# std::ranges::copy, std::ranges::copy_if, std::ranges::copy_result, std::ranges::copy_if_result

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< cpp‎ | algorithm‎ | ranges

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> constexpr copy_result<I, O> copy( I first, S last, O result ); | (1) | (since C++20)
template< ranges::input_range R, std::weakly_incrementable O > requires std::indirectly_copyable<ranges::iterator_t<R>, O> constexpr copy_result<ranges::borrowed_iterator_t<R>, O> copy( R&& r, O result ); | (2) | (since C++20)
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 copy_if_result<I, O> copy_if( I first, S last, O result, Pred pred, Proj proj = {} ); | (3) | (since C++20)
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 copy_if_result<ranges::borrowed_iterator_t<R>, O> copy_if( R&& r, O result, Pred pred, Proj proj = {} ); | (4) | (since C++20)
Helper types |  |
template< class I, class O > using copy_result = ranges::in_out_result<I, O>; | (5) | (since C++20)
template< class I, class O > using copy_if_result = ranges::in_out_result<I, O>; | (6) | (since 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 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 (C++20) | copies a range of elements in backwards order (niebloid)
ranges::reverse_copy (C++20) | creates a copy of a range that is reversed (niebloid)
ranges::copy_n (C++20) | copies a number of elements to a new location (niebloid)
ranges::fill (C++20) | assigns a range of elements a certain value (niebloid)
ranges::remove_copy ranges::remove_copy_if (C++20)(C++20) | copies a range of elements omitting those that satisfy specific criteria (niebloid)
copy copy_if (C++11) | copies a range of elements to a new location (function template)

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