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<title>Issue 2377: std::align requirements overly strict</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#C++17">C++17</a> status.</em></p>
<h3 id="2377"><a href="lwg-defects.html#2377">2377</a>. <code>std::align</code> requirements overly strict</h3>
<p><b>Section:</b> 20.2.5 <a href="https://wg21.link/ptr.align">[ptr.align]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Peter Dimov <b>Opened:</b> 2014-03-30 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#ptr.align">active issues</a> in [ptr.align].</p>
<p><b>View all other</b> <a href="lwg-index.html#ptr.align">issues</a> in [ptr.align].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::align</code> requires that its <code>alignment</code> argument shall be "a fundamental alignment value or an 
extended alignment value supported by the implementation in this context".
<p/>
This requirement is overly strict. There are use cases that require a buffer aligned at values that are not tied 
to the C++ implementation, such as page size, cache line size, sector size. These come from the hardware or the 
OS and are generally not known until run time. The implementation of <code>std::align</code> does not depend on the 
requirement that <code>alignment</code> be a fundamental or an extended alignment value; any power of two would be handled 
the same way.
<p/>
In addition, it is not possible for the user to even determine whether a value is "a fundamental alignment value 
or an extended alignment value supported by the implementation in this context". One would expect values coming 
from <code>alignof</code> to be fine, but I'm not sure whether even that is guaranteed in the presence of <code>alignas</code>.
<p/>
Therefore, I propose that
</p>
<blockquote><p>
<i>Requires</i>:
</p>
<ul>
<li><p><code>alignment</code> shall be a fundamental alignment value or an extended alignment value supported by
the implementation in this context</p></li>
</ul>
</blockquote>
<p>
be changed to
</p>
<blockquote><p>
<i>Requires</i>:
</p>
<ul>
<li><p><code>alignment</code> shall be a power of two</p></li>
</ul>
</blockquote>

<p><i>[2014-06-16 Rapperswil]</i></p>

<p>
Move to Ready
</p>



<p id="res-2377"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3936.</p>

<ol>
<li><p>Edit 20.2.5 <a href="https://wg21.link/ptr.align">[ptr.align]</a> p2 as indicated:</p>

<blockquote>
<pre>
void* align(std::size_t alignment, std::size_t size,
  void*&amp; ptr, std::size_t&amp; space);
</pre><blockquote>
<p>
-1- [&hellip;]
<p/>
-2- <i>Requires</i>:
</p>
<ul>
<li><p>
<code>alignment</code> shall be a <del>fundamental alignment value or an extended alignment value supported by
the implementation in this context</del><ins>power of two</ins>
</p></li>
</ul>
</blockquote>
</blockquote>
</li>
</ol>






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