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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 [firstlast) 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.

If d_first + [0std::distance(first, last)) overlaps with [firstlast), the behavior is undefined.

(since C++20)
2) Same as (1), but executed according to policy. This overload participates in overload resolution only if

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

#include <cstdlib>
#include <iostream>
#include <memory>
#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 866 C++98 given T as the value type of NoThrowForwardIt, if
T::operator new exists, the program might be ill-formed
uses global replacement-
new instead
LWG 2133 C++98 the effect description used a for loop with the iteration
expression ++d_first, ++first, which results
in an argument-dependent lookup of operator,
discards the value
of one operand
to disable that ADL
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

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