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<title>Issue 3064: How do uninitialized memory algorithms obtain pointer without undefined behavior?</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#Resolved">Resolved</a> status.</em></p>
<h3 id="3064"><a href="lwg-defects.html#3064">3064</a>. How do uninitialized memory algorithms obtain pointer without undefined behavior?</h3>
<p><b>Section:</b> 26.11 <a href="https://wg21.link/specialized.algorithms">[specialized.algorithms]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2018-02-12 <b>Last modified:</b> 2024-11-28</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#specialized.algorithms">active issues</a> in [specialized.algorithms].</p>
<p><b>View all other</b> <a href="lwg-index.html#specialized.algorithms">issues</a> in [specialized.algorithms].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
A typical specification of the algorithms for initializing raw memory in <code>&lt;memory&gt;</code> looks like:
</p>
<blockquote>
<p>
<i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
for (; first != last; ++first)
  ::new (static_cast&lt;void*&gt;(addressof(*first)))
    typename iterator_traits&lt;ForwardIterator&gt;::value_type;
</pre></blockquote>
</blockquote>
<p>
However, this hides a nasty question:
<p/>
How do we bind a reference to an uninitialized object when dereferencing our iterator,
so that <code>static_cast&lt;void*&gt;(addressof(*first))</code> does not trigger undefined behavior on
the call to <code>*first</code>?
<p/>
When pointers are the only iterators we cared about, we could simply cast the iterator
value to <code>void*</code> 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 <code>void*</code> in order to get the address of the element.
</p>

<p><i>[2018-02-20, Priority set to 3 after mailing list discussion]</i></p>

<p><i>[2024-11-28 Status changed: New &rarr; Resolved.]</i></p>

<p>
Resolved by
<a href="https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#453"
title='References may only bind to “valid” objects'>CWG 453</a>,
accepted as a DR in March 2024.
</p>


<p id="res-3064"><b>Proposed resolution:</b></p>





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