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<title>Issue 3381: begin and data must agree for contiguous_range</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++20">C++20</a> status.</em></p>
<h3 id="3381"><a href="lwg-defects.html#3381">3381</a>. <code>begin</code> and <code>data</code> must agree for <code>contiguous_range</code></h3>
<p><b>Section:</b> 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-01-25 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.refinements">issues</a> in [range.refinements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The definition of the <code>contiguous_range</code> concept in 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a>/2 
requires that <code>ranges::data(r)</code> be valid for a <code>contiguous_range r</code>, but fails to 
impose the obvious semantic requirement that <code>to_address(ranges::begin(r)) == ranges::data(r)</code>. 
In other words, <code>data</code> and <code>begin</code> must agree so that <code>[begin(r), end(r))</code> and 
the counted range <code>data(r) + [0, size(r))</code> (this is the new "counted range" specification 
syntax per <a href="https://github.com/cplusplus/draft/issues/2932">working draft issue 2932</a>) denote 
the same sequence of elements.
</p>

<p><i>[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]</i></p>



<p id="res-3381"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.</p>

<ol>
<li><p>Modify 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a> as indicated:</p>

<blockquote>
<p>
-2- <code>contiguous_range</code> additionally requires that the <code>ranges::data</code> customization point 
(25.3.14 <a href="https://wg21.link/range.prim.data">[range.prim.data]</a>) is usable with the range.
</p>
<pre>
template&lt;class T&gt;
  concept contiguous_range =
    random_access_range&lt;T&gt; &amp;&amp; contiguous_iterator&lt;iterator_t&lt;T&gt;&gt; &amp;&amp;
    requires(T&amp; t) {
      { ranges::data(t) } -&gt; same_as&lt;add_pointer_t&lt;range_reference_t&lt;T&gt;&gt;&gt;;
    };
</pre>
<p>
<ins>-?- Given an expression <code>t</code> such that <code>decltype((t))</code> is <code>T&amp;</code>, <code>T</code> 
models <code>contiguous_range</code> only if <code>(to_address(ranges::begin(t)) == ranges::data(t))</code>.</ins>
<p/>
-3- The <code>common_range</code> concept [&hellip;]
</p>
</blockquote>
</li>
</ol>





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