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 >
                        (since
   requires std::indirectly_copyable<I, O> (1) 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
   constexpr copy_result<ranges::borrowed_iterator_t<R>, O> C++20)
   copy( R&& r, O result );
   template< std::input_iterator I, std::sentinel_for<I> S,
   std::weakly_incrementable O,
   class Proj = std::identity, (since
   std::indirect_unary_predicate<std::projected<I, Proj>> Pred > (3) 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>, (since
   Proj>> Pred > (4) 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
   using copy_result = ranges::in_out_result<I, O>; C++20)
   template< class I, class O > (6) (since
   using copy_if_result = ranges::in_out_result<I, O>; C++20)
   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 may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   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 std::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, std::ranges::copy is appropriate when copying to
   the left (beginning of the destination range is outside the source range) while
   std::ranges::copy_backward is appropriate when copying to the right (end of the
   destination range is outside the source range).

Possible implementation

First version
  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;

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

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