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std::copy, std::copy_if

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< cpp‎ | algorithm
 
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Defined in header <algorithm>
template< class InputIt, class OutputIt >

OutputIt copy( InputIt first, InputIt last,

               OutputIt d_first );
 (1) (constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2 >
ForwardIt2 copy( ExecutionPolicy&& policy,
                 ForwardIt1 first, ForwardIt1 last,

                 ForwardIt2 d_first );
 (2) (since C++17)
template< class InputIt, class OutputIt, class UnaryPred >

OutputIt copy_if( InputIt first, InputIt last,

                  OutputIt d_first, UnaryPred pred );
 (3) (since C++11)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class UnaryPred >
ForwardIt2 copy_if( ExecutionPolicy&& policy,
                    ForwardIt1 first, ForwardIt1 last,

                    ForwardIt2 d_first, UnaryPred pred );
 (4) (since C++17)

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 d_first is in [first, last), the behavior is undefined. In this case, std::copy_backward 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:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)
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:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

[edit] Parameters

first, last - the pair of iterators defining the source range of elements to copy
d_first - the beginning of the destination range
policy - the execution policy to use
pred - unary predicate which returns ​true for the required elements.

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). ​

Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.
-
ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
-
UnaryPred must meet the requirements of Predicate.

[edit] Return value

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

[edit] Complexity

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

1,2) Exactly N assignments.
3,4) Exactly N applications of the predicate pred, and at most N assignments.

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

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;
}
copy_if (3) 
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

The following code uses std::copy both to 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> 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>(