# std::unique

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt > ForwardIt unique( ForwardIt first, ForwardIt last ); |  | (until C++20)
template< class ForwardIt > constexpr ForwardIt unique( ForwardIt first, ForwardIt last ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt unique( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last ); | (2) | (since C++17)
| (3) |
template< class ForwardIt, class BinaryPredicate > ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p ); |  | (until C++20)
template< class ForwardIt, class BinaryPredicate > constexpr ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate > ForwardIt unique( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, BinaryPredicate p ); | (4) | (since C++17)

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.

1) Elements are compared using `operator==`. The behavior is undefined if it is not an equivalence relation.
3) Elements are compared using the given binary predicate p. The behavior is undefined if it is not an equivalence relation.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to process
policy | - | the execution policy to use. See execution policy for details.
p | - | binary predicate which returns ​true if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for `Type1` a move is equivalent to a copy (since C++11)). The types Type1 and Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- The type of dereferenced `ForwardIt` must meet the requirements of MoveAssignable.

### 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.

unique (1)
    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;
    }
unique (3)
    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

Run this code

    #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);
    }

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

### Defect reports

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

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 202 | C++98 | the behavior was unclear if the elements are compared using a non-equivalence relation | the behavior is undefined in this case

### See also

adjacent_find | finds the first two adjacent items that are equal (or satisfy a given predicate) (function template)
unique_copy | creates a copy of some range of elements that contains no consecutive duplicates (function template)
remove remove_if | removes elements satisfying specific criteria (function template)
unique | removes consecutive duplicate elements (public member function of `std::list<T,Allocator>`)
unique (C++11) | removes consecutive duplicate elements (public member function of `std::forward_list<T,Allocator>`)
ranges::unique (C++20) | removes consecutive duplicate elements in a range (niebloid)

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