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<title>Issue 2256: On vector iterator invalidation</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">NAD</a> status.</em></p>
<h3 id="2256"><a href="lwg-closed.html#2256">2256</a>. On <code>vector</code> iterator invalidation</h3>
<p><b>Section:</b> 23.3.13.5 <a href="https://wg21.link/vector.modifiers">[vector.modifiers]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2013-04-29 <b>Last modified:</b> 2020-05-08</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#vector.modifiers">issues</a> in [vector.modifiers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>

<p>
23.3.13.5 <a href="https://wg21.link/vector.modifiers">[vector.modifiers]</a>/p3 says:
</p>
<blockquote><pre>
iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);
</pre>
<blockquote>
<p>
<i>Effects</i>: Invalidates iterators and references at or after the point of the erase.
</p>
</blockquote></blockquote>
<p>
Consider this example:
</p>
<blockquote><pre>
#include &lt;vector&gt;
#include &lt;cassert&gt;

int main()
{
  typedef std::vector&lt;int&gt; C;
  C c = {1, 2, 3, 4};
  C::iterator i = c.begin() + 1;
  C::iterator j = c.end() - 1;
  assert(*i == 2);
  assert(*j == 4);
  c.erase(c.begin());
  <span style="color:#C80000;font-weight:bold">assert(*i == 3); // Why is this not perfectly fine?!</span>
}
</pre></blockquote>
<p>
Why has the iterator <code>i</code> been invalidated? It still refers to a perfectly reasonable, fully constructed object. 
If <code>vector::iterator</code> were to be implemented as a pointer (which is legal), it is not possible for that last 
line to do anything but run fine.
<p/>
The iterator <code>j</code> on the other hand now points at end, and any iterators that may now point beyond <code>end()</code>, 
into uninitialized memory, are clearly invalid.
<p/>
But why do we say that an iterator that <em>must</em> point to a valid object is invalid? This looks to me like we 
simply got sloppy in our specification.
</p>

<p><i>[2016-05 Issues Telecon]</i></p>

<p>
This is related to <a href="lwg-defects.html#2698" title="Effect of assign() on iterators/pointers/references (Status: C++17)">2698</a><sup><a href="https://cplusplus.github.io/LWG/issue2698" title="Latest snapshot">(i)</a></sup>
</p>

<p><i>[2017-03-04, Kona]</i></p>

<p>NAD. "Works as designed"  Also, the example does not work in today's world of <code>launder</code>.</p>


<p id="res-2256"><b>Proposed resolution:</b></p>






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