std::ranges::copy,std::ranges::copy_if,std::ranges::copy_result,(3)

NAME
std::ranges::copy,std::ranges::copy_if,std::ranges::copy_result, - std::ranges::copy,std::ranges::copy_if,std::ranges::copy_result,

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::sentinel_for<I> S,
   std::weakly_incrementable O >
   requires std::indirectly_copyable<I, O> (1) (since C++20)
   constexpr copy_result<I, O>
       copy( I first, S last, O result );
   template< ranges::input_range R, std::weakly_incrementable O >
   requires std::indirectly_copyable<ranges::iterator_t<R>, O> (2) (since C++20)
   constexpr copy_result<ranges::borrowed_iterator_t<R>, O>
       copy( R&& r, O result );
   template< std::input_iterator I, std::sentinel_for<I> S,
   std::weakly_incrementable O,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>>
   Pred > (3) (since C++20)
   requires std::indirectly_copyable<I, O>
   constexpr copy_if_result<I, O>
       copy_if( I first, S last, O result, Pred pred, Proj proj = {}
   );
   template< ranges::input_range R, std::weakly_incrementable O,
             class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>> Pred > (4) (since C++20)
   requires std::indirectly_copyable<ranges::iterator_t<R>, O>
   constexpr copy_if_result<ranges::borrowed_iterator_t<R>, O>
       copy_if( R&& r, O result, Pred pred, Proj proj = {} );

Helper types
   template< class I, class O > (5) (since C++20)
   using copy_result = ranges::in_out_result<I, O>;
   template< class I, class O > (6) (since C++20)
   using copy_if_result = ranges::in_out_result<I, O>;
   Copies the elements in the range, defined by [first, last), to another range
   beginning at result.
   1) Copies all elements in the range [first, last) starting from first and proceeding
   to last - 1. The behavior is undefined if result is within the range [first, last).
   In this case, ranges::copy_backward may be used instead.
   3) Only copies the elements for which the predicate pred returns true. The relative
   order of the elements that are copied is preserved. The behavior is undefined if the
   source and the destination ranges overlap.
   2,4) Same as (1,3), but uses r as the source range, as if using ranges::begin(r) as
   first and ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - the range of elements to copy
   r - the range of elements to copy
   result - the beginning of the destination range.
   pred - predicate to apply to the projected elements
   proj - projection to apply to the elements

Return value
   A ranges::in_out_result containing an input iterator equal to last and an output
   iterator past the last element copied.

Complexity
   1,2) Exactly last - first assignments.
   3,4) Exactly last - first applications of the predicate and projection, between 0
   and last - first assignments (assignment for every element for which predicate
   returns true, dependent on predicate and input data).

Notes
   In practice, implementations of ranges::copy avoid multiple assignments and use bulk
   copy functions such as std::memmove if the value type is TriviallyCopyable and the
   iterator types satisfy contiguous_iterator.
   When copying overlapping ranges, ranges::copy is appropriate when copying to the
   left (beginning of the destination range is outside the source range) while
   ranges::copy_backward is appropriate when copying to the right (end of the
   destination range is outside the source range).

Possible implementation
                        copy
  struct copy_fn
  {
      template<std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O>
      requires std::indirectly_copyable<I, O>
      constexpr ranges::copy_result<I, O> operator()(I first, S last, O result) const
      {
          for (; first != last; ++first, (void)++result)
              *result = *first;
          return {std::move(first), std::move(result)};
      }
      template<ranges::input_range R, std::weakly_incrementable O>
      requires std::indirectly_copyable<ranges::iterator_t<R>, O>
      constexpr ranges::copy_result<ranges::borrowed_iterator_t<R>, O>
          operator()(R&& r, O result) const
      {
          return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
      }
  };
  inline constexpr copy_fn copy;
                        copy_if
  struct copy_if_fn
  {
      template<std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O,
               class Proj = std::identity,
               std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
      requires std::indirectly_copyable<I, O>
      constexpr ranges::copy_if_result<I, O>
          operator()(I first, S last, O result, Pred pred, Proj proj = {}) const
      {
          for (; first != last; ++first)
              if (std::invoke(pred, std::invoke(proj, *first)))
              {
                  *result = *first;
                  ++result;
              }
          return {std::move(first), std::move(result)};
      }
      template<ranges::input_range R, std::weakly_incrementable O,
               class Proj = std::identity,
               std::indirect_unary_predicate<
                   std::projected<ranges::iterator_t<R>, Proj>> Pred>
      requires std::indirectly_copyable<ranges::iterator_t<R>, O>
      constexpr ranges::copy_if_result<ranges::borrowed_iterator_t<R>, O>
          operator()(R&& r, O result, Pred pred, Proj proj = {}) const
      {
          return (*this)(ranges::begin(r), ranges::end(r),
                        std::move(result),
                        std::ref(pred), std::ref(proj));
      }
  };
  inline constexpr copy_if_fn copy_if;

Example
   The following code uses ranges::copy to both copy the contents of one std::vector to
   another and to display the resulting std::vector: // Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 #include <numeric>
 #include <vector>
 int main()
 {
     std::vector<int> source(10);
     std::iota(source.begin(), source.end(), 0);
     std::vector<int> destination;
     std::ranges::copy(source.begin(), source.end(),
                       std::back_inserter(destination));
 // or, alternatively,
 // std::vector<int> destination(source.size());
 // std::ranges::copy(source.begin(), source.end(), destination.begin());
 // either way is equivalent to
 // std::vector<int> destination = source;
     std::cout << "destination contains: ";
     std::ranges::copy(destination, std::ostream_iterator<int>(std::cout, " "));
     std::cout << '\n';
     std::cout << "odd numbers in destination are: ";
     std::ranges::copy_if(destination, std::ostream_iterator<int>(std::cout, " "),
                        [](int x) { return (x % 2) == 1; });
     std::cout << '\n';
 }

Output:
 destination contains: 0 1 2 3 4 5 6 7 8 9
 odd numbers in destination are: 1 3 5 7 9

See also
   ranges::copy_backward copies a range of elements in backwards order
   (C++20) (niebloid)
   ranges::reverse_copy creates a copy of a range that is reversed
   (C++20) (niebloid)
   ranges::copy_n copies a number of elements to a new location
   (C++20) (niebloid)
   ranges::fill assigns a range of elements a certain value
   (C++20) (niebloid)
   ranges::remove_copy copies a range of elements omitting those that satisfy
   ranges::remove_copy_if specific criteria
   (C++20) (niebloid)
   (C++20)
   copy copies a range of elements to a new location
   copy_if (function template)
   (C++11)