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<title>Issue 3275: Why does time_get::do_get require a valid pointer when none of the others do?</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#New">New</a> status.</em></p>
<h3 id="3275"><a href="lwg-active.html#3275">3275</a>. Why does <code>time_get::do_get</code> require a valid pointer when none of the others do?</h3>
<p><b>Section:</b> 28.3.4.6.2.3 <a href="https://wg21.link/locale.time.get.virtuals">[locale.time.get.virtuals]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Marshall Clow <b>Opened:</b> 2019-09-09 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#locale.time.get.virtuals">active issues</a> in [locale.time.get.virtuals].</p>
<p><b>View all other</b> <a href="lwg-index.html#locale.time.get.virtuals">issues</a> in [locale.time.get.virtuals].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
According to 28.3.4.6.2.3 <a href="https://wg21.link/locale.time.get.virtuals">[locale.time.get.virtuals]</a> p11:
</p>
<blockquote><p>
<i>Requires:</i> <code>t</code> shall point to an object
</p></blockquote>
<p>
[Note: In my "Mandates" paper, I changed this to "<i>Expects:</i> <code>t</code> points to an object"]
<p/>
Nevertheless, it's odd, and inconsistent. <code>time_get::get</code> does not have any such stated requirement, 
and it calls <code>do_get</code>. None of the other "time" calls in <code>time_get</code> have such a (stated) 
requirement.
<p/>
I believe that this requirement is redundant, that it is implied by the wording in P12 and P14.
<p/>
P12: "or until it has extracted and assigned those <code>struct tm</code> members"
<p/>
P14: "It is unspecified whether multiple calls to <code>do_get()</code> with the address of the same 
<code>struct tm</code> object will update the current contents of the object or simply overwrite its members."
<p/>
If the pointer is invalid (null, or points to unmapped memory, say), you've got UB anyway.
<p/>
All the other calls in [locale.time.get.virtuals] were from C++98. <code>do_get_time</code> was added in C++11, 
and p11 originally said "<code>t</code> shall be dereferenceable". 
<p/>
This was changed to "<code>t</code> shall point to an object" as part of the resolution of CWG issue 
<a href="https://wg21.link/cwg342">342</a>
</p>

<p><i>[2019-10 Priority set to 3 after reflector discussion]</i></p>



<p id="res-3275"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 28.3.4.6.2.3 <a href="https://wg21.link/locale.time.get.virtuals">[locale.time.get.virtuals]</a> as indicated:</p>

<blockquote>
<pre>
iter_type do_get(iter_type s, iter_type end, ios_base&amp; f,
                 ios_base::iostate&amp; err, tm* t, char format, char modifier) const;
</pre>
<blockquote>
<p>
<del>-11- <i>Requires:</i> <code>t</code> shall point to an object.</del>
<p/>
-12- <i>Effects:</i> [&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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