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<title>Issue 707: null pointer constant for exception_ptr</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="707"><a href="lwg-closed.html#707">707</a>. null pointer constant for <code>exception_ptr</code></h3>
<p><b>Section:</b> 17.9.7 <a href="https://wg21.link/propagation">[propagation]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Jens Maurer <b>Opened:</b> 2007-07-20 <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#propagation">issues</a> in [propagation].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>

<p>
From the Toronto Core wiki:
</p>

<p>
What do you mean by "null pointer constant"? How do you guarantee that
<code>exception_ptr() == 1</code> doesn't work?  Do you even want to prevent that?
What's the semantics?  What about <code>void *p = 0; exception_ptr() == p</code>?
Maybe disallow those in the interface, but how do you do that with
portable C++? Could specify just "make it work".
</p>

<p>
Peter's response:
</p>

<p>
null pointer constant as defined in 7.3.12 <a href="https://wg21.link/conv.ptr">[conv.ptr]</a>. Intent is "just make it
work", can be implemented as assignment operator taking a unique pointer
to member, as in the unspecified bool type idiom.
</p>

<p><i>[
Bellevue:
]</i></p>


<blockquote>
<p>
Original implementation was possible using the "unspecified-null-pointer" idiom, similar to unspecified-bool.
</p>
<p>
Even simpler now with nullptr_t.
</p>
<p>
NAD Rationale : null pointer constant is a perfectly defined term, and
while API is clearly implementable there is no need to spell out
implementation details.
</p>
</blockquote>



<p id="res-707"><b>Proposed resolution:</b></p>
<p>
</p>





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