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<title>Issue 4253: basic_const_iterator should provide iterator_type</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#Ready">Tentatively Ready</a> status.</em></p>
<h3 id="4253"><a href="lwg-active.html#4253">4253</a>. <code class='backtick'>basic_const_iterator</code> should provide <code class='backtick'>iterator_type</code></h3>
<p><b>Section:</b> 24.5.3.3 <a href="https://wg21.link/const.iterators.iterator">[const.iterators.iterator]</a> <b>Status:</b> <a href="lwg-active.html#Ready">Tentatively Ready</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-04-29 <b>Last modified:</b> 2025-06-13</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#const.iterators.iterator">active issues</a> in [const.iterators.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#const.iterators.iterator">issues</a> in [const.iterators.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively Ready">Tentatively Ready</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, iterator adaptors in <code>&lt;iterator&gt;</code> that wrap a single iterator
such as <code class='backtick'>reverse_iterator</code>, <code class='backtick'>move_iterator</code>, and <code class='backtick'>counted_iterator</code> all provide a
public <code class='backtick'>iterator_type</code> member for users to access the underlying iterator type, except for
<code class='backtick'>basic_const_iterator</code> (<a href="https://godbolt.org/z/4hY4qhnPG">demo</a>):
</p>
<blockquote><pre>
#include &lt;iterator&gt;

using I  = int*;
using RI = std::reverse_iterator&lt;I&gt;;
using MI = std::move_iterator&lt;I&gt;;
using CI = std::counted_iterator&lt;I&gt;;
using BI = std::basic_const_iterator&lt;I&gt;;

static_assert(std::same_as&lt;RI::iterator_type, I&gt;);
static_assert(std::same_as&lt;MI::iterator_type, I&gt;);
static_assert(std::same_as&lt;CI::iterator_type, I&gt;);
static_assert(std::same_as&lt;BI::iterator_type, I&gt;); // <span  style="color:#C80000;font-weight:bold">error</span>
</pre></blockquote>
<p>
It seems reasonable to add one for <code class='backtick'>basic_const_iterator</code> for consistency.
</p>

<p><i>[2025-06-12; Reflector poll]</i></p>

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



<p id="res-4253"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>
<ol>

<li><p>Modify 24.5.3.3 <a href="https://wg21.link/const.iterators.iterator">[const.iterators.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  [&hellip;]
  template&lt;input_iterator Iterator&gt;
  class basic_const_iterator {
    Iterator <i>current_</i> = Iterator();                             // <i>exposition only</i>
    using <i>reference</i> = iter_const_reference_t&lt;Iterator&gt;;         // <i>exposition only</i>
    using <i>rvalue-reference</i> =                                    // <i>exposition only</i>
      <i>iter-const-rvalue-reference-t</i>&lt;Iterator&gt;;
          
    public:
      <ins>using iterator_type = Iterator;</ins>
      using iterator_concept = <i>see below</i>;
      using iterator_category = <i>see below</i>;  // <i>not always present</i>
      using value_type = iter_value_t&lt;Iterator&gt;;
      using difference_type = iter_difference_t&lt;Iterator&gt;;
      [&hellip;]
  };
}
</pre>
</blockquote>
</li>
</ol>





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