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Reword is_clock and add is_clock_v

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Howard Hinnant
2017-06-17 10:28:00 -04:00
parent 8c34cf1e8f
commit 7ffdc2450a
1 changed files with 21 additions and 13 deletions
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d0355r3.html
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@@ -37,7 +37,7 @@
Document number: D0355R3<br/>
<br/>
<a href="mailto:howard.hinnant@gmail.com">Howard E. Hinnant</a><br/>
2017-06-16<br/>
2017-06-17<br/>
</address>
<hr/>
<h1>Extending <code>&lt;chrono&gt;</code> to Calendars and Time Zones</h1>
@@ -805,6 +805,7 @@ namespace chrono {
// ...
template &lt;class T&gt; struct is_clock;
template &lt;class T&gt; inline constexpr bool is_clock_v = is_clock&lt;T&gt;::value;
// duration I/O
template &lt;class charT, class traits, class Rep, class Period&gt;
@@ -1571,22 +1572,29 @@ template &lt;class T&gt; struct is_clock;
</pre>
<p>
<code>is_clock</code> is a type property query trait that publicly derives from
<code>true_type</code> if <code>T</code> meets the Clock requirements ([time.clock.req]),
else it derives from <code>false_type</code>. For the purposes of the specification of
this trait, <code>T</code> meets the clock requirements if it has all of the following
properties:
<code>is_clock</code> is a <code>UnaryTypeTrait</code> ([meta.rqmts]) with a base
characteristic of <code>true_type</code> if <code>T</code> meets the <code>Clock</code>
requirements ([time.clock.req]), otherwise <code>false_type</code>. For the purposes of
the specification of this trait, the extent to which an implementation determines that a
type cannot meet the clock requirements is unspecified, except that as a minimum a type
<code>T</code> shall not qualify as a clock unless it satisfies all of the following
conditions:
</p>
<ul>
<li>Has a nested type named <code>rep</code>.</li>
<li>Has a nested type named <code>period</code>.</li>
<li>Has a nested type named <code>duration</code>.</li>
<li>Has a nested type named <code>time_point</code>.</li>
<li>Has a static data member named <code>is_steady</code>.</li>
<li>Has a static member function named <code>now</code> with zero parameters.</li>
<li>the <i>qualified-id</i>s <tt>T::rep</tt>, <tt>T::period</tt>,
<tt>T::duration</tt>, and <tt>T::time_point</tt> are (all) valid and
each denotes a type ([temp.deduct]),</li>
<li>the expression <tt>T::is_steady</tt> is well-formed when treated as
an unevaluated operand,</li>
<li>the expression <tt>T::now()</tt> is well-formed when treated as an
unevaluated operand.</li>
</ul>
<p>
The behavior of a program that adds specializations for <code>is_clock</code> is undefined.
</p>
</blockquote>
<a name="time.duration.io"></a><p class = note>
@@ -10273,7 +10281,7 @@ requirements. If a template parameter is named <code>Predicate</code>,
<code>operator()</code> applied to the template argument shall return a value that is
convertible to <code>bool</code>. <ins>A program that instantiates a template with a
template parameter named <code>Clock</code> with a type for which
<code>chrono::is_clock&lt;Clock&gt;::value</code> is <code>false</code>, is ill-formed.
<code>chrono::is_clock_v&lt;Clock&gt;</code> is <code>false</code>, is ill-formed.
</p>
</blockquote>