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<title>Issue 3880: Clarify operator+= complexity for {chunk,stride}_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="3880"><a href="lwg-defects.html#3880">3880</a>. Clarify <code>operator+=</code> complexity for <code>{chunk,stride}_view::<i>iterator</i></code></h3>
<p><b>Section:</b> 25.7.29.7 <a href="https://wg21.link/range.chunk.fwd.iter">[range.chunk.fwd.iter]</a>, 25.7.32.3 <a href="https://wg21.link/range.stride.iterator">[range.stride.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2023-02-09 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
The intent was that the precondition allows the call to <code>ranges::advance</code>,
which otherwise would have time linear in the argument of <code>operator+=</code>,
to actually be implemented using <code>operator+=</code> or equivalent for all but 
the last step. This is at best very non-obvious and should be clarified.
</p>

<p><i>[Issaquah 2023-02-10; LWG issue processing]</i></p>

<p>Move to Immediate for C++23</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3880"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>
<ol>
<li><p>Modify 25.7.29.7 <a href="https://wg21.link/range.chunk.fwd.iter">[range.chunk.fwd.iter]</a> p13 as indicated:</p>
<blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator+=(difference_type x)
  requires random_access_range&lt;<i>Base</i>&gt;;
</pre>
<blockquote>
<p> -12- <i>Preconditions:</i> If <code>x</code> is positive, 
<code>ranges::distance(<i>current_</i>, <i>end_</i>) > <i>n_</i> * (x - 1)</code> is <code>true</code>.
<p/>
[<i>Note 1</i>: If <code>x</code> is negative, 
the <i>Effects</i> paragraph implies a precondition. &mdash; <i>end note</i>]
<p/>
-13- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
if (x > 0) {
  <ins>ranges::advance(<i>current_</i>, <i>n_</i> * (x - 1));</ins>
  <i>missing_</i> = ranges::advance(<i>current_</i>, <i>n_</i><del> * x</del>, <i>end_</i>);
} else if (x &lt; 0) {
  ranges::advance(<i>current_</i>, <i>n_</i> * x + <i>missing_</i>);
  <i>missing_</i> = 0;
}
return *this;
</pre>
</blockquote>
</blockquote>
</blockquote>
</li>
<li><p>Modify 25.7.32.3 <a href="https://wg21.link/range.stride.iterator">[range.stride.iterator]</a> p14 as indicated:</p>
<blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator+=(difference_type n) requires random_access_range&lt;<i>Base</i>&gt;;
</pre>
<blockquote>
<p> -13- <i>Preconditions:</i> If <code>n</code> is positive, 
<code>ranges::distance(<i>current_</i>, <i>end_</i>) > <i>stride_</i> * (n - 1)</code> is <code>true</code>.
<p/>
[<i>Note 1</i>: If <code>n</code> is negative, 
the <i>Effects</i> paragraph implies a precondition. &mdash; <i>end note</i>]
<p/>
-14- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
if (n > 0) {
  <ins>ranges::advance(<i>current_</i>, <i>stride_</i> * (n - 1));</ins>
  <i>missing_</i> = ranges::advance(<i>current_</i>, <i>stride_</i><del> * n</del>, <i>end_</i>);
} else if (n &lt; 0) {
  ranges::advance(<i>current_</i>, <i>stride_</i> * n + <i>missing_</i>);
  <i>missing_</i> = 0;
}
return *this;
</pre>
</blockquote>
</blockquote>
</blockquote>
</li>
</ol>





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