Defined in header <algorithm>  

(1)  
template< class ForwardIt > ForwardIt unique( ForwardIt first, ForwardIt last );  (until C++20)  
template< class ForwardIt > constexpr ForwardIt unique( ForwardIt first, ForwardIt last );  (since C++20)  
template< class ExecutionPolicy, class ForwardIt > ForwardIt unique( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );  (2)  (since C++17) 
(3)  
template< class ForwardIt, class BinaryPredicate > ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p );  (until C++20)  
template< class ForwardIt, class BinaryPredicate > constexpr ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p );  (since C++20)  
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate > ForwardIt unique( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, BinaryPredicate p );  (4)  (since C++17) 
Eliminates all except the first element from every consecutive group of equivalent elements from the range [first, last)
and returns a pasttheend 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.
operator==
. The behavior is undefined if it is not an equivalence relation.p
. The behavior is undefined if it is not an equivalence relation.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.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:
While the signature does not need to have 
Type requirements  
ForwardIt must meet the requirements of LegacyForwardIterator. 

The type of dereferenced ForwardIt must meet the requirements of MoveAssignable. 
A ForwardIt
to the new end of the range.
For nonempty ranges, exactly std::distance(first,last) 1
applications of the corresponding predicate.
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. 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.
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; } 
#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
finds the first two adjacent items that are equal (or satisfy a given predicate) (function template) 

creates a copy of some range of elements that contains no consecutive duplicates (function template) 

removes elements satisfying specific criteria (function template) 

removes consecutive duplicate elements (public member function of std::list<T,Allocator> ) 

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