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Showing content from https://en.cppreference.com/w/cpp/language/../../cpp/symbol_index/../algorithm/ranges/copy.html below:

std::ranges::copy, std::ranges::copy_if, std::ranges::copy_result, std::ranges::copy_if_result - cppreference.com

Call signature

Helper types

Copies the elements in the range, defined by [first, last), to another range beginning at result.

Copies all elements in the range

starting from

and proceeding to

. The behavior is undefined if

is within the range

. In this case,

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.

The function-like entities described on this page are algorithm function objects (informally known as 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.

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

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).

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).

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;

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;

The following code uses ranges::copy to both copy the contents of one std::vector to another and to display the resulting std::vector.

#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

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