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<title>Issue 3796: movable-box as member should use default-initialization instead of copy-initialization</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="3796"><a href="lwg-defects.html#3796">3796</a>. <code><i>movable-box</i></code> as member should use default-initialization instead of copy-initialization</h3>
<p><b>Section:</b> 25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a>, 25.7.31.2 <a href="https://wg21.link/range.chunk.by.view">[range.chunk.by.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-10-20 <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#range.repeat.view">issues</a> in [range.repeat.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>
Currently, the member variable <code><i>value_</i></code> of <code>repeat_view</code> is initialized with the following expression:
</p>
<blockquote><pre>
<i>movable-box</i>&lt;W&gt; <i>value_</i> = W();
</pre></blockquote>
<p>
which will result in the following unexpected error in <a href="https://godbolt.org/z/jK7djP6GT">libstdc++</a>:
</p>
<blockquote><pre>
#include &lt;ranges&gt;

int main() {
  std::ranges::repeat_view&lt;int&gt; r; // <span style="color:red;font-weight:bolder">error: could not convert '0' from 'int' to 'std::ranges::__detail::__box&lt;int&gt;'</span>
}
</pre></blockquote>
<p>
This is because the single-argument constructors of the simple version of <code><i>movable-box</i></code> in libstdc++ 
are declared as <code>explicit</code>, which is different from the conditional <code>explicit</code> declared by the 
<code>optional</code>'s constructor, resulting in no visible conversion between those two types.
<p/>
For MSVC-STL, the simple version of <code><i>movable-box</i></code> does not provide a single-argument constructor, 
so this form of initialization will also produce a hard error.
</p>
<p>
This may be a bug of existing implementations, but we don't need such "copy-initialization", the default-constructed 
<code><i>movable-box</i></code> already value-initializes the underlying type.
<p/>
Simply using default initialization, as most other range adaptors do, guarantees consistency, which also 
eliminates extra move construction.
</p>

<p><i>[2022-11-01; Reflector poll]</i></p>

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

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



<p id="res-3796"><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 25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;move_constructible W, semiregular Bound = unreachable_sentinel_t&gt;
    requires (is_object_v&lt;W&gt; &amp;&amp; same_as&lt;W, remove_cv_t&lt;W&gt;&gt; &amp;&amp;
              (<i>is-integer-like</i>&lt;Bound&gt; || same_as&lt;Bound, unreachable_sentinel_t&gt;))
   class repeat_view : public view_interface&lt;repeat_view&lt;W, Bound&gt;&gt; {
   private:
     // <i>25.6.5.3 <a href="https://wg21.link/range.repeat.iterator">[range.repeat.iterator]</a>, class repeat_view::<i>iterator</i></i>
     struct <i>iterator</i>;                            // <i>exposition only</i>

     <i>movable-box</i>&lt;W&gt; <i>value_</i><del> = W()</del>;                // <i>exposition only, see 25.7.3 <a href="https://wg21.link/range.move.wrap">[range.move.wrap]</a></i>
     Bound <i>bound_</i> = Bound();                     // <i>exposition only</i>
     [&hellip;]
   };
   [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.31.2 <a href="https://wg21.link/range.chunk.by.view">[range.chunk.by.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, indirect_binary_predicate&lt;iterator_t&lt;V&gt;, iterator_t&lt;V&gt;&gt; Pred&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;Pred&gt;
  class chunk_by_view : public view_interface&lt;chunk_by_view&lt;V, Pred&gt;&gt; {
    V <i>base_</i> = V();                                          // <i>exposition only</i>
    <i>movable-box</i>&lt;Pred&gt; <i>pred_</i><del> = Pred()</del>;                       // <i>exposition only</i>

    // <i>25.7.31.3 <a href="https://wg21.link/range.chunk.by.iter">[range.chunk.by.iter]</a></i>, <i>class</i> chunk_by_view::<i>iterator</i>
    class <i>iterator</i>;                                         // <i>exposition only</i>
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>

</li>
</ol>





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