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<title>Issue 3958: ranges::to should prioritize the "reserve" branch</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">Tentatively NAD</a> status.</em></p>
<h3 id="3958"><a href="lwg-active.html#3958">3958</a>. <code>ranges::to</code> should prioritize the "<code>reserve</code>" branch</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#NAD">Tentatively NAD</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-07-17 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>Not Prioritized
</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#Tentatively NAD">Tentatively NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When the constructed range object has no range version constructor, <code>ranges::to</code> falls into a 
branch designed specifically for C++17-compliant containers, which calls the legacy constructor that 
accepts an iterator pair with <code>C(ranges::begin(r), ranges::end(r), std::forward&lt;Args&gt;(args)...)</code>.    
<p/>
However, this kind of initialization may bring some performance issues, because we split the original 
range into pairs of iterators, which may lose information contained in the original range, for example:
</p>
<blockquote><pre>
#include &lt;boost/container/vector.hpp&gt;
#include &lt;sstream&gt;
#include &lt;ranges&gt;

int main() {
  std::istringstream ints("1 2 3 4 5");
  std::ranges::subrange s(std::istream_iterator&lt;int&gt;(ints),
                          std::istream_iterator&lt;int&gt;(),
                          5);
  auto r = std::ranges::to&lt;boost::container::vector&gt;(s); // <span style="color:red;font-weight:bolder">discard size info</span>
}
</pre></blockquote>
<p>
Above, <code>subrange</code> saves the size information of the stream, but <code>ranges::to</code> only extracts 
its iterator pair to create the object, so that the original size information is discarded, resulting in 
unnecessary allocations.
<p/>
I believe we should prefer to use the "<code>reserve</code>" branch here because it is really designed for this situation.
</p>

<p><i>[2023-10-30; Reflector poll]</i></p>

<p>
Set status to Tentatively NAD after reflector poll.
"This optimizes exotic cases at the expense of realistic cases."
</p>



<p id="res-3958"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</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>Mandates</i>: <code>C</code> is a cv-unqualified class type.
</p>
<p>
-2- <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>(2.1) &mdash; If <code>C</code> does not satisfy <code>input_range</code> or <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>(2.1.1) &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>(2.1.2) &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><del>(2.1.3) &mdash; Otherwise, if</del></p>
<ol style="list-style-type: none">
<li><p><del>(2.1.3.1) &mdash; <code>common_range&lt;R&gt;</code> is <code>true</code>,</del></p></li>
<li><p><del>(2.1.3.2) &mdash; the <i>qualified-id</i>  
<code>iterator_traits&lt;iterator_t&lt;R&gt;&gt;::iterator_category</code> is valid and denotes a type that models 
<code>derived_from&lt;input_iterator_tag&gt;</code>, and</del></p></li>
<li><p><del>(2.1.3.3) &mdash; <code>constructible_from&lt;C, iterator_t&lt;R&gt;, sentinel_t&lt;R&gt;, Args...&gt;</code> 
is <code>true</code>:</del></p>
<blockquote><pre><del>C(ranges::begin(r), ranges::end(r), std::forward&lt;Args&gt;(args)...)</del></pre></blockquote></li>
</ol>
</li>
<li><p>(2.1.4) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(2.1.4.1) &mdash; <code>constructible_from&lt;C, Args...&gt;</code> is <code>true</code>, and</p></li>
<li><p>(2.1.4.2) &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;)
  c.reserve(static_cast&lt;range_size_t&lt;C&gt;&gt;(ranges::size(r)));
ranges::copy(r, <i>container-inserter</i>&lt;range_reference_t&lt;R&gt;&gt;(c));
</pre></blockquote></li>
</ol>
</li>
<li><p><ins>(?.?.?) &mdash; Otherwise, if</ins></p>
<ol style="list-style-type: none">
<li><p><ins>(?.?.?.?) &mdash; <code>common_range&lt;R&gt;</code> is <code>true</code>,</ins></p></li>
<li><p><ins>(?.?.?.?) &mdash; the <i>qualified-id</i> <code>iterator_traits&lt;iterator_t&lt;R&gt;&gt;::iterator_category</code> 
is valid and denotes a type that models <code>derived_from&lt;input_iterator_tag&gt;</code>, and</ins></p></li>
<li><p><ins>(?.?.?.?) &mdash; <code>constructible_from&lt;C, iterator_t&lt;R&gt;, sentinel_t&lt;R&gt;, Args...&gt;</code> 
is <code>true</code>:</ins></p>
<blockquote><pre><ins>C(ranges::begin(r), ranges::end(r), std::forward&lt;Args&gt;(args)...)</ins></pre></blockquote></li>
</ol>
</li>
</ol>
</li>
<li><p>(2.2) &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>(2.3) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol>
</blockquote>
</blockquote>
</li>

</ol>





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