std::unique() algorithm

since C++20

// (1)
template< class ForwardIt >
constexpr ForwardIt unique( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class BinaryPredicate >
constexpr ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt unique( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate >
ForwardIt unique( ExecutionPolicy&& policy,
                  ForwardIt first, ForwardIt last, BinaryPredicate p );

since C++17

// (1)
template< class ForwardIt >
ForwardIt unique( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class BinaryPredicate >
ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt unique( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate >
ForwardIt unique( ExecutionPolicy&& policy,
                  ForwardIt first, ForwardIt last, BinaryPredicate p );

until C++17

// (1)
template< class ForwardIt >
ForwardIt unique( ForwardIt first, ForwardIt last );

// (3)
template< class ForwardIt, class BinaryPredicate >
ForwardIt unique( 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.

• (1) Elements are compared using operator==.

• (2) Elements are compared using the given binary predicate p.

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

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) is true.

important

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

Undefined Behaviour

The behavior is undefined

if it is not an equivalence relation.

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 function should be equivalent to the following: bool fun(const Type1& a, const Type2& b); The signature does not need to have const&. The function must not modify the objects passed to it. Must accept all values of type (possibly const) Type1 and Type2, regardless of value category (so 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 them.

Type requirements

ForwardIt	LegacyForwardIterator
Dereferenced ForwardIt	MoveAssignable

Return value

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

Complexity

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

At most N applications of 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

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 (2)

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

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 std::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.

Examples

The following code uniques all spaces from a string by shifting all non-space characters to the left and then erasing the extra. This is an example of Erase-remove idiom.

Main.cpp

#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