Defined in header
template<classForwardIt>ForwardItunique(ForwardItfirst,ForwardItlast); (1)(constexpr since C++20)template<classForwardIt,classBinaryPred>ForwardItunique(ForwardItfirst,ForwardItlast,BinaryPredp); (2)(constexpr since C++20)template<classExecutionPolicy,classForwardIt>ForwardItunique(ExecutionPolicy&&policy,ForwardItfirst,ForwardItlast); (3) (since C++17)template<classExecutionPolicy,classForwardIt,classBinaryPred>ForwardItunique(ExecutionPolicy&&policy,ForwardItfirst,ForwardItlast,BinaryPredp); (4) (since C++17)“Removes” all except the first element from every group of consecutive equivalent elements from the target range [first, last).
1) Elements are compared using operator==.
If operator== does not establish an
, the behavior is undefined.
2) Elements are compared using the given binary predicate p.
If p does not establish an equivalence relation, the behavior is undefined.
3,4) Same as (1,2), but executed according to policy.
These overloads participate in overload resolution only if the value of the following expression is true:
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)Removing is done by partitioning the elements in the target range. Given the partition point result, the leading elements of every group appear in [first, result), while other elements can only appear in [result, last).
The underlying sequence of the target range is not shortened by the removing operation.
Elements are shifted by
(until C++11)
(since C++11).
All iterators in [result, last) are still
, and each element of [result, last) has a valid but unspecified state(since C++11).
The removing operation is stable: the relative order of the elements not to be removed stays the same.
Parameters
first, last - the pair of iterators defining the target
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:
boolpred(constType1&a,constType2&b);
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
(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.
policy - the
to use Type requirements -ForwardIt must meet the requirements of
. Return value
The iterator result mentioned above.
Complexity
Given N as std::distance(first,last):
1) Exactly max(0,N-1) comparisons using operator==.
2) Exactly max(0,N-1) applications of the predicate p.
3)𝓞(N) comparisons using operator==.
4)𝓞(N) applications of the predicate p.
Exceptions
3,4) During the execution process:
If the temporary memory resources required for parallelization are not available,
is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for
,
is invoked).
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.
Possible implementation
See also the implementations in
,
, and
.
template<classForwardIt>ForwardItunique(ForwardItfirst,ForwardItlast){if(first==last)returnlast;ForwardItresult=first;while(++first!=last)if(!(*result==*first)&&++result!=first)*result=std::move(*first);return++result;}
template<classForwardIt,classBinaryPredicate>ForwardItunique(ForwardItfirst,ForwardItlast,BinaryPredicatep){if(first==last)returnlast;ForwardItresult=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>voidprint(intid,conststd::vector<int>&v,std::vector<int>::const_iteratorlast){std::cout<<id<<") ";for(autofirst{v.cbegin()};first!=last;++first)std::cout<<*first<<' ';for(;last!=v.end();++last)std::cout<<"? ";std::cout<<'\n';}intmain(){// a vector containing several duplicate elementsstd::vector<int>v{1,2,1,1,3,3,3,4,5,4};print(1,v,v.cend());// remove consecutive (adjacent) duplicatesautolast=std::unique(v.begin(),v.end());print(2,v,last);v.erase(last,v.end());print(3,v,v.cend());// sort followed by unique, to remove all duplicatesstd::sort(v.begin(),v.end());print(4,v,v.cend());last=std::unique(v.begin(),v.end());print(5,v,last);v.erase(last,v.end());print(6,v,v.cend());}Output:
1) 1 2 1 1 3 3 3 4 5 4 2) 1 2 1 3 4 5 4 ? ? ? 3) 1 2 1 3 4 5 4 4) 1 1 2 3 4 4 5 5) 1 2 3 4 5 ? ? 6) 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
C++98 the behavior was unclear if the elements are
compared using a non-equivalence relation the behavior is
undefined in this case See also
(C++20)
removes consecutive duplicate elements in a range
(algorithm function object)
finds the first two adjacent items that are equal (or satisfy a given predicate)
(function template & algorithm function object)
(C++20)
creates a copy of some range of elements that contains no consecutive duplicates
(function template & algorithm function object)
(C++20)
removes elements satisfying specific criteria
(function template & algorithm function object)
ranges::removeranges::remove_if
(C++20)(C++20)
removes consecutive duplicate elements
(public member function of std::list<T,Allocator>)
removes consecutive duplicate elements
(public member function of std::forward_list<T,Allocator>)
removes consecutive duplicate elements
(public member function of std::hive<T,Allocator>)