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Minor clarifications and updates

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Howard Hinnant
2017-08-13 22:27:21 -04:00
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@@ -37,7 +37,7 @@
Document number: D0355R4<br>
<br>
<a href="mailto:howard.hinnant@gmail.com">Howard E. Hinnant</a><br>
2017-08-12<br>
2017-08-13<br>
</address>
<hr>
<h1>Extending <code>&lt;chrono&gt;</code> to Calendars and Time Zones</h1>
@@ -216,10 +216,9 @@ Everything proposed herein has been fully implemented here:
</blockquote>
<p>
The implementation includes full documentation, and an
<a href="http://www.timqian.com/star-history/#HowardHinnant/date&google/cctz">active
community of users with positive field experience.</a> The implementation has been ported
to Windows, Linux, Android, macOS and iOS.
The implementation includes full documentation, and an active community of users with
positive field experience. The implementation has been ported to Windows, Linux, Android,
macOS and iOS.
</p>
<p>
@@ -652,80 +651,6 @@ please see:
<a href="http://howardhinnant.github.io/date/iso_week.html">http://howardhinnant.github.io/date/iso_week.html</a>
</blockquote>
<a name="Issues"></a><h2>Issues</h2>
<p>
This is a collection of issues that could be changed one way or the other with this
proposal.
</p>
<ol>
<li>
<p>
Can the database be updated by the program while the program is running?
</p>
<p>
This is probably the most important issue to be decided. This decision, one way or
the other, leads (or doesn't) to many other decisions. If the database can be updated
while the program is running:
</p>
<ol type="a">
<li>Is the client responsible for thread safety issues? This proposal says yes.</li>
<li>What is the format of the database? This proposal says it is the text format
of the IANA database.</li>
<li>Can the database be updated to the latest version at a remote server on application
startup? This proposal says yes. This means Egypt-like time-zone changes (3 days
notice) can be handled seamlessly.</li>
</ol>
<p>
Not allowing the database to be dynamically updated is by far the simpler solution.
This proposal shows you what dynamic updating could look like. It is far easier to
remove this feature from a proposal than to add it. This proposal is designed in such
a way that it is trivial to remove this functionality.
</p>
</li>
<li>
<p>
Currently this library passes <code>time_zone</code>s around with
<code>const time_zone*</code>. Each <code>time_zone</code> is a non-copyable
<code>const</code> singleton in the application (much like a
<code>type_info</code>). Passing them around by pointers allows syntax such as:
</p>
<blockquote><pre>
auto tz = current_zone();
cout &lt;&lt; tz-&gt;name() &lt;&lt; '\n';
</pre></blockquote>
<p>
But source functions such as <code>current_zone</code> and <code>locate_zone</code>
<i>never</i> return <code>nullptr</code>. So it has been suggested that the library
traffic in <code>const time_zone&amp;</code> instead. This would change the above
code snippet to:
</p>
<blockquote><pre>
auto&amp; tz = current_zone();
cout &lt;&lt; tz.name() &lt;&lt; '\n';
</pre></blockquote>
<p>
Either solution is workable. And whichever we choose, the client can get the other
with <code>*current_zone()</code> or <code>&amp;current_zone()</code>. And whichever
we choose, we <i>will</i> make the library API self-consistent so that things like
the following work no matter what with this syntax:
</p>
<blockquote><pre>
cout &lt;&lt; zoned_time{current_zone(), system_clock::now()} &lt;&lt; '\n';
</pre></blockquote>
<p>
We simply need to decide if the default style guide for passing <code>time_zone</code>s
around is <code>const time_zone*</code> or <code>const time_zone&amp;</code>. And yes,
it is ok for a client to have a <code>const time_zone*</code> which equals
<code>nullptr</code>. And no, the library never provides a <code>const time_zone*</code>
which is equal to <code>nullptr</code>.
</p>
</li>
</ol>
<a name="Wording"></a><h2>Proposed Wording</h2>
<h3>Table of Contents for Proposed Wording</h3>
@@ -9117,7 +9042,7 @@ struct tzdb
class tzdb_list
{
std::atomic<tzdb*> head_{nullptr}; // exposition only
std::atomic&lt;tzdb*&gt; head_{nullptr}; // exposition only
public:
class const_iterator;
@@ -9234,8 +9159,8 @@ const time_zone* current_zone();
</p>
</blockquote>
<code>tzdb_list::const_iterator</code> is a non-mutating iterator which meets the
forward iterator requirements.
<code>tzdb_list::const_iterator</code> is a constant iterator which meets the
forward iterator requirements and has a value type of <code>tzdb</code>.
<pre>
const tzdb&amp; tzdb_list::front() const noexcept;