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<title>Issue 4170: contiguous_iterator should require to_address(I{})</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="4170"><a href="lwg-defects.html#4170">4170</a>. <code class='backtick'>contiguous_iterator</code> should require <code class='backtick'>to_address(I{})</code></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#WP">WP</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2024-11-01 <b>Last modified:</b> 2024-11-28</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#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The design intent of the <code class='backtick'>contiguous_iterator</code> concept is that iterators can be converted
to pointers denoting the same sequence of elements. This enables a common range <code class='backtick'>[i, j)</code>
or counted range <code class='backtick'>i + [0, n)</code> to be processed with extremely efficient low-level C
or assembly code that operates on <code class='backtick'>[to_address(i), to_address(j))</code> (respectively
<code class='backtick'>to_address(i) + [0, n)</code>).
</p><p>
A value-initialized iterator <code class='backtick'>I{}</code> can be used to denote the empty ranges <code class='backtick'>[I{}, I{})</code>
and <code class='backtick'>I{} + [0, 0)</code>. While the existing semantic requirements of <code class='backtick'>contiguous_iterator</code> enable us
to convert both dereferenceable and past-the-end iterators with <code class='backtick'>to_address</code>, converting
ranges involving value-initialized iterators to pointer ranges additionally needs
<code class='backtick'>to_address(I{})</code> to be well-defined. Note that <code class='backtick'>to_address</code> is already implicitly
equality-preserving for <code class='backtick'>contiguous_iterator</code> arguments. Given this additional requirement
<code class='backtick'>to_address(I{}) == to_address(I{})</code> and <code class='backtick'>to_address(I{}) == to_address(I{)) + 0</code>
both hold, so the two types of empty ranges involving value-initialized iterators convert
to empty pointer ranges as desired.
</p>

<p><i>[2024-11-13; Reflector poll]</i></p>

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

<p><i>[Wrocław 2024-11-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4170"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4993" title=" Working Draft, Programming Languages — C++">N4993</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>
<p>
-1- The <code class='backtick'>contiguous_iterator</code> concept provides a guarantee that the denoted elements are stored contiguously
in memory.
</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 class='backtick'>a</code> and <code class='backtick'>b</code> be dereferenceable iterators and <code class='backtick'>c</code> be a non-dereferenceable iterator of type
<code class='backtick'>I</code> such that <code class='backtick'>b</code> is reachable from <code class='backtick'>a</code> and <code class='backtick'>c</code> is reachable from <code class='backtick'>b</code>, and let <code class='backtick'>D</code> be
<code>iter_difference_t&lt;I&gt;</code>. The type <code class='backtick'>I</code> models <code class='backtick'>contiguous_iterator</code> only if
</p>

<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code class='backtick'>to_address(a) == addressof(*a)</code>,</p></li>
<li><p>(2.2) &mdash; <code class='backtick'>to_address(b) == to_address(a) + D(b - a)</code>,</p></li>
<li><p>(2.3) &mdash; <code class='backtick'>to_address(c) == to_address(a) + D(c - a)</code>,</p></li>
<li><p><ins>(2.?) &mdash; <code class='backtick'>to_address(I{})</code> is well-defined,</ins></p></li>
<li><p>(2.4) &mdash; <code class='backtick'>ranges::iter_move(a)</code> has the same type, value category, and effects as
<code class='backtick'>std::move(*a)</code>, and</p></li>
<li><p>(2.5) &mdash; if <code class='backtick'>ranges::iter_swap(a, b)</code> is well-formed, it has effects equivalent to
<code class='backtick'>ranges::swap(*a, *b)</code>.</p></li>
</ol>
</blockquote>

</li>
</ol>




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