std::unique(3)

NAME
std::unique - std::unique

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt > (1) (constexpr since C++20)
   ForwardIt unique( ForwardIt first, ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt >
   ForwardIt unique( ExecutionPolicy&& policy, (2) (since C++17)
                     ForwardIt first, ForwardIt last );
   template< class ForwardIt, class BinaryPred >
   ForwardIt unique( ForwardIt first, ForwardIt last, (3) (constexpr since C++20)
   BinaryPred p );
   template< class ExecutionPolicy,
             class ForwardIt, class BinaryPred >
   ForwardIt unique( ExecutionPolicy&& policy, (4) (since C++17)
                     ForwardIt first, ForwardIt last,
   BinaryPred p );
   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
   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 (since C++11)
       elements that were originally in that range.

Parameters
   first, last - the range of elements to process
   policy - the execution policy to use. See execution policy for details.
                 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);
   p - 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
                      the behavior was unclear if the elements the behavior is
   LWG 202 C++98 are undefined in this case
                      compared using a non-equivalence relation

See also
                  finds the first two adjacent items that are equal (or satisfy a given
   adjacent_find predicate)
                  (function template)
                  creates a copy of some range of elements that contains no consecutive
   unique_copy duplicates
                  (function template)
   remove removes elements satisfying specific criteria
   remove_if (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 removes consecutive duplicate elements in a range
   (C++20) (niebloid)