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<title>Issue 4036: __alignof_is_defined is only implicitly specified in C++ and not yet deprecated</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#WP">WP</a> status.</em></p>
<h3 id="4036"><a href="lwg-defects.html#4036">4036</a>. <code>__alignof_is_defined</code> is only implicitly specified in C++ and not yet deprecated</h3>
<p><b>Section:</b> D.11 <a href="https://wg21.link/depr.c.macros">[depr.c.macros]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-01-12 <b>Last modified:</b> 2024-11-01</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently 17.15.4 <a href="https://wg21.link/stdalign.h.syn">[stdalign.h.syn]</a> states
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
The contents of the C++ header <code>&lt;stdalign.h&gt;</code> are the same as the C standard library header
<code>&lt;stdalign.h&gt;</code>, with the following changes: The header <code>&lt;stdalign.h&gt;</code> does not
define a macro named <code>alignas</code>.
<p/>
<span style="font-variant: small-caps">See also</span>: ISO/IEC 9899:2018, 7.15
</p>
</blockquote>
<p>
which implicitly specifies that <code>__alignof_is_defined</code> is also provided in C++, because C17
specified that the macro is provided in <code>&lt;stdaligh.h&gt;</code>.
<p/>
However, there's no explicit mentioning of <code>__alignof_is_defined</code> in the C++ standard wording.
And D.11 <a href="https://wg21.link/depr.c.macros">[depr.c.macros]</a>/1 (added by LWG <a href="lwg-defects.html#3827" title="Deprecate &lt;stdalign.h&gt; and &lt;stdbool.h&gt; macros (Status: C++23)">3827</a><sup><a href="https://cplusplus.github.io/LWG/issue3827" title="Latest snapshot">(i)</a></sup>) seemingly contradicts with
17.15.4 <a href="https://wg21.link/stdalign.h.syn">[stdalign.h.syn]</a> and only makes <code>__alignas_is_defined</code> deprecated.
<p/>
It seems that we should explicitly mention <code>__alignof_is_defined</code> in D.11 <a href="https://wg21.link/depr.c.macros">[depr.c.macros]</a>
at this moment.
</p>

<p><i>[2024-03-12; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

<p><i>[Tokyo 2024-03-23; Status changed: Voting &rarr; WP.]</i></p>


<p><i>[2024-10-22; Note: this should have been handled by LWG <a href="lwg-defects.html#2241" title="&lt;cstdalign&gt; and #define of alignof (Status: Resolved)">2241</a><sup><a href="https://cplusplus.github.io/LWG/issue2241" title="Latest snapshot">(i)</a></sup> but that issue was closed as Resolved without properly resolving it.]</i></p>




<p id="res-4036"><b>Proposed resolution:</b></p>
<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 D.11 <a href="https://wg21.link/depr.c.macros">[depr.c.macros]</a> as indicated:</p>

<blockquote>
<p>
-1- The header <code>&lt;stdalign.h&gt;</code> has the following macro<ins>s</ins>:
</p>
<blockquote>
<pre>
#define __alignas_is_defined 1
<ins>#define __alignof_is_defined 1</ins>
</pre>
</blockquote>
</blockquote>

</li>

</ol>







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