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 but 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. 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.
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>>
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. 
Forward iterator 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. 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 <iostream> #include <algorithm> #include <vector> #include <string> #include <cctype> int main() { // remove duplicate elements 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"; }
Output:
1 2 3 4 5 6 7
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> ) 
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