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<title>Issue 3722: ranges::to reserves the wrong size</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#New">New</a> status.</em></p>
<h3 id="3722"><a href="lwg-active.html#3722">3722</a>. <code>ranges::to</code> <code>reserve</code>s the wrong size</h3>
<p><b>Section:</b> 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-06-20 <b>Last modified:</b> 2025-02-27</p>
<p><b>Priority: </b>4
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.utility.conv.to">active issues</a> in [range.utility.conv.to].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.utility.conv.to">issues</a> in [range.utility.conv.to].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In bullet 1.1.4 of <code>ranges::to</code>, if the <code>Container</code> satisfies <code><i>container-insertable</i></code> 
and <code>R</code> models <code>sized_range</code>, it will first construct the <code>Container</code> with <code>args...</code> 
and then preallocate memory by calling <code>c.reserve()</code>.
</p>
<p>
However, this only makes sense when <code>c</code> is default-initialized. If instead the size of the <code>Container</code> 
created by <code>args...</code> is not <code>0</code>, the value passed into <code>c.reserve()</code> will be wrong, for example:
</p>
<blockquote><pre>
ranges::to&lt;std::string&gt;(std::views::single('o'), "hell");
</pre></blockquote>
<p>
The size of the <code>string</code> created by <code>"hell"</code> is already <code>4</code>, whereas the size of 
<code>R</code> is only <code>1</code>, which makes <code>c.reserve(1)</code> useless.
</p>

<p><i>[2022-07-08; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll.
Some suggestions for NAD.
</p>
<ul>
<li>
"The intended use of those additional arguments is for allocators,
comparators, max load factors, and the like. Not...this. I can't really bring
myself to care about this example - and it adds some (admittedly small)
overhead for the by far common case."
</li>
<li>
"And it's not the right fix imo. if we wanted to put *some* constraints on Args, we could attempt to do so, although that might be challenging."
</li>
<li>
"calling reserve() is just an attempt at an optimization."
</li>
<li>
"I disagree with NAD given the fact that we have specifically designed
the "reserve" branch of the decision tree."
</li>
</ul>

<p><i>[2025-02-27; post-Hagenberg status]</i></p>

<p>
The proposed resolution needs rebasing after <a href="https://wg21.link/P2846R6" title=" reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges">P2846R6</a>
was approved in Hagenberg.
It also conflicts with LWG <a href="lwg-active.html#4066" title="ranges::to should reserve when sized_sentinel_for is satisfied (Status: New)">4066</a><sup><a href="https://cplusplus.github.io/LWG/issue4066" title="Latest snapshot">(i)</a></sup>.
</p>



<p id="res-3722"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;C&gt;)
  constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Returns</i>: An object of type <code>C</code> constructed from the elements of <code>r</code> in the following manner:
</p>
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; If <code>convertible_to&lt;range_reference_t&lt;R&gt;, range_value_t&lt;C&gt;&gt;</code> is <code>true</code>:</p>
<ol style="list-style-type: none">
<li><p>(1.1.1) &mdash; If <code>constructible_from&lt;C, R, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
<li><p>(1.1.2) &mdash; Otherwise, if <code>constructible_from&lt;C, from_range_t, R, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(from_range, std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
<li><p>(1.1.3) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(1.1.3.1) &mdash; <code>common_range&lt;R&gt;</code> is <code>true</code>,</p></li>
<li><p>(1.1.3.2) &mdash; <code><i>cpp17-input-iterator</i>&lt;iterator_t&lt;R&gt;&gt;</code> is <code>true</code>, and</p></li>
<li><p>(1.1.3.3) &mdash; <code>constructible_from&lt;C, iterator_t&lt;R&gt;, sentinel_t&lt;R&gt;, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(ranges::begin(r), ranges::end(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
</ol>
</li>
<li><p>(1.1.4) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(1.1.4.1) &mdash; <code>constructible_from&lt;C, Args...&gt;</code> is <code>true</code>, and</p></li>
<li><p>(1.1.4.2) &mdash; <code><i>container-insertable</i>&lt;C, range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
C c(std::forward&lt;Args&gt;(args)...);
if constexpr (sized_range&lt;R&gt; &amp;&amp; <i>reservable-container</i>&lt;C&gt;) <ins>{
  using ST = range_size_t&lt;C&gt;;
  using CT = common_type_t&lt;ST, range_size_t&lt;R&gt;&gt;;
  auto sz = ST(CT(ranges::size(c)) + CT(ranges::size(r)));</ins>
  c.reserve(<ins>sz</ins><del>ranges::size(r)</del>);
<ins>}</ins>
ranges::copy(r, <i>container-inserter</i>&lt;range_reference_t&lt;R&gt;&gt;(c));
</pre></blockquote></li>
</ol>
</li>
</ol>
</li>
<li><p>(1.2) &mdash; Otherwise, if <code>input_range&lt;range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
to&lt;C&gt;(r | views::transform([](auto&amp;&amp; elem) {
  return to&lt;range_value_t&lt;C&gt;&gt;(std::forward&lt;decltype(elem)&gt;(elem));
}), std::forward&lt;Args&gt;(args)...);
</pre></blockquote></li>
<li><p>(1.3) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol>
</blockquote>
</blockquote>
</li>

</ol>





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