# std::unique

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

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.
These overloads participate in overload resolution only if all following conditions are satisfied:

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)

### 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.

  - Each element of `[`result`,` last`)` has a valid but unspecified state, because move assignment can eliminate elements by moving from elements that were originally in that range.

(since C++11)

### Parameters

first, last | - | the range of elements to process
policy | - | the execution policy to use

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

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

1,2) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) comparisons using operator==.
3,4) Exactly \(\scriptsize max(0,N-1)\)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.

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;
    }

### 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

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)
removeremove_if | removes elements satisfying specific criteria (function template)
unique | removes consecutive duplicate elements (public member function of `std::list<T,Allocator>`)
unique | removes consecutive duplicate elements (public member function of `std::forward_list<T,Allocator>`)
ranges::unique (C++20) | removes consecutive duplicate elements in a range (algorithm function object)

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