Difference between revisions of "cpp/algorithm/copy"

Latest revision as of 22:09, 25 March 2024

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

[edit] Parameters

[edit] Return value

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

[edit] Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

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

[edit] 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 any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

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;
}

template<class InputIt, class OutputIt, class UnaryPred>
OutputIt copy_if(InputIt first, InputIt last,
                 OutputIt d_first, UnaryPred pred)
{
    for (; first != last; ++first)
        if (pred(*first))
        {
            *d_first = *first;
            ++d_first;
        }
 
    return d_first;
}

[edit] 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).

[edit] Example

#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 (const int x : to_vector)
        std::cout << x << ' ';
    std::cout << '\n';
}

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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also