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3064. How do uninitialized memory algorithms obtain pointer without undefined behavior?

Section: 19.10.11 [specialized.algorithms] Status: New Submitter: Alisdair Meredith Opened: 2018-02-12 Last modified: 2018-03-03

Priority: 3

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Discussion:

A typical specification of the algorithms for initializing raw memory in <memory> looks like:

Effects: Equivalent to:

for (; first != last; ++first)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type;

However, this hides a nasty question:

How do we bind a reference to an uninitialized object when dereferencing our iterator, so that static_cast<void*>(addressor(*first)) does not trigger undefined behavior on the call to *first?

When pointers are the only iterators we cared about, we could simply cast the iterator value to void* without dereferencing. I don't see how to implement this spec safely without introducing another customization point for iterators that performs the same function as casting a pointer to void* in order to get the address of the element.

[2018-02-20, Priority set to 3 after mailing list discussion]

Proposed resolution: