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<title>Issue 2353: std::next is over-constrained</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++17">C++17</a> status.</em></p>
<h3 id="2353"><a href="lwg-defects.html#2353">2353</a>. <code>std::next</code> is over-constrained</h3>
<p><b>Section:</b> 24.4.3 <a href="https://wg21.link/iterator.operations">[iterator.operations]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2013-12-24 <b>Last modified:</b> 2017-09-07</p>
<p><b>Priority: </b>4
</p>
<p><b>View other</b> <a href="lwg-index-open.html#iterator.operations">active issues</a> in [iterator.operations].</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.operations">issues</a> in [iterator.operations].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In LWG <a href="lwg-defects.html#1011" title="next&#47;prev wrong iterator type (Status: C++11)">1011</a><sup><a href="https://cplusplus.github.io/LWG/issue1011" title="Latest snapshot">(i)</a></sup>, <code>std::next</code> and <code>std::prev</code> were changed 
from accepting <code>InputIterator</code> to accepting
<code>ForwardIterator</code>. This needlessly excludes perfectly legitimate use
cases. Consider the following hypothetical range-based implementation of
drop, which creates a view of a range without the first <code>n</code> elements:
</p>
<blockquote><pre>
template&lt;typename Distance, typename InputRange&gt;
iterator_range&lt;range_iterator_t&lt;InputRange&gt;&gt;
drop(Distance n, InputRange&amp; rng)
{
  return make_iterator_range(
    std::next(std::begin(rng), n),
    std::end(rng)
  );
}
</pre></blockquote>
<p>
I believe this to be a legitimate use case that is currently outlawed by
the standard without cause. See the discussion beginning at
<a href="http://listarchives.isocpp.org/cgi-bin/wg21/message?wg=lib&amp;msg=35313">c++std-lib-35313</a> 
for an in-depth discussion of the issue, in which
Howard Hinnant agreed that it was a defect.
<p/>
(Some discussion then ensued about whether an overload should be added
that only accepts rvalue <code>InputIterators</code> to avoid the surprise that issue
<a href="lwg-defects.html#1011" title="next&#47;prev wrong iterator type (Status: C++11)">1011</a><sup><a href="https://cplusplus.github.io/LWG/issue1011" title="Latest snapshot">(i)</a></sup> sought to address. I make no such attempt, nor do I believe it to
be necessary.)
<p/>
Suggested resolution:
<p/>
Back out the resolution of <a href="lwg-defects.html#1011" title="next&#47;prev wrong iterator type (Status: C++11)">1011</a><sup><a href="https://cplusplus.github.io/LWG/issue1011" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[Lenexa 2015-05-07: Move to Ready]</i></p>



<p id="res-2353"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3797.</p>

<ol>
<li><p>Change 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a>, header <code>&lt;iterator&gt;</code> synopsis, and 24.4.3 <a href="https://wg21.link/iterator.operations">[iterator.operations]</a>
before p.6 as indicated:</p>

<blockquote><pre>
template &lt;class <del>Forward</del><ins>Input</ins>Iterator&gt;
  <del>Forward</del><ins>Input</ins>Iterator next(<del>Forward</del><ins>Input</ins>Iterator x,
    typename std::iterator_traits&lt;<del>Forward</del><ins>Input</ins>Iterator&gt;::difference_type n = 1);
</pre></blockquote>
</li>

</ol>





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