# std::experimental::ranges::copy, std::experimental::ranges::copy_if

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< cpp‎ | experimental‎ | ranges

Defined in header `<experimental/ranges/algorithm>` |  |
template< InputIterator I, Sentinel<I> S, WeaklyIncrementable O > requires IndirectlyCopyable<I, O> ranges::tagged_pair<tag::in(I), tag::out(O)> copy( I first, S last, O result ); | (1) | (ranges TS)
template< InputRange R, WeaklyIncrementable O > requires IndirectlyCopyable<ranges::iterator_t<R>, O> ranges::tagged_pair<tag::in(ranges::safe_iterator_t<R>), tag::out(O)> copy( R&& r, O result ); | (2) | (ranges TS)
template< InputIterator I, Sentinel<I> S, WeaklyIncrementable O, class Proj = ranges::identity, IndirectUnaryPredicate<projected<I, Proj>> Pred > requires IndirectlyCopyable<I, O> ranges::tagged_pair<tag::in(I), tag::out(O)> copy_if( I first, S last, O result, Pred pred, Proj proj = Proj{} ); | (3) | (ranges TS)
template< InputRange R, WeaklyIncrementable O, class Proj = ranges::identity, IndirectUnaryPredicate<projected<ranges::iterator_t<R>, Proj>> Pred > requires IndirectlyCopyable<iterator_t<R>, O> ranges::tagged_pair<tag::in(ranges::safe_iterator_t<R>), tag::out(O)> copy_if( R&& r, O result, Pred pred, Proj proj = Proj{} ); | (4) | (ranges TS)

Copies elements in the source range (`[`first`,` last`)` or r) into the destination range beginning at result, starting from the first element in the source range and proceeding to the last one.

1) Copies all elements in the range `[`first`,` last`)`. For each non-negative integer `n < (last - first)`, performs *(result + n) = *(first + n). The behavior is undefined if result is within the range `[`first`,` last`)`. In this case, ranges::copy_backward may be used instead.
2) Same as (1), but uses r as the source range, as if by ranges::copy(ranges::begin(r), ranges::end(r), result); except that result may not be copied.
3) Only copies the elements for which the predicate pred returns true when applied to the element's value as projected by the projection proj. The order of the elements that are copied is preserved. The behavior is undefined if the source and the destination ranges overlap.
4) Same as (3), but uses r as the source range, as if by ranges::copy_if(ranges::begin(r), ranges::end(r), result, pred, proj); except that result, pred and proj may not be copied.

Notwithstanding the declarations depicted above, the actual number and order of template parameters for algorithm declarations is unspecified. Thus, if explicit template arguments are used when calling an algorithm, the program is probably non-portable.

### 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 `tagged_pair` object containing the following two members:

- The first member, with the tag `tag::in`, is the past-the-end iterator of the source range (that is, an iterator of type `I` that compares equal to the sentinel last).
- The second member, with the tag `tag::out`, is the past-the-end iterator of the result range.

### Complexity

1) Exactly ranges::distance(first, last) assignments.
2) Exactly ranges::distance(r) assignments.
3) Exactly ranges::distance(first, last) applications of the corresponding projection and predicate.
4) Exactly ranges::distance(r) applications of the corresponding projection and predicate.

### Possible implementations

First version
    template<InputIterator I, Sentinel<I> S, WeaklyIncrementable O>
        requires IndirectlyCopyable<I, O>()
    ranges::tagged_pair<tag::in(I), tag::out(O)>
        copy(I first, S last, O result)
    {
        for (; first != last; ++first, (void)++result)
            *result = *first;
        return {first, result};
    }
Second version
    template<InputRange R, WeaklyIncrementable O>
        requires IndirectlyCopyable<ranges::iterator_t<R>, O>()
    ranges::tagged_pair<tag::in(ranges::safe_iterator_t<R>), tag::out(O)>
        copy(R&& r, O result)
    {
       return ranges::copy(ranges::begin(r), ranges::end(r), result);
    }
Third version
    template<InputIterator I, Sentinel<I> S, WeaklyIncrementable O,
             class Proj = ranges::identity,
             IndirectUnaryPredicate<projected<I, Proj>> Pred>
        requires IndirectlyCopyable<I, O>()
    ranges::tagged_pair<tag::in(I), tag::out(O)>
        copy_if(I first, S last, O result, Pred pred, Proj proj = Proj{})
    {
        for (; first != last; ++first)
            if (ranges::invoke(pred, ranges::invoke(proj, *first)))
            {
                *result = *first;
                ++result;
            }
        return {first, result};
    }
Fourth version
    template<InputRange R, WeaklyIncrementable O,
             class Proj = ranges::identity,
             IndirectUnaryPredicate<projected<ranges::iterator_t<R>, Proj>> Pred>
        requires IndirectlyCopyable<ranges::iterator_t<R>, O>()
    ranges::tagged_pair<tag::in(ranges::safe_iterator_t<R>), tag::out(O)>
        copy_if(R&& r, O result, Pred pred, Proj proj = Proj{})
    {
        return ranges::copy_if(ranges::begin(r), ranges::end(r), result, pred, proj);
    }

### 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 <experimental/ranges/algorithm>
    #include <experimental/ranges/iterator>
    #include <iostream>
    #include <numeric>
    #include <vector>

    int main()
    {
        // see https://en.cppreference.com/w/cpp/language/namespace_alias
        namespace ranges = std::experimental::ranges;

        std::vector<int> from_vector(10);
        std::iota(from_vector.begin(), from_vector.end(), 0);

        std::vector<int> to_vector;
        ranges::copy_if(from_vector.begin(), from_vector.end(),
                        ranges::back_inserter(to_vector),
                        [](const auto i)
                        {
                           return i % 3;
                        });
    // or, alternatively,
    //  std::vector<int> to_vector(from_vector.size());
    //  std::copy(from_vector, to_vector.begin());

        std::cout << "to_vector contains: ";

        ranges::copy(to_vector, ranges::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';
    }

Output:

    to_vector contains: 1 2 4 5 7 8

### See also

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

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