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<title>Issue 3715: view_interface::empty is overconstrained</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="3715"><a href="lwg-defects.html#3715">3715</a>. <code>view_interface::empty</code> is overconstrained</h3>
<p><b>Section:</b> 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-06-12 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#view.interface.general">issues</a> in [view.interface.general].</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>
Currently, <code>view_interface::empty</code> has the following constraints
</p>
<blockquote><pre>
constexpr bool empty() requires forward_range&lt;D&gt; {
  return ranges::begin(<i>derived</i>()) == ranges::end(<i>derived</i>());
}
</pre></blockquote>
<p>
which seems reasonable, since we need to guarantee the equality preservation of the expression 
<code>ranges::begin(r)</code>.
<p/>
However, this prevents a more efficient way in some cases, i.e., when <code>D</code> models 
<code>sized_range</code>, we only need to determine whether the value of <code>ranges::size</code> is <code>0</code>.
Since <code>sized_range</code> and <code>forward_range</code> are orthogonal to each other, this also 
prevents any range that models <code>sized_range</code> but not <code>forward_range</code>.
<p/>
Consider:
</p>
<blockquote><pre>
#include &lt;iostream&gt;
#include &lt;ranges&gt;

int main() {
  auto f = std::views::iota(0, 5)
         | std::views::filter([](int) { return true; });
  auto r = std::views::counted(f.begin(), 4)
         | std::views::slide(2);
  std::cout &lt;&lt; (r.size() == 0) &lt;&lt; "\n"; // #1
  std::cout &lt;&lt; r.empty() &lt;&lt; "\n";       // #2, <span style="color:red;font-weight:bolder">calls r.begin() == r.end()</span>
}
</pre></blockquote>
<p>
Since <code>r</code> models <code>sized_range</code>, <code>#1</code> will invoke <code>slide_view::size</code>, 
which mainly invokes <code>ranges::distance</code>; However, <code>#2</code> invokes <code>view_interface::empty</code> 
and evaluates <code>r.begin() == r.end()</code>, which constructs the iterator, invokes <code>ranges::next</code>, 
and caches the result, which is unnecessary.
<p/>
Also consider:
</p> 
<blockquote><pre>
#include &lt;iostream&gt;
#include &lt;ranges&gt;

int main() {
  auto i = std::views::istream&lt;int&gt;(std::cin);
  auto r = std::views::counted(i.begin(), 4)
         | std::views::chunk(2);
  std::cout &lt;&lt; (r.size() == 0) &lt;&lt; "\n"; // #1
  std::cout &lt;&lt; !r &lt;&lt; "\n";              // #2, equivalent to r.size() == 0
  std::cout &lt;&lt; r.empty() &lt;&lt; "\n";       // #3, <span style="color:red;font-weight:bolder">ill-formed</span>
}
</pre></blockquote>
<p>
Since <code>r</code> is still <code>sized_range</code>, <code>#1</code> will invoke <code>chunk_view::size</code>.
<code>#2</code> is also well-formed since <code>view_interface::operator bool</code> only requires the 
expression <code>ranges::empty(r)</code> to be well-formed, which first determines the validity of 
<code>r.empty()</code>, and ends up evaluating <code>#1</code>; However, <code>#3</code> is ill-formed since 
<code>r</code> is not a <code>forward_range</code>. 
<p/>
Although we can still use <code>ranges::empty</code> to determine whether <code>r</code> is empty, this 
inconsistency of the validity of <code>!r</code> and <code>r.empty()</code> is quite unsatisfactory.
</p>
<p>
I see no reason to prevent <code>view_interface::empty</code> when <code>D</code> is <code>sized_range</code>, 
since checking whether <code>ranges::size(r) == 0</code> is an intuitive way to check for empty, as 
<code>ranges::empty</code> does.
</p>

<p><i>[2022-06-21; Reflector poll]</i></p>

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

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3715"><b>Proposed resolution:</b></p>
<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 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;class D&gt;
    requires is_class_v&lt;D&gt; &amp;&amp; same_as&lt;D, remove_cv_t&lt;D&gt;&gt;
  class view_interface {
  private:
    constexpr D&amp; <i>derived</i>() noexcept {                  <i>// exposition only</i>
      return static_cast&lt;D&amp;&gt;(*this);
    }
    constexpr const D&amp; <i>derived</i>() const noexcept {      <i>// exposition only</i>
      return static_cast&lt;const D&amp;&gt;(*this);
    }

  public:
    constexpr bool empty() requires <ins>sized_range&lt;D&gt; ||</ins> forward_range&lt;D&gt; {
      <ins>if constexpr (sized_range&lt;D&gt;)
        return ranges::size(<i>derived</i>()) == 0;
      else</ins>
        return ranges::begin(<i>derived</i>()) == ranges::end(<i>derived</i>());
    }
    constexpr bool empty() const requires <ins>sized_range&lt;const D&gt; ||</ins> forward_range&lt;const D&gt; {
      <ins>if constexpr (sized_range&lt;const D&gt;)
        return ranges::size(<i>derived</i>()) == 0;
      else</ins>
        return ranges::begin(<i>derived</i>()) == ranges::end(<i>derived</i>());
    }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

</ol>





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