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<title>Issue 2791: string_view objects and strings should yield the same hash values</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="2791"><a href="lwg-defects.html#2791">2791</a>. <code>string_view</code> objects and strings should yield the same hash values</h3>
<p><b>Section:</b> 27.3.6 <a href="https://wg21.link/string.view.hash">[string.view.hash]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Nicolai Josuttis  <b>Opened:</b> 2016-11-09 <b>Last modified:</b> 2016-11-21</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>
Under certain conditions we want to be able to use <code>string_view</code> instead
of <code>string</code>. As a consequence both types should behave the same as long we
talk about value specific behavior (if possible). For this reason, we should require 
that both strings and <code>string_view</code> yield the same hash values.
<p/>
Should be solved in C++17 to ensure that following this use
does not require to change hash values (which is allowed but possibly
unfortunate).
</p>

<p><i>[2016-11-12, Issaquah]</i></p>

<p>Resolved by P0513R0</p>


<p id="res-2791"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4606.</p>

<blockquote class="note">
<p>
A more formal formulation would be:
</p>
<blockquote>
<p>
For any <code>sv</code> of type <code>SV</code> (being <code>string_view</code>, <code>u16string_view</code>,
<code>u32string_view</code>, or <code>wstring_view</code>) and <code>s</code> of the corresponding
type <code>S</code> (being <code>string</code>, <code>u16string</code>, <code>u32string</code>, or <code>wstring</code>),
<code>hash&lt;SV&gt;()(sv) == hash&lt;S&gt;()(s)</code>.
</p>
</blockquote>
</blockquote>

<ol>
<li><p>Edit 27.3.6 <a href="https://wg21.link/string.view.hash">[string.view.hash]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;&gt; struct hash&lt;string_view&gt;;
template&lt;&gt; struct hash&lt;u16string_view&gt;;
template&lt;&gt; struct hash&lt;u32string_view&gt;;
template&lt;&gt; struct hash&lt;wstring_view&gt;>;
</pre>
<blockquote>
<p>
-1- The template specializations shall meet the requirements of class template <code>hash</code> (22.10.19 <a href="https://wg21.link/unord.hash">[unord.hash]</a>). 
<ins>The hash values shall be the same as the hash values for the corresponding string class 
(27.4.6 <a href="https://wg21.link/basic.string.hash">[basic.string.hash]</a>).</ins>
</p>
</blockquote>
</blockquote>
</li>
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