Defined in header <algorithm>  

template< class InputIt, class OutputIt > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );  (1)  
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first );  (2)  (since C++17) 
template< class InputIt, class OutputIt, class BinaryPredicate > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first, BinaryPredicate p );  (3)  
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, BinaryPredicate p );  (4)  (since C++17) 
Copies the elements from the range [first, last)
, to another range beginning at d_first
in such a way that there are no consecutive equal elements. Only the first element of each group of equal elements is copied.
operator==
.p
.policy
. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
is truefirst, last    the range of elements to process 
d_first    the beginning of the destination range 
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:
The signature does not need to have 
Type requirements  
InputIt must meet the requirements of InputIterator . 

OutputIt must meet the requirements of OutputIterator . 

ForwardIt1, ForwardIt2 must meet the requirements of ForwardIterator . 

The type of dereferenced InputIt must meet the requirements of CopyAssignable . if InputIt does not satisfy ForwardIterator 

The type of dereferenced InputIt must meet the requirements of CopyConstructible . if neither InputIt nor OutputIt satisfies ForwardIterator , or if InputIt does not satisfy ForwardIterator and the value type of InputIt differs from that of OutputIt 
Output iterator to the element past the last written element.
For nonempty ranges, exactly std::distance(first, last)  1
applications of the corresponding comparator.
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the three standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. If InputIt
satisfies ForwardIterator
, this function rereads the input in order to detect duplicates.
Otherwise, if OutputIt
satisfies ForwardIterator
, and the value type of InputIt
is the same as that of OutputIt
, this function compare *d_first
to *first
.
Otherwise, this function compares *first
to a local element copy.
For the overloads with an ExecutionPolicy, there may be a performance cost if the value type of ForwardIterator1
is not both CopyConstructible
and CopyAssignable
.
#include <string> #include <iostream> #include <algorithm> #include <iterator> int main() { std::string s1 = "The string with many spaces!"; std::cout << "before: " << s1 << '\n'; std::string s2; std::unique_copy(s1.begin(), s1.end(), std::back_inserter(s2), [](char c1, char c2){ return c1 == ' ' && c2 == ' '; }); std::cout << "after: " << s2 << '\n'; }
Output:
before: The string with many spaces! after: The string with many spaces!
finds the first two adjacent items that are equal (or satisfy a given predicate) (function template) 

removes consecutive duplicate elements in a range (function template) 
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