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<title>Issue 2573: [fund.ts.v2] std::hash&lt;std::experimental::shared_ptr&lt;T&gt;&gt; does not work for arrays</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#TS">TS</a> status.</em></p>
<h3 id="2573"><a href="lwg-defects.html#2573">2573</a>. [fund.ts.v2] <code>std::hash&lt;std::experimental::shared_ptr&lt;T&gt;&gt;</code> does not work for arrays</h3>
<p><b>Section:</b> 8.2.1 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#memory.smartptr.shared">[fund.ts.v2::memory.smartptr.shared]</a> <b>Status:</b> <a href="lwg-active.html#TS">TS</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2015-12-13 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TS">TS</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: fund.ts.v2</b></p>
<p>
The library fundamentals TS does not provide a separate specification for <code>std::hash&lt;std::experimental::shared_ptr&lt;T&gt;&gt;</code>, 
deferring to the C++14 specification in &sect;20.8.2.7/3:
</p>
<blockquote>
<pre>
template &lt;class T&gt; struct hash&lt;shared_ptr&lt;T&gt; &gt;;
</pre>
<blockquote>
<p>
-3- The template specialization shall meet the requirements of class template <code>hash</code> (20.9.13). For an object <code>p</code> 
of type <code>shared_ptr&lt;T&gt;</code>, <code>hash&lt;shared_ptr&lt;T&gt; &gt;()(p)</code> shall evaluate to the same value as
<code>hash&lt;T*&gt;()(p.get())</code>.
</p>
</blockquote>
</blockquote>
<p>
That specification doesn't work if <code>T</code> is an array type (<code>U[N]</code> or <code>U[]</code>), as in this case <code>get()</code> 
returns <code>U*</code>, which cannot be hashed by <code>std::hash&lt;T*&gt;</code>.
</p>


<p id="res-2573"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4562.html">N4562</a>.
</p>

<ol>
<li><p>Insert a new subclause after 8.2.1.3 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#memory.smartptr.shared.cast">[fund.ts.v2::memory.smartptr.shared.cast]</a>:</p>
<blockquote class="note">
<p>
[<i>Note for the editor</i>: The synopses in [header.memory.synop] and [memory.smartptr.shared] should be updated to refer 
to the new subclause rather than C++14 &sect;20.8.2.7]
</p>
</blockquote>

<blockquote>
<p><ins>?.?.?.? <code>shared_ptr</code> hash support [memory.smartptr.shared.hash]</ins></p>
<pre>
<ins>template &lt;class T&gt; struct hash&lt;experimental::shared_ptr&lt;T&gt;&gt;;</ins>
</pre>
<blockquote>
<p>
<ins>-1- The template specialization shall meet the requirements of class template <code>hash</code> (C++14 &sect;20.9.12). 
For an object <code>p</code> of type <code>experimental::shared_ptr&lt;T&gt;</code>, <code>hash&lt;experimental::shared_ptr&lt;T&gt;&gt;()(p)</code> 
shall evaluate to the same value as <code>hash&lt;typename experimental::shared_ptr&lt;T&gt;::element_type*&gt;()(p.get())</code>.</ins>
</p>
</blockquote>
</blockquote>

</li>
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