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<title>Issue 3607: contiguous_iterator should not be allowed to have custom iter_move and iter_swap behavior</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="3607"><a href="lwg-defects.html#3607">3607</a>. <code>contiguous_iterator</code> should not be allowed to have custom <code>iter_move</code> and <code>iter_swap</code> behavior</h3>
<p><b>Section:</b> 24.3.4.14 <a href="https://wg21.link/iterator.concept.contiguous">[iterator.concept.contiguous]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-09-28 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.concept.contiguous">issues</a> in [iterator.concept.contiguous].</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>
The <code>iter_move</code> and <code>iter_swap</code> customization points were introduced
primarily for proxy iterators. Whatever their application to non-proxy
iterators in general, they should not be allowed to have custom
behavior for contiguous iterators &mdash; this new iterator category was
introduced in large part to permit better optimizations, and allowing
custom <code>iter_move/iter_swap</code> prevents such optimizations for a wide
variety of algorithms that are specified to call them.
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>
<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3607"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 24.3.4.14 <a href="https://wg21.link/iterator.concept.contiguous">[iterator.concept.contiguous]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class I&gt;
  concept contiguous_iterator =
    random_access_iterator&lt;I&gt; &amp;&amp;
    derived_from&lt;<i>ITER_CONCEPT</i>(I), contiguous_iterator_tag&gt; &amp;&amp;
    is_lvalue_reference_v&lt;iter_reference_t&lt;I&gt;&gt; &amp;&amp;
    same_as&lt;iter_value_t&lt;I&gt;, remove_cvref_t&lt;iter_reference_t&lt;I&gt;&gt;&gt; &amp;&amp;
    requires(const I&amp; i) {
      { to_address(i) } -&gt; same_as&lt;add_pointer_t&lt;iter_reference_t&lt;I&gt;&gt;&gt;;
    };
</pre>
<blockquote>
<p>
-2- Let <code>a</code> and <code>b</code> be dereferenceable iterators and <code>c</code> be a non-dereferenceable 
iterator of type <code>I</code> such that <code>b</code> is reachable from <code>a</code> and <code>c</code> is 
reachable from <code>b</code>, and let <code>D</code> be <code>iter_difference_t&lt;I&gt;</code>. The type <code>I</code> 
models <code>contiguous_iterator</code> only if
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code>to_address(a) == addressof(*a)</code>,</p></li>
<li><p>(2.2) &mdash; <code>to_address(b) == to_address(a) + D(b - a)</code>, <del>and</del></p></li>
<li><p>(2.3) &mdash; <code>to_address(c) == to_address(a) + D(c - a)</code><del>.</del><ins>,</ins></p></li>
<li><p><ins>(2.?) &mdash; <code>ranges::iter_move(a)</code> has the same type, value category, and effects as 
<code>std::move(*a)</code>, and</ins></p></li>
<li><p><ins>(2.?) &mdash; if <code>ranges::iter_swap(a, b)</code> is well-formed, it has effects
equivalent to <code>ranges::swap(*a, *b)</code>.</ins></p></li>
</ol>
</blockquote>
</blockquote>
</li>

</ol>





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