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<title>Issue 306: offsetof macro and non-POD types</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#CD1">CD1</a> status.</em></p>
<h3 id="306"><a href="lwg-defects.html#306">306</a>. offsetof macro and non-POD types</h3>
<p><b>Section:</b> 17.2 <a href="https://wg21.link/support.types">[support.types]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Steve Clamage <b>Opened:</b> 2001-02-21 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#support.types">issues</a> in [support.types].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p> 
<p>Spliced together from reflector messages c++std-lib-8294 and -8295:</p>

<p>18.1, paragraph 5, reads: &quot;The macro <code>offsetof</code>
accepts a restricted set of <i>type</i> arguments in this
International Standard. <i>type</i> shall be a POD structure or a POD
union (clause 9). The result of applying the offsetof macro to a field
that is a static data member or a function member is
undefined.&quot;</p>

<p>For the POD requirement, it doesn't say &quot;no diagnostic
required&quot; or &quot;undefined behavior&quot;. I read 4.1 <a href="https://wg21.link/intro.compliance">[intro.compliance]</a>, paragraph 1, to mean that a diagnostic is required.
It's not clear whether this requirement was intended.  While it's
possible to provide such a diagnostic, the extra complication doesn't
seem to add any value.
</p>


<p id="res-306"><b>Proposed resolution:</b></p>
<p>Change 18.1, paragraph 5, to &quot;If <i>type</i> is not a POD
structure or a POD union the results are undefined.&quot;</p>

<p><i>[Copenhagen: straw poll was 7-4 in favor.  It was generally
agreed that requiring a diagnostic was inadvertent, but some LWG
members thought that diagnostics should be required whenever
possible.]</i></p>






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