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<title>Issue 4108: lazy_split_view should be sized_range when pattern is empty tiny-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#SG9">SG9</a> status.</em></p>
<h3 id="4108"><a href="lwg-active.html#4108">4108</a>. <code>lazy_split_view</code> should be <code>sized_range</code> when pattern is empty <code><i>tiny-range</i></code></h3>
<p><b>Section:</b> 25.7.16.2 <a href="https://wg21.link/range.lazy.split.view">[range.lazy.split.view]</a>, 25.7.17.2 <a href="https://wg21.link/range.split.view">[range.split.view]</a> <b>Status:</b> <a href="lwg-active.html#SG9">SG9</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-05-23 <b>Last modified:</b> 2024-08-02</p>
<p><b>Priority: </b>4
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.lazy.split.view">active issues</a> in [range.lazy.split.view].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.lazy.split.view">issues</a> in [range.lazy.split.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#SG9">SG9</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When the pattern range is empty, <code>lazy_split_view</code> will split each element into individual subranges, 
which means its size is equal to the size of the underlying range.
<p/>
Since we already have a <code><i>tiny-range</i></code> that can determine whether the range is empty by its type, 
it seems valuable to provide a <code>size</code> for <code>lazy_split_view</code> in this case, given that we already 
specifically checked for it by <code>Pattern::size() == 0</code> in <code><i>inner-iterator</i>::operator++()</code>.
</p>

<p><i>[2024-08-02; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll. Set Status to SG9.
"Design change".
"<code>tiny-range</code> should accept <code>span&lt;T, 0|1&gt;</code> or <code>ref_view&lt;array&lt;T, 0|1&gt;&gt;</code>,
see related paper <a href="https://wg21.link/P1419" title=" A SFINAE-friendly trait to determine the extent of statically sized containers">P1419</a>."
</p>



<p id="res-4108"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>

<li><p>Modify 25.7.16.2 <a href="https://wg21.link/range.lazy.split.view">[range.lazy.split.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;auto&gt; struct <i>require-constant</i>;                        // <i>exposition only</i>

  template&lt;class R&gt;
  concept <i>tiny-range</i> =                                           // <i>exposition only</i>
     sized_range&lt;R&gt; &amp;&amp;
     requires { typename <i>require-constant</i>&lt;remove_reference_t&lt;R&gt;::size()&gt;; } &amp;&amp;
     (remove_reference_t&lt;R&gt;::size() &lt;= 1);
  
  template&lt;input_range V, forward_range Pattern&gt;
    requires view&lt;V&gt; &amp;&amp; view&lt;Pattern&gt; &amp;&amp;
             indirectly_comparable&lt;iterator_t&lt;V&gt;, iterator_t&lt;Pattern&gt;, ranges::equal_to&gt; &amp;&amp;
             (forward_range&lt;V&gt; || <i>tiny-range</i>&lt;Pattern&gt;)
  class lazy_split_view : public view_interface&lt;lazy_split_view&lt;V, Pattern&gt;&gt; {
    [&hellip;]
    <ins>constexpr auto size()
      requires sized_range&lt;V&gt; &amp;&amp;
               <i>tiny-range</i>&lt;Pattern&gt; &amp;&amp; (Pattern::size() == 0) {
      return ranges::size(<i>base_</i>);
    }</ins>

    <ins>constexpr auto size() const
      requires sized_range&lt;const V&gt; &amp;&amp;
               <i>tiny-range</i>&lt;Pattern&gt; &amp;&amp; (Pattern::size() == 0) {
      return ranges::size(<i>base_</i>);
    }</ins>
  };
  [&hellip;]
}
</pre>
</blockquote>

</li>

<li><p>Modify 25.7.17.2 <a href="https://wg21.link/range.split.view">[range.split.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, forward_range Pattern&gt;
    requires view&lt;V&gt; &amp;&amp; view&lt;Pattern&gt; &amp;&amp;
             indirectly_comparable&lt;iterator_t&lt;V&gt;, iterator_t&lt;Pattern&gt;, ranges::equal_to&gt;
  class split_view : public view_interface&lt;split_view&lt;V, Pattern&gt;&gt; {
    [&hellip;]
    <ins>constexpr auto size()
      requires sized_range&lt;V&gt; &amp;&amp;
               <i>tiny-range</i>&lt;Pattern&gt; &amp;&amp; (Pattern::size() == 0) {
      return ranges::size(<i>base_</i>);
    }</ins>

    <ins>constexpr auto size() const
      requires sized_range&lt;const V&gt; &amp;&amp;
               <i>tiny-range</i>&lt;Pattern&gt; &amp;&amp; (Pattern::size() == 0) {
      return ranges::size(<i>base_</i>);
    }</ins>
  };
  [&hellip;]
}
</pre>
</blockquote>

</li>
</ol>





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