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 last, (1) (constexpr since C++20)
   OutputIt d_first );
   template< class ExecutionPolicy, class ForwardIt1, class
   ForwardIt2 >
   ForwardIt2 unique_copy( ExecutionPolicy&& policy, (2) (since C++17)
   ForwardIt1 first,
                        ForwardIt1 last, ForwardIt2
   d_first );
   template< class InputIt, class OutputIt, class
   BinaryPred >
                        (3) (constexpr since C++20)
   OutputIt unique_copy( InputIt first, InputIt last,
                        OutputIt d_first, BinaryPred p );
   template< class ExecutionPolicy, class ForwardIt1,
             class ForwardIt2, class BinaryPred >
   ForwardIt2 unique_copy( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 (4) (since C++17)
   last,
                        ForwardIt2 d_first, BinaryPred 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==.
   If operator== does not establish an equivalence relation, the behavior is undefined.
   3) Elements are compared using the given binary predicate p.
   If p does not establish an equivalence relation, the behavior is undefined.
   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)
   If
   *d_first = *first is invalid
   (until C++20)
   *first is not writable to d_first
   (since C++20), the program is ill-formed.
   If source and destination ranges overlap, the behavior is undefined.
   Given T as the value type of InputIt, if overload (1) or (3) does not satisfy all of
   the following conditions, the behavior is undefined:
     * InputIt meets the requirements of LegacyForwardIterator. (until C++20)
     * InputIt models forward_iterator. (since C++20)
     * T is both CopyConstructible and CopyAssignable.
     * All following conditions are satisfied:
     * OutputIt meets the requirements of LegacyForwardIterator.
     * The value type of OutputIt is also T.
     * T is CopyAssignable.

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.

Return value
   Output iterator to the element past the last written element.

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.
   For overloads (2,4), there may be a performance cost if the value type of ForwardIt1
   is not both CopyConstructible and CopyAssignable.

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.

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

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.

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 #include <string>
 int main()
 {
     std::string s1 {"A string with mmmany letters!"};
     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 == 'm' && 'm' == c2; });
     std::cout << "After: " << s2 << '\n';
 }

Output:
 Before: A string with mmmany letters!
 After: A string with many letters!
   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 applied one time fewer
                       std::distance(first, last) times (for non-empty ranges)
   LWG 241 C++98 the value type of InputIt was not conditionally required
                       required to be CopyConstructible
   LWG 538 C++98 the value type of InputIt was not conditionally required
                       required to be CopyAssignable
                       the value type of InputIt was not
   LWG 2439 C++98 required to be conditionally required
                       CopyConstructible if OutputIt is a
                       LegacyForwardIterator

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)