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

std::copy, std::copy_if - cppreference.com

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

1) Copies all elements in the range [first, last) starting from first and proceeding to last.

If

is in

, the behavior is undefined. In this case,

may be used instead.

2) Copies the elements, but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

If [first, last) and the copy destination range overlaps, the behavior is undefined.

3) Only copies the elements for which the predicate pred returns true. This copy algorithm is stable: the relative order of the elements that are copied is preserved.

If [first, last) and the copy destination range overlaps, the behavior is undefined.

4) Same as (3), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

The expression pred(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​

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

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

1,2) Exactly \(\scriptsize N\)N assignments.

3,4) Exactly \(\scriptsize N\)N applications of the predicate pred, and at most \(\scriptsize N\)N assignments.

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

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.

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;
}

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).

The following code uses std::copy both to 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> 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';
}

Possible 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

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

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