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std::unique(3) C++ Standard Libary std::unique(3) NAME std::unique - std::unique Synopsis Defined in header <algorithm> template< class ForwardIt > (1) (const- expr since C++20) ForwardIt unique( ForwardIt first, ForwardIt last ); template< class ExecutionPolicy, class ForwardIt > ForwardIt unique( ExecutionPolicy&& policy, (2) (since C++17) ForwardIt first, ForwardIt last ); template< class ForwardIt, class BinaryPred > ForwardIt unique( ForwardIt first, ForwardIt last, (3) (const- expr since C++20) BinaryPred p ); template< class ExecutionPolicy, class ForwardIt, class BinaryPred > ForwardIt unique( ExecutionPolicy&& policy, (4) (since C++17) ForwardIt first, ForwardIt last, BinaryPred p ); Removes all except the first element from every consecutive group of equivalent elements from the range [first, last) and returns a past-the-end it- erator for the new end of the range. 1) Elements are compared using operator==. If operator== does not establish an equivalence relation, the behav- ior is undefined. 3) Elements are compared using the given binary predicate p. If p does not establish an equivalence relation, the behavior is un- defined. 2,4) Same as (1,3), but executed according to policy. These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since C++20) 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 by copy assignment (until C++11) move assignment (since C++11). * The removing operation is stable: the relative order of the ele- ments not to be removed stays the same. * The underlying sequence of [first, last) is not shortened by the removing operation. Given result as the returned iterator: * All iterators in [result, last) are still dereferenceable. * Each element of [result, last) has a valid but unspecified state, because move assignment can eliminate elements by moving from (since C++11) elements that were originally in that range. Parameters first, last - the range of elements to process policy - the execution policy to use. See execution policy for details. 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: bool pred(const Type1 &a, const Type2 &b); p - 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 re- gardless of value category (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 con- verted to both of them. Type requirements - ForwardIt must meet the requirements of LegacyForwardIterator. - The type of dereferenced ForwardIt must meet the requirements of MoveAssignable. Return value A ForwardIt to the new end of the range. Complexity Given \(\scriptsize N\)N as std::distance(first, last): 1,2) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) comparisons using operator==. 3,4) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) applications of the predicate p. Exceptions The overloads with a template parameter named ExecutionPolicy report errors as follows: * If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::termi- nate is called. For any other ExecutionPolicy, the behavior is implementation- defined. * If the algorithm fails to allocate memory, std::bad_alloc is thrown. Possible implementation See also the implementations in libstdc++, libc++, and MSVC STL. unique (1) 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; } unique (3) 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; } 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. Example // Run this code #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 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior the behavior was unclear if the elements the behavior is LWG 202 C++98 are unde- fined in this case compared using a non-equivalence relation See also finds the first two adjacent items that are equal (or satisfy a given adjacent_find predicate) (function template) creates a copy of some range of elements that con- tains no consecutive unique_copy duplicates (function template) remove removes elements satisfying specific criteria remove_if (function template) unique removes consecutive duplicate elements (public member function of std::list<T,Allocator>) unique removes consecutive duplicate elements (public member function of std::forward_list<T,Allo- cator>) ranges::unique removes consecutive duplicate elements in a range (C++20) (niebloid) http://cppreference.com 2024.06.10 std::unique(3)
NAME | Synopsis | Explanation | Parameters | Type requirements | Return value | Complexity | Exceptions | Possible implementation | Notes | Example | Output: | See also
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