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<title>Issue 458: 24.1.5 contains unintended limitation for operator-</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="458"><a href="lwg-closed.html#458">458</a>. 24.1.5 contains unintended limitation for <code>operator-</code></h3>
<p><b>Section:</b> 24.3.5.7 <a href="https://wg21.link/random.access.iterators">[random.access.iterators]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Daniel Frey <b>Opened:</b> 2004-02-27 <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#random.access.iterators">issues</a> in [random.access.iterators].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In 24.1.5 [lib.random.access.iterators], table 76 the operational
semantics for the expression "<code>r -= n</code>" are defined as "<code>return r += -n</code>".
This means, that the expression <code>-n</code> must be valid, which is not the case
for unsigned types.
</p>

<p><i>[
Sydney: Possibly not a real problem, since difference type is required
to be a signed integer type. However, the wording in the standard may
be less clear than we would like.
]</i></p>


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


<blockquote>
<p>
This issue refers to a requirements table we have removed.
</p>
<p>
The issue might now relate to 24.3.5.7 <a href="https://wg21.link/random.access.iterators">[random.access.iterators]</a> p5.
However, the rationale in the issue already recognises that the
<code>difference_type</code> must be signed, so this really looks NAD.
</p>
</blockquote>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
We agree with Alisdair's observations.
</p>
<p>
Move to NAD.
</p>
</blockquote>

<p><i>[
2009-07 Frankfurt:
]</i></p>


<blockquote>
<p>
Need to look at again without concepts.
</p>
<p>
There was a question about this phrase in the discussion: "the
expression <code>-n</code> must be valid, which is not the case for unsigned types."
If <code>n</code> is an object of the iterator <code>difference_type</code> (eg <code>ptrdiff_t</code>), 
then it is never unsigned.
</p>
</blockquote>

<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
The group reviewed the wording in the draft and agreed that <code>n</code> is of
difference type, the difference type is signed, and the current wording
is correct.  Moved to NAD.
</p></blockquote>



<p id="res-458"><b>Proposed resolution:</b></p>
<p>
To remove this limitation, I suggest to change the
operational semantics for this column to:
</p>
<blockquote><pre>
    { Distance m = n;
      if (m >= 0)
        while (m--) --r;
      else
        while (m++) ++r;
      return r; }
</pre></blockquote>






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