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<title>Issue 2333: [fund.ts] Hashing disengaged optional&lt;T&gt; objects</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="2333"><a href="lwg-defects.html#2333">2333</a>. [fund.ts] Hashing disengaged <code>optional&lt;T&gt;</code> objects</h3>
<p><b>Section:</b> 5.11 <a href="https://cplusplus.github.io/fundamentals-ts/v1.html#optional.hash">[fund.ts::optional.hash]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2013-10-03 <b>Last modified:</b> 2015-10-26</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: fund.ts</b></p>

<p>
The spec for <code>hash&lt;optional&lt;T&gt;&gt;</code> doesn't say anything about disengaged
objects, so 3.65 <a href="https://wg21.link/defns.undefined">[defns.undefined]</a> would imply it's undefined behaviour,
but that's very unhelpful to users.
<p/>
If hashing disengaged optional objects is undefined there should be a <i>Requires</i>, otherwise 
there should be some statement saying it's OK.
<p/>
It would be possible to specify the value, e.g. saying it returns the
same value as something like <code>std::hash&lt;void*&gt;()(nullptr)</code>, but leaving
it unspecified would permit users to specialize <code>hash&lt;optional&lt;UserDefinedType&gt;&gt;</code> so 
that the hash value for a disengaged object is distinct from any value returned by
<code>hash&lt;UserDefinedType&gt;</code>.
</p>

<p><i>[2014-06-06 pre-Rapperswill]</i></p>

<p>
This issue has been reopened as fundamentals-ts.
</p>

<p><i>[2014-06-07 Daniel comments]</i></p>

<p>
This issue should be set to <b>Resolved</b>, because the wording fix is already applied in the 
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4023.html#optional.hash">last fundamentals working draft</a>.
</p>

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

<p>
Confirmed that this issue is resolved in the current Library Fundamentals working paper.
</p>



<p id="res-2333"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3691.</p>

<ol>
<li><p>Add to 5.11 <a href="https://cplusplus.github.io/fundamentals-ts/v1.html#optional.hash">[fund.ts::optional.hash]</a>/3</p>

<blockquote><pre>
template &lt;class T&gt; struct hash&lt;optional&lt;T&gt;&gt;;
</pre><blockquote>
<p>
[&hellip;]
<p/>
-3- For an object <code>o</code> of type <code>optional&lt;T&gt;</code>, if <code>bool(o) == true</code>, 
<code>hash&lt;optional&lt;T&gt;&gt;()(o)</code> shall evaluate to the same value as 
<code>hash&lt;T&gt;()(*o)</code> <ins>otherwise it evaluates to an unspecified value</ins>.
</p>
</blockquote></blockquote>
</li>

</ol>





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