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<title>Issue 4125: move_iterator's default constructor should be constrained</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#New">New</a> status.</em></p>
<h3 id="4125"><a href="lwg-active.html#4125">4125</a>. <code>move_iterator</code>'s default constructor should be constrained</h3>
<p><b>Section:</b> 24.5.4.2 <a href="https://wg21.link/move.iterator">[move.iterator]</a>, 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-22 <b>Last modified:</b> 2025-02-07</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#move.iterator">active issues</a> in [move.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#move.iterator">issues</a> in [move.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Although it is unclear why <a href="https://wg21.link/P2325" title=" Views should not be required to be default constructible">P2325</a> did not apply to <code>move_iterator</code>, there is 
implementation divergence among the current libraries (<a href="https://godbolt.org/z/WdoPcjjv4">demo</a>):
</p>
<blockquote><pre>
#include &lt;istream&gt;
#include &lt;iterator&gt;
#include &lt;ranges&gt;

using R = std::ranges::istream_view&lt;int&gt;;
using I = std::ranges::iterator_t&lt;R&gt;;
using MI = std::move_iterator&lt;I&gt;;
static_assert(std::default_initializable&lt;MI&gt;); // <span  style="color:#C80000;font-weight:bold">libstdc++ passes, libc++ fails</span>
</pre></blockquote>
<p>
As libc++ additionally requires that its default constructors satisfy <code>is_constructible_v&lt;Iterator&gt;</code>.
<p/>
Although this is not current standard-conforming behavior, such constraint does make sense since <code>move_iterator</code> 
only requires the underlying iterator to be <code>input_iterator</code> which may not be <code>default_initializable</code>.
</p>

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

<p>
Set priority to 3 after reflector poll.
Six P0 (tentatively ready) votes but one for NAD:
"design change, needs paper. Not constraining this seems to be intended by
<a href="https://wg21.link/P2325R3" title=" Views should not be required to be default constructible">P2325R3</a>. Even if we want to constrain it,
why <code class='backtick'>default_initializable</code> rather than just <code class='backtick'>is_constructible_v</code>?"
Author of <a href="https://wg21.link/P2325R3" title=" Views should not be required to be default constructible">P2325R3</a> had no such intent and voted P0.
Issue submitter used <code class='backtick'>default_initializable</code> to align with <code class='backtick'>forward_iterator</code>
requirements, but would not object to implementers using <code class='backtick'>is_constructible_v</code>
for backported DRs.
</p>



<p id="res-4125"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
</p>

<ol>
<li><p>Modify 24.5.4.2 <a href="https://wg21.link/move.iterator">[move.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Iterator&gt;
  class move_iterator {
  public:
    [&hellip;]
    constexpr move_iterator() <ins>requires default_initializable&lt;Iterator&gt; = default</ins>;
    [&hellip;]
  private:
    Iterator current <ins>= Iterator()</ins>;   // <i>exposition only</i>
  };
}
</pre>
</blockquote>

</li>

<li><p>Modify 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> as indicated:</p>

<blockquote>
<pre>
<del>constexpr move_iterator();</del>
</pre>
<blockquote>
<p>
<del>-1- <i>Effects</i>: Value-initializes <code>current</code>.</del>
</p>
</blockquote>
</blockquote>

</li>
</ol>





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