std::unique_copy(3)

NAME
std::unique_copy - std::unique_copy

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class OutputIt >
   OutputIt unique_copy( InputIt first, InputIt (until C++20)
   last,
   OutputIt d_first );
   template< class InputIt, class OutputIt >
   constexpr OutputIt unique_copy( InputIt first, (since C++20)
   InputIt last,
   OutputIt d_first );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2 >
   ForwardIt2 unique_copy( ExecutionPolicy&& (2) (since C++17)
   policy, ForwardIt1 first, ForwardIt1 last,
   ForwardIt2 d_first );
   template< class InputIt, class OutputIt, class
   BinaryPredicate > (1)
   OutputIt unique_copy( InputIt first, InputIt (until C++20)
   last,
   OutputIt d_first, BinaryPredicate p );
   template< class InputIt, class OutputIt, class
   BinaryPredicate >
   constexpr OutputIt unique_copy( InputIt first, (3) (since C++20)
   InputIt last,
   OutputIt d_first, BinaryPredicate p );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2, class
   BinaryPredicate >
                        (4) (since C++17)
   ForwardIt2 unique_copy( ExecutionPolicy&&
   policy, ForwardIt1 first, ForwardIt1 last,
   ForwardIt2 d_first, BinaryPredicate p );
   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>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) 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.
                    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 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 if 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 ForwardIterator1 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!

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