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<title>Issue 4064: Clarify that std::launder is not needed when using the result of std::memcpy</title>
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<h3 id="4064"><a href="lwg-defects.html#4064">4064</a>. Clarify that <code>std::launder</code> is not needed when using the result of <code>std::memcpy</code></h3>
<p><b>Section:</b> 27.5.1 <a href="https://wg21.link/cstring.syn">[cstring.syn]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Jan Schultke <b>Opened:</b> 2024-04-05 <b>Last modified:</b> 2024-11-28</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<blockquote><pre>
int x = 0;
alignas(int) std::byte y[sizeof(int)];
int z = *static_cast&lt;int*&gt;(std::memcpy(y, &amp;x, sizeof(int)));
</pre></blockquote>
<p>
This example should be well-defined, even without the use of <code>std::launder</code>.
<code>std::memcpy</code> implicitly creates an <code>int</code> inside <code>y</code>, and 
<a href="https://www.iso-9899.info/n3047.html#7.26.2.1p3">https://www.iso-9899.info/n3047.html#7.26.2.1p3</a> 
states that
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
The <code>memcpy</code> function returns the value of [the destination operand].
</p>
</blockquote>
<p>
In conjunction with 27.5.1 <a href="https://wg21.link/cstring.syn">[cstring.syn]</a> p3, this presumably means that <code>std::memcpy</code> 
returns a pointer to the (first) implicitly-created object, and no use of <code>std::launder</code> 
is necessary.
<p/>
The wording should be clarified to clearly support this interpretation or reject it.
</p>

<p><i>[2024-06-24; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/N4971" title=" Working Draft, Programming Languages — C++">N4971</a>.
</p>

<ol>
<li><p>Modify 27.5.1 <a href="https://wg21.link/cstring.syn">[cstring.syn]</a> as indicated:</p>

<blockquote>
<p>
-3- The functions <code>memcpy</code> and <code>memmove</code> are signal-safe (17.14.5 <a href="https://wg21.link/support.signal">[support.signal]</a>). 
Both functions implicitly create objects (6.8.2 <a href="https://wg21.link/intro.object">[intro.object]</a>) in the destination region of 
storage immediately prior to copying the sequence of characters to the destination. <ins>Both functions 
return a pointer to a suitable created object.</ins>
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[St. Louis 2024-06-26; CWG suggested improved wording]</i></p>

<p><i>[St. Louis 2024-06-28; LWG: move to Ready]</i></p>

<p><i>[Wrocław 2024-11-18; approved by Core (again)]</i></p>


<p><i>[Wrocław 2024-11-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4064"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>
<li><p>Modify 27.5.1 <a href="https://wg21.link/cstring.syn">[cstring.syn]</a> as indicated:</p>

<blockquote>
<p>
-3-
The functions <code>memcpy</code> and <code>memmove</code> are signal-safe
(17.14.5 <a href="https://wg21.link/support.signal">[support.signal]</a>).
<del>Both</del> <ins>Each of these</ins> functions implicitly
<del>create</del> <ins>creates</ins> objects (6.8.2 <a href="https://wg21.link/intro.object">[intro.object]</a>)
in the destination region of storage immediately prior to copying
the sequence of characters to the destination.
<ins>Each of these functions returns a pointer to a suitable created object,
if any, otherwise the value of the first parameter.</ins>
</p>
</blockquote>
</li>
</ol>





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