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<title>Issue 4235: cache_latest_view and to_input_view miss reserve_hint</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#WP">WP</a> status.</em></p>
<h3 id="4235"><a href="lwg-defects.html#4235">4235</a>. <code>cache_latest_view</code> and <code>to_input_view</code> miss <code>reserve_hint</code></h3>
<p><b>Section:</b> 25.7.34.2 <a href="https://wg21.link/range.cache.latest.view">[range.cache.latest.view]</a>, 25.7.35.2 <a href="https://wg21.link/range.to.input.view">[range.to.input.view]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-03-25 <b>Last modified:</b> 2025-06-23</p>
<p><b>Priority: </b>2
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Intuitively, both view classes should also have <code>reserve_hint</code> members.
</p>

<p><i>[2025-06-12; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>


<p><i>[2025-06-13; Reflector poll]</i></p>

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

<p><i>[Sofia 2025-06-21; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4235"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>

<li><p>Modify 25.7.34.2 <a href="https://wg21.link/range.cache.latest.view">[range.cache.latest.view]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt;
  class cache_latest_view : public view_interface&lt;cache_latest_view&lt;V&gt;&gt; {
    [&hellip;]
    constexpr auto size() requires sized_range&lt;V&gt;;
    constexpr auto size() const requires sized_range&lt;const V&gt;;

    <ins>constexpr auto reserve_hint() requires approximately_sized_range&lt;V&gt;;
    constexpr auto reserve_hint() const requires approximately_sized_range&lt;const V&gt;;</ins>
  };
  [&hellip;]
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr auto size() requires sized_range&lt;V&gt;;
constexpr auto size() const requires sized_range&lt;const V&gt;;
</pre>
<blockquote>
<p>
-4- <i>Effects</i>: Equivalent to: <code>return ranges::size(<i>base_</i>);</code>
</p>
</blockquote>
<pre><ins>
constexpr auto reserve_hint() requires approximately_sized_range&lt;V&gt;;
constexpr auto reserve_hint() const requires approximately_sized_range&lt;const V&gt;;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return ranges::reserve_hint(<i>base_</i>);</code></ins>
</p>
</blockquote>
</blockquote>

</li>

<li><p>Modify 25.7.35.2 <a href="https://wg21.link/range.to.input.view">[range.to.input.view]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
template&lt;input_range V&gt;
  requires view&lt;V&gt;
class to_input_view : public view_interface&lt;to_input_view&lt;V&gt;&gt; {
  [&hellip;]
  constexpr auto size() requires sized_range&lt;V&gt;;
  constexpr auto size() const requires sized_range&lt;const V&gt;;

  <ins>constexpr auto reserve_hint() requires approximately_sized_range&lt;V&gt;;
  constexpr auto reserve_hint() const requires approximately_sized_range&lt;const V&gt;;</ins>
};
[&hellip;]
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr auto size() requires sized_range&lt;V&gt;;
constexpr auto size() const requires sized_range&lt;const V&gt;;
</pre>
<blockquote>
<p>
-5- <i>Effects</i>: Equivalent to: <code>return ranges::size(<i>base_</i>);</code>
</p>
</blockquote>
<pre><ins>
constexpr auto reserve_hint() requires approximately_sized_range&lt;V&gt;;
constexpr auto reserve_hint() const requires approximately_sized_range&lt;const V&gt;;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return ranges::reserve_hint(<i>base_</i>);</code></ins>
</p>
</blockquote>
</blockquote>

</li>

</ol>





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