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<title>Issue 3520: iter_move and iter_swap are inconsistent for transform_view::iterator</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="3520"><a href="lwg-defects.html#3520">3520</a>. <code>iter_move</code> and <code>iter_swap</code> are inconsistent for <code>transform_view::<i>iterator</i></code></h3>
<p><b>Section:</b> 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-02-03 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.transform.iterator">issues</a> in [range.transform.iterator].</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>
For <code>transform_view::<i>iterator</i></code>, <code>iter_move</code> is specified to operate on the
transformed value but <code>iter_swap</code> is specified to operate on the underlying iterator.
<p/>
Consider the following test case:
</p>
<blockquote><pre>
struct X { int x; int y; };
std::vector&lt;X&gt; v = {...};
auto t = v | views::transform(&amp;X::x);
ranges::sort(t);
</pre></blockquote>
<p>
<code>iter_swap</code> on <code>t</code>'s iterators would swap the whole <code>X</code>, including the 
<code>y</code> part, but <code>iter_move</code> will only move the <code>x</code> data member and leave 
the <code>y</code> part intact. Meanwhile, <code>ranges::sort</code> can use both <code>iter_move</code> and
<code>iter_swap</code>, and does so in at least one implementation. The mixed behavior means that we 
get neither "sort <code>X</code>s by their <code>x</code> data member" (as <code>ranges::sort(v, {}, &amp;X::x)</code> 
would do), nor "sort the <code>x</code> data member of these <code>X</code>s and leave the rest unchanged", 
as one might expect, but instead some arbitrary permutation of <code>y</code>. This seems like a
questionable state of affairs.
</p>

<p><i>[2021-03-12; Reflector poll]</i></p>

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


<p><i>[2021-03-12; LWG telecon]</i></p>

<p>
Set status to Tentatively Ready after discussion and poll.
<table>
<tr><th>F</th><th>A</th><th>N</th></tr>
<tr><td>9</td><td>0</td><td>0</td></tr>
</table>
</p>

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3520"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4878">N4878</a>. 
</p>

<ol>
<li><p>Modify 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, copy_constructible F&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
             regular_invocable&lt;F&amp;, range_reference_t&lt;V&gt;&gt; &amp;&amp;
             <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;V&gt;&gt;&gt;
  template&lt;bool Const&gt;
  class transform_view&lt;V, F&gt;::<i>iterator</i> {
    [&hellip;]
    <del>friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
      noexcept(noexcept(ranges::iter_swap(x.<i>current_</i>, y.<i>current_</i>)))
      requires indirectly_swappable&lt;iterator_t&lt;<i>Base</i>&gt;&gt;;</del>
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
<del>friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  noexcept(noexcept(ranges::iter_swap(x.<i>current_</i>, y.<i>current_</i>)))
  requires indirectly_swappable&lt;iterator_t&lt;<i>Base</i>&gt;&gt;;</del>
</pre>
<blockquote>
<p>
<del>-23- <i>Effects:</i> Equivalent to <code>ranges::iter_swap(x.<i>current_</i>, y.<i>current_</i>)</code>.</del>
</p>
</blockquote>
</blockquote>
</li>

</ol>





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