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<title>Issue 4006: chunk_view::outer-iterator::value_type should provide empty</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#NAD">Tentatively NAD</a> status.</em></p>
<h3 id="4006"><a href="lwg-active.html#4006">4006</a>. <code>chunk_view::<i>outer-iterator</i>::value_type</code> should provide <code>empty</code></h3>
<p><b>Section:</b> 25.7.29.4 <a href="https://wg21.link/range.chunk.outer.value">[range.chunk.outer.value]</a> <b>Status:</b> <a href="lwg-active.html#NAD">Tentatively NAD</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-11-05 <b>Last modified:</b> 2024-03-11</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.chunk.outer.value">issues</a> in [range.chunk.outer.value].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively NAD">Tentatively NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>chunk_view::<i>outer-iterator</i>::value_type</code> can determine whether it is empty by simply checking whether the 
<code>chunk_view</code>'s <code><i>remainder_</i></code> is <code>0</code>, which makes it valuable to explicitly provide a 
<code>noexcept empty</code> member.
<p/>
Otherwise, the <code>view_interface::empty</code> is synthesized only through the <code>size</code> member when the original 
sentinel and iterator type model <code>sized_sentinel_for</code>, <a href="https://godbolt.org/z/v3MbzfT9G">which seems overkill</a>:
</p>
<blockquote><pre>
#include &lt;cassert&gt;
#include &lt;iostream&gt;
#include &lt;sstream&gt;
#include &lt;ranges&gt;

int main() {
  auto ints = std::istringstream{"1 2 3 4 5 6 7 8 9 10"};
  for (auto chunk : std::views::istream&lt;int&gt;(ints) | std::views::chunk(3)) {
    for (auto elem : chunk) {
      assert(!chunk.empty()); // <span style="color:red;font-weight:bolder">no matching function for call to 'empty()'</span>
      std::cout &lt;&lt; elem &lt;&lt; " ";
    }
    assert(chunk.empty()); // <span style="color:red;font-weight:bolder">ditto</span>
    std::cout &lt;&lt; "\n";
  }
}
</pre></blockquote>

<p><i>[2024-03-11; Reflector poll]</i></p>

<p>
Set status to Tentatively NAD after reflector poll in November 2023.
</p>
<p>
"The example shows you could use it if it existed, but not why that would be useful."
</p>
<p>
"This is a bad idea - the fact that the chunk 'shrinks' as it is iterated over
is an implementation detail and not supposed to be observable."
</p>



<p id="res-4006"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4964" title=" Working Draft, Programming Languages — C++">N4964</a>.
</p>

<ol>

<li><p>Modify 25.7.29.4 <a href="https://wg21.link/range.chunk.outer.value">[range.chunk.outer.value]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
  namespace std::ranges {
    template&lt;view V&gt;
      requires input_range&lt;V&gt;
    struct chunk_view&lt;V&gt;::<i>outer-iterator</i>::value_type : view_interface&lt;value_type&gt; {
    private:
      chunk_view* <i>parent_</i>;                                        // <i>exposition only</i>

      constexpr explicit value_type(chunk_view&amp; parent);          // <i>exposition only</i>

    public:
      constexpr <i>inner-iterator</i> begin() const noexcept;
      constexpr default_sentinel_t end() const noexcept;

      <ins>constexpr bool empty() const noexcept;</ins>
      constexpr auto size() const
        requires sized_sentinel_for&lt;sentinel_t&lt;V&gt;, iterator_t&lt;V&gt;&gt;;
    };
  }
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr default_sentinel_t end() const noexcept;
</pre>
<blockquote>
<p>
-3- <i>Returns</i>: <code>default_sentinel</code>.
</p>
</blockquote>
<pre>
<ins>constexpr bool empty() const noexcept;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return <i>parent_</i>-&gt;<i>remainder_</i> == 0;</code></ins>
</p>
</blockquote>
</blockquote>

</li>

</ol>





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