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<title>Issue 3717: common_view::end should improve random_access_range case</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="3717"><a href="lwg-defects.html#3717">3717</a>. <code>common_view::end</code> should improve <code>random_access_range</code> case</h3>
<p><b>Section:</b> 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-06-15 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.common.view">issues</a> in [range.common.view].</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>
This issue is part of NB comment <b><a href="https://github.com/cplusplus/nbballot/issues/524">US 47-109</a></b>
26 [ranges] Resolve open issues
</p>

<p>
In view of the fact that random access iterators are only required to work with its difference type, 
<a href="https://wg21.link/P2393R1" title=" Cleaning up integer-class types">P2393R1</a> improves the wording of <code>take_view</code>, which first convert the integer type 
to difference type and then operate with the iterator. However, the paper omits the handling of random 
access iterators in <code>common_view::end</code>, which directly operates on the return types of 
<code>ranges::begin</code> and <code>ranges::size</code>. We should improve this, too.
</p>

<p><i>[2022-07-06; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;view V&gt;
    requires (!common_range&lt;V&gt; &amp;&amp; copyable&lt;iterator_t&lt;V&gt;&gt;)
  class common_view : public view_interface&lt;common_view&lt;V&gt;&gt; {
  private:
    V <i>base_</i> = V();  <i>// exposition only</i>
  public:
    [&hellip;]
    constexpr auto begin() {
      if constexpr (random_access_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
        return ranges::begin(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;V&gt;, sentinel_t&lt;V&gt;&gt;(ranges::begin(<i>base_</i>));
    }

    constexpr auto begin() const requires range&lt;const V&gt; {
      if constexpr (random_access_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
        return ranges::begin(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;const V&gt;, sentinel_t&lt;const V&gt;&gt;(ranges::begin(<i>base_</i>));
    }
    
    constexpr auto end() {
      if constexpr (random_access_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
        return ranges::begin(<i>base_</i>) + <ins>range_difference_t&lt;V&gt;(size())</ins><del>ranges::size(<i>base_</i>)</del>;
      else
        return common_iterator&lt;iterator_t&lt;V&gt;, sentinel_t&lt;V&gt;&gt;(ranges::end(<i>base_</i>));
    }

    constexpr auto end() const requires range&lt;const V&gt; {
      if constexpr (random_access_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
        return ranges::begin(<i>base_</i>) + <ins>range_difference_t&lt;const V&gt;(size())</ins><del>ranges::size(<i>base_</i>)</del>;
      else
        return common_iterator&lt;iterator_t&lt;const V&gt;, sentinel_t&lt;const V&gt;&gt;(ranges::end(<i>base_</i>));
    }

    constexpr auto size() requires sized_range&lt;V&gt; {
      return ranges::size(<i>base_</i>);
    }
    constexpr auto size() const requires sized_range&lt;const V&gt; {
      return ranges::size(<i>base_</i>);
    }
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[Kona 2022-11-08; Discussed at joint LWG/SG9 session. Move to Open]</i></p>

<p><i>[2022-11-09 Tim updates wording per LWG discussion]</i></p>

<p><i>[Kona 2022-11-10; Move to Immediate]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3717"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;view V&gt;
    requires (!common_range&lt;V&gt; &amp;&amp; copyable&lt;iterator_t&lt;V&gt;&gt;)
  class common_view : public view_interface&lt;common_view&lt;V&gt;&gt; {
  private:
    V <i>base_</i> = V();  <i>// exposition only</i>
  public:
    [&hellip;]
    constexpr auto begin() {
      if constexpr (random_access_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
        return ranges::begin(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;V&gt;, sentinel_t&lt;V&gt;&gt;(ranges::begin(<i>base_</i>));
    }

    constexpr auto begin() const requires range&lt;const V&gt; {
      if constexpr (random_access_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
        return ranges::begin(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;const V&gt;, sentinel_t&lt;const V&gt;&gt;(ranges::begin(<i>base_</i>));
    }
    
    constexpr auto end() {
      if constexpr (random_access_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
        return ranges::begin(<i>base_</i>) + ranges::<del>size</del><ins>distance</ins>(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;V&gt;, sentinel_t&lt;V&gt;&gt;(ranges::end(<i>base_</i>));
    }

    constexpr auto end() const requires range&lt;const V&gt; {
      if constexpr (random_access_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
        return ranges::begin(<i>base_</i>) + ranges::<del>size</del><ins>distance</ins>(<i>base_</i>);
      else
        return common_iterator&lt;iterator_t&lt;const V&gt;, sentinel_t&lt;const V&gt;&gt;(ranges::end(<i>base_</i>));
    }

    constexpr auto size() requires sized_range&lt;V&gt; {
      return ranges::size(<i>base_</i>);
    }
    constexpr auto size() const requires sized_range&lt;const V&gt; {
      return ranges::size(<i>base_</i>);
    }
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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