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<title>Issue 3646: std::ranges::view_interface::size returns a signed type</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#C++23">C++23</a> status.</em></p>
<h3 id="3646"><a href="lwg-defects.html#3646">3646</a>. <code>std::ranges::view_interface::size</code> returns a signed type</h3>
<p><b>Section:</b> 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2021-11-29 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#view.interface.general">issues</a> in [view.interface.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
According to 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a>, <code>view_interface::size</code> returns 
the difference between the sentinel and the beginning iterator, which always has a 
signed-integer-like type. However, IIUC the decision that a <code>size</code> member function 
should return an unsigned type by default was made when adopting <a href="https://wg21.link/P1227R2" title=" Signed ssize() functions, unsigned size() functions">P1227R2</a>, 
and the relative changes of the ranges library were done in <a href="https://wg21.link/P1523R1" title=" Views and Size Types">P1523R1</a>. 
I don't know why <code>view_interface::size</code> was unchanged, while <code>ranges::size</code> 
returns an unsigned type in similar situations (25.3.10 <a href="https://wg21.link/range.prim.size">[range.prim.size]</a> (2.5)).
<p/>
If we want to change <code>views_interface::size</code> to return an unsigned type, the both 
overloads should be changed as below:
</p>
<blockquote><pre>
constexpr auto size() requires forward_range&lt;D&gt; &amp;&amp;
  sized_sentinel_for&lt;sentinel_t&lt;D&gt;, iterator_t&lt;D&gt;&gt; {
    return <i>to-unsigned-like</i>(ranges::end(<i>derived</i>()) - ranges::begin(<i>derived</i>()));
  }
constexpr auto size() const requires forward_range&lt;const D&gt; &amp;&amp;
  sized_sentinel_for&lt;sentinel_t&lt;const D&gt;, iterator_t&lt;const D&gt;&gt; {
    return <i>to-unsigned-like</i>(ranges::end(<i>derived</i>()) - ranges::begin(<i>derived</i>()));
  }
</pre></blockquote>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>

<p><i>[2022-06-22; Reflector poll]</i></p>

<p>
LEWG poll approved the proposed resolution
</p>

<p><i>[2022-09-23; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll in July 2022.
</p>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3646"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

<ol>

<li><p>Modify 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a>, class template <code>view_interface</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
constexpr auto size() requires forward_range&lt;D&gt; &amp;&amp;
  sized_sentinel_for&lt;sentinel_t&lt;D&gt;, iterator_t&lt;D&gt;&gt; {
    return <ins><i>to-unsigned-like</i>(</ins>ranges::end(<i>derived</i>()) - ranges::begin(<i>derived</i>())<ins>)</ins>;
  }
constexpr auto size() const requires forward_range&lt;const D&gt; &amp;&amp;
  sized_sentinel_for&lt;sentinel_t&lt;const D&gt;, iterator_t&lt;const D&gt;&gt; {
    return <ins><i>to-unsigned-like</i>(</ins>ranges::end(<i>derived</i>()) - ranges::begin(<i>derived</i>())<ins>)</ins>;
  }
[&hellip;]
</pre>
</blockquote>
</li>

</ol>





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