std::unique

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

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

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

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 group of consecutive equivalent elements from the target range [first, last).

Removing is done by partitioning the elements in the target range. Given the partition point result, the leading elements of every group appear in [first, result), while other elements can only appear in [result, last).

• The underlying sequence of the target range is not shortened by the removing operation.
• Elements are shifted by copy assignment(until C++11)move assignment(since C++11).
• All iterators in [result, last) are still dereferenceable, and each element of [result, last) has a valid but unspecified state(since C++11).
• The removing operation is stable: the relative order of the elements not to be removed stays the same.

Parameters

Return value

The iterator result mentioned above.

Complexity

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

Exceptions

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.

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;
}

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