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<title>Issue 1087: Incorrect OutputIterator concept requirements for replace algorithms</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#NAD_Concepts">NAD Concepts</a> status.</em></p>
<h3 id="1087"><a href="lwg-closed.html#1087">1087</a>. Incorrect <code>OutputIterator</code> concept requirements for <code>replace</code> algorithms</h3>
<p><b>Section:</b> 26.7.5 <a href="https://wg21.link/alg.replace">[alg.replace]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Concepts">NAD Concepts</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2009-03-22 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.replace">issues</a> in [alg.replace].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Concepts">NAD Concepts</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses UK 301 [CD1]</b></p>

<p>
<code>replace</code> and <code>replace_if</code> have the requirement: <code>OutputIterator&lt;Iter,
Iter::reference&gt;</code> Which implies they need to copy some values in the
range the algorithm is iterating over. This is not however the case, the
only thing that happens is <code>const T&amp;</code>s might be copied over existing
elements (hence the <code>OutputIterator&lt;Iter, const T&amp;&gt;</code>.
</p>

<p>
Remove <code>OutputIterator&lt;Iter, Iter::reference&gt;</code> from <code>replace</code>
and <code>replace_if</code>.
</p>

<p><i>[
Summit:
]</i></p>

<blockquote><p>
We agree. To be handled by Howard.
</p></blockquote>


<p id="res-1087"><b>Proposed resolution:</b></p>
<p>
Change in  [algorithms.syn] and 26.7.5 <a href="https://wg21.link/alg.replace">[alg.replace]</a>:
</p>

<blockquote><pre>
template&lt;ForwardIterator Iter, class T&gt; 
  requires <del>OutputIterator&lt;Iter, Iter::reference&gt; 
        &amp;&amp;</del> OutputIterator&lt;Iter, const T&amp;&gt; 
        &amp;&amp; HasEqualTo&lt;Iter::value_type, T&gt; 
  void replace(Iter first, Iter last, 
               const T&amp; old_value, const T&amp; new_value); 

template&lt;ForwardIterator Iter, Predicate&lt;auto, Iter::value_type&gt; Pred, class T&gt; 
  requires <del>OutputIterator&lt;Iter, Iter::reference&gt; 
        &amp;&amp;</del> OutputIterator&lt;Iter, const T&amp;&gt; 
        &amp;&amp; CopyConstructible&lt;Pred&gt; 
  void replace_if(Iter first, Iter last,
                  Pred pred, const T&amp; new_value);
</pre></blockquote>





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