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<title>Issue 1072: Is std::hash a constrained template or not?</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#NAD_Concepts">NAD Concepts</a> status.</em></p>
<h3 id="1072"><a href="lwg-closed.html#1072">1072</a>. Is <code>std::hash</code> a constrained template or not?</h3>
<p><b>Section:</b> 22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Concepts">NAD Concepts</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-19 <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#unord.hash">issues</a> in [unord.hash].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Concepts">NAD Concepts</a> status.</p>
<p><b>Discussion:</b></p>

<p>
Is <code>std::hash</code> a constrained template or not?
</p>
<p>
According to class template <code>hash</code> 22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a>, the definition is:
</p>

<blockquote><pre>
template &lt;class T&gt;
struct hash : public std::unary_function&lt;T, std::size_t&gt; {
  std::size_t operator()(T val) const;
};
</pre></blockquote>

<p>
And so unconstrained.
</p>
<p>
According to the <code>&lt;functional&gt;</code> synopsis in p2 Function objects
22.10 <a href="https://wg21.link/function.objects">[function.objects]</a> the template is declared as:
</p>

<blockquote><pre>
template &lt;ReferentType T&gt; struct hash;
</pre></blockquote>

<p>
which would make hash a constrained template.
</p>

<p><i>[
2009-03-22 Daniel provided wording.
]</i></p>


<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
Alisdair is not certain that Daniel's proposed resolution is sufficient,
and recommends we leave the hash template unconstrained for now.
</p>
<p>
Recommend that the Project Editor make the constrained declaration consistent
with the definition in order to make the Working Paper internally consistent,
and that the issue then be revisited.
</p>
<p>
Move to Open.
</p>
</blockquote>


<p id="res-1072"><b>Proposed resolution:</b></p>

<p>
[To the editor: This resolution is merge-compatible to the
resolution of <a href="lwg-closed.html#1078" title="DE-17: Remove class type_index (Status: NAD Concepts)">1078</a><sup><a href="https://cplusplus.github.io/LWG/issue1078" title="Latest snapshot">(i)</a></sup>]
</p>

<ol>
<li>
<p>
In 22.10 <a href="https://wg21.link/function.objects">[function.objects]</a>/2, header <code>&lt;functional&gt;</code> synopsis, change as indicated:
</p>

<blockquote><pre>
// 20.6.17, hash function base template:
template &lt;ReferentType T&gt; struct hash; <ins>// undefined</ins>
</pre></blockquote>
</li>
<li>
<p>
In 22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a>/1 change as indicated:
</p>
<blockquote><pre>
namespace std {
 <del>template &lt;class T&gt;
 struct hash : public std::unary_function&lt;T, std::size_t&gt; {
 std::size_t operator()(T val) const;
 };</del>
 <ins>template &lt;ReferentType T&gt; struct hash; // undefined</ins>
}
</pre></blockquote>
</li>
<li>
<p>
In 22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a>/2 change as indicated:
</p>

<blockquote><p>
-2-  <ins>For all library-provided specializations, the template
instantiation <code>hash&lt;T&gt;</code>
  shall provide a public <code>operator()</code> with return type <code>std::size_t</code> to
satisfy the concept
  requirement <code>Callable&lt;const hash&lt;T&gt;, const T&amp;&gt;</code>. If <code>T</code> is an object
type or reference to
  object, <code>hash&lt;T&gt;</code> shall be publicly derived from
<code>std::unary_function&lt;T, std::size_t&gt;</code>.
  </ins> The return value of <code>operator()</code> is unspecified, except that
equal arguments
  shall yield the same result. <code>operator()</code> shall not throw exceptions.
</p></blockquote>
</li>
<li>
<p>
In 17.7 <a href="https://wg21.link/support.rtti">[support.rtti]</a>/1, header <code>&lt;typeinfo&gt;</code> synopsis change as indicated:
</p>
<blockquote><pre>
namespace std {
  class type_info;
  class type_index;
  template &lt;<del>class</del><ins>ReferentType</ins> T&gt; struct hash;
</pre></blockquote>
</li>
</ol>





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