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<title>Issue 4035: single_view 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#WP">WP</a> status.</em></p>
<h3 id="4035"><a href="lwg-defects.html#4035">4035</a>. <code>single_view</code> should provide <code>empty</code></h3>
<p><b>Section:</b> 25.6.3.2 <a href="https://wg21.link/range.single.view">[range.single.view]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-12-31 <b>Last modified:</b> 2024-04-02</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.single.view">issues</a> in [range.single.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Although <code>single_view::empty</code> can be synthesized through <code>view_interface</code>,
it seems more worthwhile to provide a static <code>empty</code> for it which eliminates the 
need to pass in an object parameter, guarantees <code>noexcept</code>-ness, and is consistent 
with the design of <code>empty_view</code> (<a href="https://godbolt.org/z/Yr6bhM11n">demo</a>):
</p>
<blockquote>
<pre>
#include &lt;ranges&gt;
auto empty = std::views::empty&lt;int&gt;;
static_assert(noexcept(empty.empty()));
static_assert(noexcept(empty.size()));
auto single = std::views::single(0);
static_assert(noexcept(single.empty())); // <span style="color:red;font-weight:bolder">fire</span>
static_assert(noexcept(single.size()));
</pre>
</blockquote>

<p><i>[2024-03-12; Reflector poll]</i></p>

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

<p><i>[Tokyo 2024-03-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4035"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4971" title=" Working Draft, Programming Languages — C++">N4971</a>.
</p>

<ol>

<li><p>Modify 25.6.3.2 <a href="https://wg21.link/range.single.view">[range.single.view]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;move_constructible T&gt;
    requires is_object_v&lt;T&gt;
  class single_view : public view_interface&lt;single_view&lt;T&gt;&gt; {
    [&hellip;]
  public:
    [&hellip;]
    constexpr T* begin() noexcept;
    constexpr const T* begin() const noexcept;
    constexpr T* end() noexcept;
    constexpr const T* end() const noexcept;
    <ins>static constexpr bool empty() noexcept;</ins>
    static constexpr size_t size() noexcept;
    constexpr T* data() noexcept;
    constexpr const T* data() const noexcept;
  };
  [&hellip;]
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr T* end() noexcept;
constexpr const T* end() const noexcept;
</pre>
<blockquote>
<p>
-5- <i>Effects</i>: Equivalent to: <code>return data() + 1;</code>
</p>
</blockquote>
<pre>
<ins>static constexpr bool empty() noexcept;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return false;</code></ins>
</p>
</blockquote>
</blockquote>

</li>

</ol>







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