std::unique

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

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

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

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

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

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

Eliminates 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 logical end of the range.

Removing is done by shifting the elements in the range in such a way that elements to be erased are overwritten.

Parameters

Return value

A ForwardIt to the new end of the range.

Complexity

For nonempty ranges, exactly std::distance(first, last) - 1 applications of the corresponding predicate.

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.

Notes

Relative order of the elements that remain is preserved and the physical size of the container is unchanged. Iterators in [r, last) (if any), where r is the return value, are still dereferenceable, but the elements themselves have unspecified values. A call to unique is typically 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.

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