons using operator==.

3,4) Exactly max(0,N-1) applications of the predicate p.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

template<class ForwardIt>
ForwardIt unique(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return last;
    
    ForwardIt result = first;
    while (++first != last)
        if (!(*result == *first) && ++result != first)
            *result = std::move(*first);
    
    return ++result;
}

template<class ForwardIt, class BinaryPredicate>
ForwardIt unique(ForwardIt first, ForwardIt last, BinaryPredicate p)
{
    if (first == last)
        return last;
    
    ForwardIt result = first;
    while (++first != last)
        if (!p(*result, *first) && ++result != first)
            *result = std::move(*first);
    
    return ++result;
}

Notes

A call to unique is typically followed by a call to a container's erase member function to actually remove elements from the container.

Example

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    // a vector containing several duplicate elements
    std::vector<int> v{1, 2, 1, 1, 3, 3, 3, 4, 5, 4};
    auto print = [&](int id)
    {
        std::cout << "@" << id << ": ";
        for (int i : v)
            std::cout << i << ' ';
        std::cout << '\n';
    };
    print(1);

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

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

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

@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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also