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<title>Issue 3391: Problems with counted_iterator/move_iterator::base() const &amp;</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="3391"><a href="lwg-defects.html#3391">3391</a>. Problems with <code>counted_iterator/move_iterator::base() const &amp;</code></h3>
<p><b>Section:</b> 24.5.4 <a href="https://wg21.link/move.iterators">[move.iterators]</a>, 24.5.7 <a href="https://wg21.link/iterators.counted">[iterators.counted]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Patrick Palka <b>Opened:</b> 2020-02-07 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#move.iterators">issues</a> in [move.iterators].</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>
It is not possible to use the <code>const &amp;</code> overloads of <code>counted_iterator::base()</code> or
<code>move_iterator::base()</code> to get at an underlying move-only iterator in order to compare it 
against a sentinel.
<p/>
More concretely, assuming issue LWG <a href="lwg-defects.html#3389" title="A move-only iterator still does not have a counted_iterator (Status: C++20)">3389</a><sup><a href="https://cplusplus.github.io/LWG/issue3389" title="Latest snapshot">(i)</a></sup> is fixed, this means that
</p>
<blockquote><pre>
auto v = r | views::take(5);
ranges::begin(v) == ranges::end(v);
</pre></blockquote>
<p>
is invalid when <code>r</code> is a <code>view</code> whose <code>begin()</code> is a move-only input iterator.
The code is invalid because <code>ranges::take_view::sentinel::operator==()</code> must call 
<code>counted_iterator::base()</code> to compare the underlying iterator against its sentinel, and 
therefore this <code>operator==()</code> requires that the underlying iterator is <code>copy_constructible</code>.
<p/>
Suggested resolution:
<p/>
Make these <code>const &amp; base()</code> overloads return the underlying iterator by <code>const</code> reference.  
Remove the <code>copy_constructible</code> constraint on these overloads. Perhaps the <code>base()</code> 
overloads for the iterator wrappers in 25.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a> could use the same treatment?
</p>

<p><i>[2020-02 Prioritized as P2 Monday morning in Prague]</i></p>


<p><i>[2021-01-28; Reflector poll]</i></p>

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

<p><i>[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3391"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.</p>

<ol>
<li><p>Modify 24.5.4.2 <a href="https://wg21.link/move.iterator">[move.iterator]</a>, class template <code>move_iterator</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Iterator&gt;
  class move_iterator {
  public:
    using iterator_type = Iterator;
    [&hellip;]
    constexpr <ins>const</ins> iterator_type<ins>&amp;</ins> base() const &amp;;
    constexpr iterator_type base() &amp;&amp;;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 24.5.4.5 <a href="https://wg21.link/move.iter.op.conv">[move.iter.op.conv]</a> as indicated:</p>

<blockquote>
<pre>
constexpr <ins>const</ins> Iterator<ins>&amp;</ins> base() const &amp;;
</pre>
<blockquote>
<p>
<del>-1- <i>Constraints:</i> <code>Iterator</code> satisfies <code>copy_constructible</code>.</del>
<p/>
<del>-2- <i>Preconditions:</i> <code>Iterator</code> models <code>copy_constructible</code>.</del>
<p/>
-3- <i>Returns:</i> <code>current</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.5.7.1 <a href="https://wg21.link/counted.iterator">[counted.iterator]</a>, class template <code>counted_iterator</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;input_or_output_iterator I&gt;
  class counted_iterator {
  public:
    using iterator_type = I;
    [&hellip;]
    constexpr <ins>const</ins> I<ins>&amp;</ins> base() const &amp; <del>requires copy_constructible&lt;I&gt;</del>;
    constexpr I base() &amp;&amp;;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 24.5.7.3 <a href="https://wg21.link/counted.iter.access">[counted.iter.access]</a> as indicated:</p>

<blockquote>
<pre>
constexpr <ins>const</ins> I<ins>&amp;</ins> base() const &amp; <del>requires copy_constructible&lt;I&gt;</del>;
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> Equivalent to: <code>return current;</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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