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<title>Issue 51: Requirement to not invalidate iterators missing</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#TC1">TC1</a> status.</em></p>
<h3 id="51"><a href="lwg-defects.html#51">51</a>. Requirement to not invalidate iterators missing</h3>
<p><b>Section:</b> 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> David Vandevoorde <b>Opened:</b> 1998-06-23 <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#container.requirements">issues</a> in [container.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>The std::sort algorithm can in general only sort a given sequence
by moving around values. The list&lt;&gt;::sort() member on the other
hand could move around values or just update internal pointers. Either
method can leave iterators into the list&lt;&gt; dereferencable, but
they would point to different things. </p>

<p>Does the FDIS mandate anywhere which method should be used for
list&lt;&gt;::sort()?</p>

<p>Matt Austern comments:</p>

<p>I think you've found an omission in the standard. </p>

<p>The library working group discussed this point, and there was
supposed to be a general requirement saying that list, set, map,
multiset, and multimap may not invalidate iterators, or change the
values that iterators point to, except when an operation does it
explicitly. So, for example, insert() doesn't invalidate any iterators
and erase() and remove() only invalidate iterators pointing to the
elements that are being erased. </p>

<p>I looked for that general requirement in the FDIS, and, while I
found a limited form of it for the sorted associative containers, I
didn't find it for list. It looks like it just got omitted. </p>

<p>The intention, though, is that list&lt;&gt;::sort does not
invalidate any iterators and does not change the values that any
iterator points to. There would be no reason to have the member
function otherwise.</p>


<p id="res-51"><b>Proposed resolution:</b></p>
<p>Add a new paragraph at the end of 23.1:</p>

<blockquote>
  <p>Unless otherwise specified (either explicitly or by defining a function in terms of
  other functions), invoking a container member function or passing a container as an
  argument to a library function shall not invalidate iterators to, or change the values of,
  objects within that container. </p>
</blockquote>


<p><b>Rationale:</b></p>
<p>This was US issue CD2-23-011; it was accepted in London but the
change was not made due to an editing oversight. The wording in the
proposed resolution below is somewhat updated from CD2-23-011,
particularly the addition of the phrase &quot;or change the values
of&quot;</p>





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