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std::unique_copy

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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. 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 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`

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 three 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 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.

Example

#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!

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)