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<title>Issue 3564: transform_view::iterator&lt;true&gt;::value_type and iterator_category should 
use const F&amp;</title>
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use const F&amp;">
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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="3564"><a href="lwg-defects.html#3564">3564</a>. <code>transform_view::<i>iterator</i>&lt;true&gt;::value_type</code> and <code>iterator_category</code> should 
use <code>const F&amp;</code></h3>
<p><b>Section:</b> 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-06-06 <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#range.transform.iterator">issues</a> in [range.transform.iterator].</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>
Iterators obtained from a <code>const transform_view</code> invoke the
transformation function as <code>const</code>, but the <code>value_type</code> and
<code>iterator_category</code> determination uses plain <code>F&amp;</code>, i.e., non-<code>const</code>.
</p>

<p><i>[2021-06-14; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll, send to SG9 for design clarification.
Should <code>r</code> and <code>as_const(r)</code> guarantee same elements?
</p>

<p><i>[2022-07-08; Reflector poll]</i></p>

<p>
SG9 has decided to proceed with this PR in its 2021-09-13 telecon and
not block the issue for a paper on the more general const/non-const
problem.
</p>
<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3564"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, copy_constructible F&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
             regular_invocable&lt;F&amp;, range_reference_t&lt;V&gt;&gt; &amp;&amp;
             <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;V&gt;&gt;&gt;
  template&lt;bool Const&gt;
  class transform_view&lt;V, F&gt;::<i>iterator</i> {
  private:
    [&hellip;]
  public:
    using iterator_concept = <i>see below</i>;
    using iterator_category = <i>see below</i>; <i>// not always present</i>
    using value_type =
      remove_cvref_t&lt;invoke_result_t&lt;<ins><i>maybe-const</i>&lt;Const,</ins> F<ins>&gt;</ins>&amp;, 
      range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;;
    using difference_type = range_difference_t&lt;<i>Base</i>&gt;;  
    [&hellip;]
  };
}
</pre>
</blockquote>
<p>
-1- [&hellip;]
<p/>
-2- The member <i>typedef-name</i> <code>iterator_category</code> is defined if and only if <code><i>Base</i></code> 
models <code>forward_range</code>. In that case, <code>iterator::iterator_category</code> is defined as follows: Let 
<code>C</code> denote the type <code>iterator_traits&lt;iterator_t&lt;<i>Base</i>&gt;&gt;::iterator_category</code>.
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; If <code>is_lvalue_reference_v&lt;invoke_result_t&lt;<ins><i>maybe-const</i>&lt;Const,</ins> 
F<ins>&gt;</ins>&amp;, range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;</code> 
is <code>true</code>, then</p>
<ol style="list-style-type: none">
<li><p>(2.1.1) &mdash; [&hellip;]</p></li>
<li><p>(2.1.2) &mdash; [&hellip;]</p></li>
</ol>
</li>
<li><p>(2.2) &mdash; Otherwise, <code>iterator_category</code> denotes <code>input_iterator_tag</code>.</p></li>
</ol>
</blockquote>
</li>

</ol>




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