std::copy() algorithm

od C++20

// (1)
template< class InputIt, class OutputIt >
constexpr OutputIt copy( InputIt first, InputIt last, OutputIt d_first );

// (2)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 copy( ExecutionPolicy&& policy,
                 ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first );

od C++17

// (1)
template< class InputIt, class OutputIt >
OutputIt copy( InputIt first, InputIt last, OutputIt d_first );

// (2)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 copy( ExecutionPolicy&& policy,
                 ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first );

do C++17

// (1)
template< class InputIt, class OutputIt >
OutputIt copy( InputIt first, InputIt last, OutputIt d_first );

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

• (1) Copies all elements in the range [first; last) starting from first and proceeding to last - 1.

  Undefined Behaviour

  The behavior is undefined if d_first is within the range [first; last). In this case, std::copy_backward may be used instead.

• (2) Same as (1), but executed according to policy.

Overload Resolution

These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Parameters

first last	The range of elements to copy.
d_first	The beginning of the destination range.
policy	The execution policy to use. See execution policy for details.

Type requirements

InputIt	LegacyInputIterator
OutputIt	LegacyOutputIterator
ForwardIt1 ForwardIt2	LegacyForwardIterator

Return value

Output iterator to the element in the destination range, one past the last element copied.

Complexity

Exactly last - first assignments.

For the overloads with an ExecutionPolicy, there may be a performance cost if ForwardIt1's value type is not MoveConstructible.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

copy (1)

template<class InputIt, class OutputIt>
OutputIt copy(InputIt first, InputIt last,
              OutputIt d_first)
{
    for (; first != last; (void)++first, (void)++d_first)
        *d_first = *first;

    return d_first;
}

Notes

In practice, implementations of std::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy LegacyContiguousIterator.

When copying overlapping ranges, std::copy is appropriate when copying to the left (beginning of the destination range is outside the source range), while std::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>

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

    std::vector<int> to_vector;
    std::copy(from_vector.begin(), from_vector.end(),
              std::back_inserter(to_vector));
// or, alternatively,
//  std::vector<int> to_vector(from_vector.size());
//  std::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: ";

    std::copy(to_vector.begin(), to_vector.end(),
              std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';

    std::cout << "odd numbers in to_vector are: ";

    std::copy_if(to_vector.begin(), to_vector.end(),
                 std::ostream_iterator<int>(std::cout, " "),
                 [](int x) { return x % 2 != 0; });
    std::cout << '\n';

    std::cout << "to_vector contains these multiples of 3: ";

    to_vector.clear();
    std::copy_if(from_vector.begin(), from_vector.end(),
                 std::back_inserter(to_vector),
                 [](int x) { return x % 3 == 0; });

    for (int x : to_vector)
        std::cout << x << ' ';
    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
to_vector contains these multiples of 3: 0 3 6 9