std::unique(3)

NAME
std::unique - std::unique

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt >
   ForwardIt unique( ForwardIt first, ForwardIt (until C++20)
   last );
   template< class ForwardIt >
   constexpr ForwardIt unique( ForwardIt first, (since C++20)
   ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt
   > (2) (since C++17)
   ForwardIt unique( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last );
   template< class ForwardIt, class BinaryPredicate
   > (until C++20)
   ForwardIt unique( ForwardIt first, ForwardIt (1)
   last, BinaryPredicate p );
   template< class ForwardIt, class BinaryPredicate
   > (since C++20)
   constexpr ForwardIt unique( ForwardIt first,
   ForwardIt last, BinaryPredicate p ); (3)
   template< class ExecutionPolicy, class
   ForwardIt, class BinaryPredicate >
   ForwardIt unique( ExecutionPolicy&& policy, (4) (since C++17)
   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.
   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>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

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
   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.

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;
   }

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

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
   (C++11) (public member function of std::forward_list<T,Allocator>)
   ranges::unique removes consecutive duplicate elements in a range
   (C++20) (niebloid)