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<title>Issue 878: forward_list preconditions</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++11">C++11</a> status.</em></p>
<h3 id="878"><a href="lwg-defects.html#878">878</a>. <code>forward_list</code> preconditions</h3>
<p><b>Section:</b> 23.3.7 <a href="https://wg21.link/forward.list">[forward.list]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2008-08-23 <b>Last modified:</b> 2023-02-07</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#forward.list">issues</a> in [forward.list].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
       <p>

<code>forward_list</code> member functions that take
a <code>forward_list::iterator</code> (denoted <code>position</code> in the
function signatures) argument have the following precondition:

       </p>
       <blockquote><p>

<i>Requires:</i> <code>position</code> is dereferenceable or equal
to <code>before_begin()</code>.

       </p></blockquote>
       <p>

I believe what's actually intended is this:

       </p>
       <blockquote><p>

<i>Requires:</i> <code>position</code> is in the range
[<code>before_begin()</code>, <code>end()</code>).

       </p></blockquote>
       <p>

That is, when it's dereferenceable, <code>position</code> must point
into <code>*this</code>, not just any <code>forward_list</code> object.

       </p>

<p><i>[
San Francisco:
]</i></p>


<blockquote><p>
Robert suggested alternate proposed wording which had large support.
</p></blockquote>

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


<blockquote>
<p>
Walter: "position is before_begin() or a dereferenceable": add "is" after the "or"
</p>
<p>
With that minor update, Recommend Tentatively Ready.
</p>
</blockquote>



<p id="res-878"><b>Proposed resolution:</b></p>
       <p>

Change the <i>Requires</i> clauses
 [forwardlist], p21, p24, p26, p29, and,
 [forwardlist.ops], p39, p43, p47
as follows:

       </p>
       <blockquote><p>

<i>Requires:</i> <code>position</code> is <ins><code>before_begin()</code> or is a</ins>
dereferenceable
<ins>iterator in the range <code>[begin(), end())</code></ins>
<del>or equal to <code>before_begin()</code></del>. ...

       </p></blockquote>
   



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