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std::uninitialized_copy

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< cpp‎ | memory
 
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Defined in header <memory>
template< class InputIt, class NoThrowForwardIt >

NoThrowForwardIt uninitialized_copy( InputIt first, InputIt last,

                                     NoThrowForwardIt d_first );
 (1)
template< class ExecutionPolicy, class ForwardIt, class NoThrowForwardIt >

NoThrowForwardIt uninitialized_copy( ExecutionPolicy&& policy,
                                     ForwardIt first, ForwardIt last,

                                     NoThrowForwardIt d_first );
 (2) (since C++17)
1) Copies elements from the range [first, last) to an uninitialized memory area beginning at d_first as if by 
for (; first != last; ++d_first, (void) ++first)
   ::new (/*VOIDIFY*/(*d_first))
      typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);

where /*VOIDIFY*/(e) is:

static_cast<void*>(&e)
(until C++11)
static_cast<void*>(std::addressof(e))
(since C++11)
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

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 range of the 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 must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions. Applying &* to a NoThrowForwardIt value must yield a pointer to its value type. (until C++11)

[edit] Return value

Iterator to the element past the last element copied.

[edit] Complexity

Linear in the distance between first and last.

[edit] Exceptions

The overload with a template parameter named ExecutionPolicy reports 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 NoThrowForwardIt>
NoThrowForwardIt uninitialized_copy(InputIt first, InputIt last, NoThrowForwardIt d_first)
{
    using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try {
        for (; first != last; ++first, (void) ++current) {
            ::new (static_cast<void*>(std::addressof(*current))) T(*first);
        }
        return current;
    } catch (...) {
        for (; d_first != current; ++d_first) {
            d_first->~T();
        }
        throw;
    }
}

[edit] Example

Run this code
#include <iostream>
#include <memory>
#include <cstdlib>
#include <string>
 
int main()
{
    const char *v[] = {"This", "is", "an", "example"};
 
    auto sz = std::size(v);
 
    if(void *pbuf = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
    {
        try
        {
            auto first = static_cast<std::string*>(pbuf);
            auto last = std::uninitialized_copy(std::begin(v), std::end(v), first);
 
            for (auto it = first; it != last; ++it)
                std::cout << *it << '_';
            std::cout << '\n';
 
            std::destroy(first, last);
        }
        catch(...) {}
        std::free(pbuf);
    }
}

Output: 

This_is_an_example_

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 2433 C++11 this algorithm might be hijacked by overloaded operator& uses std::addressof
LWG 3870 C++20 this algorithm might create objects on a const storage kept disallowed

[edit] See also

(C++11)
 copies a number of objects to an uninitialized area of memory
(function template) [edit]
(C++20)
 copies a range of objects to an uninitialized area of memory
(niebloid) [edit]
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