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<title>Issue 208: Unnecessary restriction on past-the-end iterators</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="208"><a href="lwg-defects.html#208">208</a>. Unnecessary restriction on past-the-end iterators</h3>
<p><b>Section:</b> 24.3.4 <a href="https://wg21.link/iterator.concepts">[iterator.concepts]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Stephen Cleary <b>Opened:</b> 2000-02-02 <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#iterator.concepts">issues</a> in [iterator.concepts].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>In 24.1 paragraph 5, it is stated &quot;. . . Dereferenceable and
past-the-end values are always non-singular.&quot;</p>
<p>This places an unnecessary restriction on past-the-end iterators for
containers with forward iterators (for example, a singly-linked list). If the
past-the-end value on such a container was a well-known singular value, it would
still satisfy all forward iterator requirements.</p>
<p>Removing this restriction would allow, for example, a singly-linked list
without a &quot;footer&quot; node.</p>
<p>This would have an impact on existing code that expects past-the-end
iterators obtained from different (generic) containers being not equal.</p>


<p id="res-208"><b>Proposed resolution:</b></p>
<p>Change 24.3.4 <a href="https://wg21.link/iterator.concepts">[iterator.concepts]</a> paragraph 5, the last sentence, from:</p>
<blockquote>
<p>Dereferenceable and past-the-end values are always non-singular.</p>
</blockquote>
<p>to:</p>
<blockquote>
<p>Dereferenceable values are always non-singular.&nbsp;</p>
</blockquote>


<p><b>Rationale:</b></p>
<p>For some kinds of containers, including singly linked lists and
zero-length vectors, null pointers are perfectly reasonable past-the-end
iterators.  Null pointers are singular.
</p>




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