std::unique_copy

 

C++

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Feature test macros (C++20)

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Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)	execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)(C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)	findfind_iffind_if_not (C++11) find_end find_first_of
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

Defined in header <algorithm>
	(1)
template< class InputIt, class OutputIt > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );			(until C++20)
template< class InputIt, class OutputIt > constexpr OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );			(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first,                         ForwardIt1 last, ForwardIt2 d_first );		(2)	(since C++17)
		(3)
template< class InputIt, class OutputIt, class BinaryPredicate > OutputIt unique_copy( InputIt first, InputIt last,                       OutputIt d_first, BinaryPredicate p );				(until C++20)
template< class InputIt, class OutputIt, class BinaryPredicate > constexpr OutputIt unique_copy( InputIt first, InputIt last,                                 OutputIt d_first, BinaryPredicate p );				(since C++20)
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.

1) Elements are compared using operator==. The behavior is undefined if it is not an equivalence relation.

3) Elements are compared using the given binary predicate p. The behavior is undefined if it is not an equivalence relation.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

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)

Parameters

first, 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:  bool pred(const Type1 &a, const Type2 &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 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 InputIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- InputIt must meet the requirements of LegacyInputIterator.
- OutputIt must meet the requirements of LegacyOutputIterator.
- ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
- The type of dereferenced InputIt must meet the requirements of CopyAssignable. if InputIt does not satisfy LegacyForwardIterator
- The type of dereferenced InputIt must meet the requirements of CopyConstructible. if neither InputIt nor OutputIt satisfies LegacyForwardIterator, or InputIt does not satisfy LegacyForwardIterator and the value type of InputIt differs from that of OutputIt

Return value

Output iterator to the element past the last written element.

Complexity

For nonempty ranges, exactly std::distance(first, last) - 1 applications of the corresponding predicate.

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::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes

If InputIt satisfies LegacyForwardIterator, this function rereads the input in order to detect duplicates.

Otherwise, if OutputIt satisfies LegacyForwardIterator, 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 ForwardIt1 is not both CopyConstructible and CopyAssignable.

Possible implementation

See also the implementations in libstdc++ and libc++.

Example

Run this code

#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!

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 239	C++98	the predicate was applied last - first times	applied last - first - 1 times
LWG 241	C++98	the value type of InputIt was not required to be CopyConstructible	conditionally required
LWG 538	C++98	the value type of InputIt was not required to be CopyAssignable	conditionally required

See also

adjacent_find	finds the first two adjacent items that are equal (or satisfy a given predicate) (function template)
unique	removes consecutive duplicate elements in a range (function template)
copycopy_if (C++11)	copies a range of elements to a new location (function template)
ranges::unique_copy (C++20)	creates a copy of some range of elements that contains no consecutive duplicates (niebloid)