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<title>Issue 3618: Unnecessary iter_move for transform_view::iterator</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="3618"><a href="lwg-defects.html#3618">3618</a>. Unnecessary <code>iter_move</code> for <code>transform_view::iterator</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> Barry Revzin <b>Opened:</b> 2021-10-12 <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.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>
 <code>transform_view</code>'s iterator currently specifies a customization point for <code>iter_move</code>. 
 This customization point does the same thing that the default implementation would do &mdash; 
 <code>std::move(*E)</code> if <code>*E</code> is an lvalue, else <code>*E</code> &mdash; except that it is only 
 there to provide the correct conditional <code>noexcept</code> specification. But for <code>iota_view</code>, 
 we instead provide a <code>noexcept</code>-specifier on the iterator's <code>operator*</code>. We should do 
 the same thing for both, and the simplest thing would be:
<p/>
Change 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> to put the whole body into <code>noexcept</code>:
</p>
<blockquote><pre>
constexpr decltype(auto) operator*() const <ins>noexcept(noexcept(invoke(*<i>parent_</i>-&gt;<i>fun_</i>, *<i>current_</i>)))</ins> {
  return invoke(*<i>parent_</i>-&gt;<i>fun_</i>, *<i>current_</i>);
} 
</pre></blockquote>
<p>
And to remove this:
</p>
<blockquote><pre>
friend constexpr decltype(auto) iter_move(const <i>iterator</i>&amp; i)
  noexcept(noexcept(invoke(*i.<i>parent_</i>-&gt;<i>fun_</i>, *i.<i>current_</i>))) {
  if constexpr (is_lvalue_reference_v&lt;decltype(*i)&gt;)
    return std::move(*i);
  else
    return *i;
}
</pre></blockquote>

<p><i>[2022-01-29; Reflector poll]</i></p>

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

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3618"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

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

<blockquote>
<pre>
[&hellip;]
constexpr decltype(auto) operator*() const <ins>noexcept(noexcept(invoke(*<i>parent_</i>-&gt;<i>fun_</i>, *<i>current_</i>)))</ins> {
  return invoke(*<i>parent_</i>-&gt;<i>fun_</i>, *<i>current_</i>);
}
[&hellip;]
<del>friend constexpr decltype(auto) iter_move(const <i>iterator</i>&amp; i)
  noexcept(noexcept(invoke(*i.<i>parent_</i>-&gt;<i>fun_</i>, *i.<i>current_</i>))) {
  if constexpr (is_lvalue_reference_v&lt;decltype(*i)&gt;)
    return std::move(*i);
  else
    return *i;
}</del>
[&hellip;]
</pre>
</blockquote>
</li>
</ol>





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