std::unique

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

Removes all consecutive duplicate 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. Relative order of the elements that remain is preserved and the physical size of the container is unchanged. Iterators pointing to an element between the new logical end and the physical end of the range 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 method, which erases the unspecified values and reduces the physical size of the container to match its new logical size.

1) Elements are compared using operator==.

3) Elements are compared using the given binary predicate p.

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

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); The signature does not need to have const &, but the function must not modify the objects passed to it. 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 `ForwardIterator`.
- The type of dereferenced `ForwardIt` must meet the requirements of `MoveAssignable`.

[edit] Return value

Forward iterator to the new end of the range

[edit] Complexity

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

[edit] 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,

if policy is std::parallel_vector_execution_policy, std::terminate is called
if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
if policy is some other type, the behavior is implementation-defined

If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Possible implementation

First version

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;
}

Second version

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;
}

[edit] Example

Run this code

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <cctype>
 
int main() 
{
    // remove duplicate elements (normal use)
    std::vector<int> v{1,2,3,1,2,3,3,4,5,4,5,6,7};
    std::sort(v.begin(), v.end()); // 1 1 2 2 3 3 3 4 4 5 5 6 7 
    auto last = std::unique(v.begin(), v.end());
    // v now holds {1 2 3 4 5 6 7 x x x x x x}, where 'x' is indeterminate
    v.erase(last, v.end()); 
    for (int i : v)
      std::cout << i << " ";
    std::cout << "\n";
 
    // remove consecutive spaces
    std::string s = "wanna go    to      space?";
    auto end = std::unique(s.begin(), s.end(), [](char l, char r){
        return std::isspace(l) && std::isspace(r) && l == r;
    });
    // s now holds "wanna go to space?xxxxxxxx", where 'x' is indeterminate
    std::cout << std::string(s.begin(), end) << '\n';
}

Output:

1 2 3 4 5 6 7
wanna go to space?

[edit] 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`)
std::experimental::parallel::unique (parallelism TS)	parallelized version of `std::unique` (function template)

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