Removes 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 end of the range.

[edit]Explanation

Removing is done by shifting the elements in the range in such a way that the elements that are not to be removed appear in the beginning of the range.

• Shifting is done bycopy assignment(until C++11)move assignment(since C++11).
• The removing operation is stable: the relative order of the elements not to be removed stays the same.
• The underlying sequence of[first, last) is not shortened by the removing operation. Givenresult as the returned iterator:

• All iterators in[result, last) are stilldereferenceable.

[edit]Parameters

[edit]Return value

AForwardIt to the new end of the range.

[edit]Complexity

Given\(\scriptsize N\)N asstd::distance(first, last):

[edit]Exceptions

The overloads with a template parameter namedExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception andExecutionPolicy is one of thestandard policies,std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory,std::bad_alloc is thrown.

[edit]Possible implementation

See also the implementations inlibstdc++,libc++, andMSVC STL.

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;}

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;}

[edit]Notes

A call tounique is typically followed by a call to a container'serase member function to actually remove elements from the container.

[edit]Example

#include <algorithm>#include <iostream>#include <vector> int main(){// a vector containing several duplicate elementsstd::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) duplicatesauto 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 duplicatesstd::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);}

@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

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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