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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>chrono fixes</title>
<style>
p {text-align:justify}
li {text-align:justify}
blockquote.note
{
background-color:#E0E0E0;
padding-left: 15px;
padding-right: 15px;
padding-top: 1px;
padding-bottom: 1px;
}
ins {color:#00A000}
del {color:#A00000}
code {white-space:pre;}
table, th, td {
border: 1px solid black;
border-collapse: collapse;
padding: 10px;
}
</style>
</head>
<body>
<address align=right>
Document number: d1466r3<br>
<br/>
<br/>
<a href="mailto:[email protected]">Howard E. Hinnant</a><br/>
2019-07-16<br/>
</address>
<hr/>
<h1 align=center>Miscellaneous minor fixes for chrono</h1>
<h2>Contents</h2>
<ul>
<li><a href="#Revision">Revision History</a></li>
<li><a href="#Introduction">Introduction</a></li>
<li><a href="#Wording">Wording</a></li>
</ul>
<a name="Revision"></a><h2>Revision History</h2>
<ul>
<li>
<p>
<b>R3:</b>
<ul>
<li>
Reword the <code>weekday(unsigned)</code> constructor to not use the word "stores."
</li>
<li>
Reword the <code>hh_mm_ss(Duration)</code> constructor to not use the word "stores."
</li>
</ul>
</li>
<li>
<p>
<b><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1466r2.html">R2:</a></b>
<ul>
<li>
Removed superflous and questionably grammatical wording from the <code>hh_mm_ss</code>
constructor <i>Effects</i> clause..
</li>
<li>
Removed <code>noexcept</code> from <code>hh_mm_ss</code> constructor.
</li>
<li>
In the description of <code>hh_mm_ss</code>, allow
<i>subseconds</i> to be based on a non-positive power of 10.
</li>
<li>
Update <code>__cpp_lib_chrono</code>.
</li>
<li>
Reference current working draft
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf">N4810</a>
and update section numbers.
</li>
<li>
Remove errant "," in [time.clock.utc.nonmembers].
</li>
<li>
Remove use of removed conversion from <code>weekday</code> to
<code>unsigned</code> in specification.
</li>
<li>
Removed use of the term "broken down".
</li>
<li>
Changed <code>floor</code> to <code>duration_cast</code> in the
<i>Ensures:</i> clause of the <code>hh_mm_ss</code> constructor.
</li>
<li>
Changed <code>make12</code> and <code>make24</code> to use
<code>const hours&</code> parameters instead of <code>hours</code>.
</li>
</ul>
</p>
</li>
<li>
<p>
<b><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1466r1.html">R1:</a></b>
<ul>
<li>
Introduced <code>struct leap_second_info</code>.
</li>
<li>
<code>is_leap_second</code> renamed to <code>get_leap_second_info</code>.
</li>
<li>
Return type of <code>is_leap_second</code> changed from
<code>pair<bool, seconds></code> to <code>leap_second_info</code>.
</li>
<li>
Corrected type-o in author's note concerning <code>weekday</code>.
</li>
<li>
Corrected type-o in <code>weekday::iso_encoding</code> <i>Returns:</i> clause.
</li>
<li>
Removed the insertion of "missing" <code>link</code> comparison operators. They
are implicitly specified by the existing standard.
</li>
<li>
Rename <code>time_of_day</code> to <code>hh_mm_ss</code>.
</li>
<li>
Move <code>make12</code>/<code>make24</code> from <code>hh_mm_ss</code> to
namespace scope.
</li>
<li>
Add <code>fractional_width</code> to <code>hh_mm_ss</code>.
</li>
<li>
Improve <code>hh_mm_ss</code> specification.
</li>
</ul>
</p>
</li>
<li>
<p>
<b><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1466r0.html">R0:</a></b> Original paper.
</li>
</ul>
<a name="Introduction"></a><h2>Introduction</h2>
<p>
This is a collection of minor fixes and upgrades to the <code><chrono></code>
library that have come to my attention since the acceptance of
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">P0355r7</a>.
This paper is written with respect to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf">N4810</a>.
</p>
<a name="Wording"></a><h2>Wording</h2>
<ol>
<!-- Update __cpp_lib_chrono -->
<li>
Update the value of the <code>__cpp_lib_chrono</code> in table
"Standard library feature-test macros" of [support.limits.general] to reflect date
of approval of this proposal.
</li>
<!-- get_leap_second_info -->
<li>
<blockquote class=note>
<p>
Feedback from the field asks for a way to identify if a <code>utc_time</code> represents
a leap second. This proposed API is the only API that has field experience in providing
that information in a way that is efficient, and provides all information the client
desires when making a query in this area. It is used in the implementation of the
conversion between <code>utc_time</code> and <code>sys_time</code>, and in the parsing
and formatting of <code>utc_time</code>, so is likely to be present in any event. If
we choose to specify it, it can be spelled without underscores, and be available to
clients of <code><chrono></code>.
</p>
</blockquote>
<p>
Insert into the synopsis 27.2 Header <code><chrono></code> synopsis [time.syn]:
</p>
<blockquote>
<pre>
struct leap_second_info;
template <class Duration>
leap_second_info get_leap_second_info(utc_time<Duration> const& ut);
</pre>
</blockquote>
<p>
Insert new paragraphs in 27.7.2.3 Non-member functions [time.clock.utc.nonmembers]:
</p>
<blockquote>
<pre>
struct leap_second_info
{
bool is_leap_second;
seconds elapsed;
};
</pre>
<p>
The type <code>leap_second_info</code> has data members and special
members specified above. It has no base classes or members other than those
specified.
</p>
<pre>
template <class Duration>
leap_second_info
get_leap_second_info(utc_time<Duration> const& ut);
</pre>
<blockquote>
<p>
<i>Returns:</i> A <code>leap_second_info</code> where
<code>is_leap_second</code> is true if <code>ut</code> is during a
leap second insertion, and otherwise false. <code>elapsed</code> is
the number of leap seconds between 1970-01-01 and <code>ut</code>. If
<code>is_leap_second</code> is true, the leap second referred to by
<code>ut</code> is included in the count.
</p>
</blockquote>
</blockquote>
</li>
<!-- %Q %q -->
<li>
<blockquote class=note>
<p>
Feedback from the field asks for a way to customize the spacing between a duration's
value and its unit (e.g. supply a space, or a Unicode custom space between the value
and the unit). This item proposes <code>%Q</code> to represent the durations's value
and <code>%q</code> to represent the duration's unit, for formatting only. Example:
<code>format("%Q %q", 45ms) == "45 ms"</code>.
</p>
</blockquote>
<p>
Add two new rows to Table 87 — Meaning of <code>format</code> conversion
specifiers:
</p>
<blockquote>
<table>
<tr>
<td><code>%Q</code></td>
<td>The duration's numeric value (as if extracted via <code>.count()</code>).</td>
</tr>
<tr>
<td><code>%q</code></td>
<td>The duration's unit suffix as specified in [time.duration.io].</td>
</tr>
</table>
</blockquote>
</li>
<!-- weekday encoding controversy -->
<li>
<blockquote class=note>
<p>
About half of the clients are upset that the currently specified encoding for
<code>weekday</code> implies that Sunday is the first day of the week (consistent
with the current C and C++ specifications for <code>tm.tm_wday</code>), and the
other half of the clients will be upset if the <code>weekday</code> encoding
follows the ISO specification of [1, 7] maps to [Monday, Sunday].
</p>
<p>
This change strikes a compromise in an attempt to please everyone (a nearly
impossible task).
</p>
<ul>
<li>
<p>
The <code>weekday{unsigned}</code> constructor accepts both mappings, which means
that [0, 6] maps to [Sunday, Saturday] <b>and</b> [1, 7] maps to [Monday, Sunday].
This is possible by simply accepting [0, 7] where [1, 6] maps to [Monday, Saturday]
and both 0 and 7 map to Sunday.
</p>
</li>
<li>
<p>
The explicit conversion to <code>unsigned</code> is removed from <code>weekday</code>
and named conversions are inserted in its place: <code>c_encoding()</code> and
<code>iso_encoding()</code>. The client can choose which mapping from <code>weekday</code>
to <code>unsigned</code> he desires by choosing one of these member functions.
</p>
</li>
</ul>
</blockquote>
<p>
Modify 27.8.6.2 [time.cal.wd.members] as indicated:
</p>
<blockquote>
<pre>
constexpr explicit weekday(unsigned wd) noexcept;
</pre>
<blockquote>
<p>
<i>Effects:</i> Constructs an object of type <code>weekday</code> by
initializing <code>wd_</code> with <code>wd<ins> == 7 ? 0 : wd</ins></code>. The value held is unspecified if wd is
not in the range [0, 255].
</p>
</blockquote>
<pre>
<del>constexpr explicit operator unsigned() const noexcept;</del>
</pre>
<blockquote>
<p>
<del><i>Returns:</i> <code>wd_</code>.</del>
</p>
</blockquote>
<pre>
<ins>constexpr unsigned c_encoding() const noexcept;</ins>
</pre>
<blockquote>
<p>
<ins><i>Returns:</i> <code>wd_</code>.</ins>
</p>
</blockquote>
<pre>
<ins>constexpr unsigned iso_encoding() const noexcept;</ins>
</pre>
<blockquote>
<p>
<ins><i>Returns:</i> <code>wd_ == 0u ? 7u : wd_</code>.</ins>
</p>
</blockquote>
</blockquote>
<p>
Modify 27.8.6.3 [time.cal.wd.nonmembers] as indicated:
</p>
<blockquote>
<pre>
constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code><del>unsigned{x} == unsigned{y}</del> <ins>x.wd_ == y.wd_</ins></code>.
</p>
</blockquote>
<pre>
constexpr weekday operator+(const weekday& x, const days& y) noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>weekday{modulo(static_cast<long long>(<del>unsigned{</del>x<ins>.wd_</ins><del>}</del>) + y.count(), 7)}</code>.
</p>
</blockquote>
<pre>
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const weekday& wd);
</pre>
<blockquote>
<p>
<i>Effects:</i> If <code>wd.ok() == true</code> inserts
<code>format(os.getloc(), fmt, wd)</code> where <code>fmt</code> is <code>"%a"</code>
widened to <code>charT</code>. Otherwise inserts
<code>unsigned{wd<ins>.wd_</ins>} << " is not a valid weekday"</code>.
</p>
</blockquote>
</blockquote>
</li>
<!-- hh_mm_ss -->
<li>
<blockquote class=note>
<p>
<code>time_of_day</code> is poorly specified. It suffers from an overly
complicated specification, and at the same time, insufficient functionality from
that specification. Additionally, guidance from the LEWG mailing list indicates
significant dissatisfaction with the name of <code>time_of_day</code> and its
internal state that changes mode between 24h and 12h time. This note changes the
name of <code>time_of_day</code> to <code>hh_mm_ss</code> to bring the semantics
away from holding just a time duration since midnight, and more towards holding
<i>any</i> duration as an <code>{hours, minutes, seconds, subseconds}</code>
data structure. Also the 24h/12h functionality is moved out of this structure
and into namespace-scope functions. And finally this change both simplifies the
specification while providing more pertinent functionality. However the basic
functionality is maintained: <code>hh_mm_ss</code> is an <code>{hours, minutes,
seconds, subseconds}</code> structure which easily converts to and from a
<code>duration</code> and provides easy formatting.
</p>
</blockquote>
<blockquote>
<p>
Modify 27.1 General [time.general]
</p>
<blockquote>
<table>
<caption>Table 85 — Time library summary</caption>
<tr>
<th></th>
<th>Subclause</th>
<th>Header(s)</th>
</tr>
<tr>
<td colspan=3>...</td>
</tr>
<tr>
<td>27.8</td>
<td>Civil calendar</td>
<td></td>
</tr>
<tr>
<td>27.9</td>
<td>Class template <del><code>time_of_day</code></del><ins><code>hh_mm_ss</code></ins></td>
<td></td>
</tr>
<tr>
<td><ins>27.10</ins></td>
<td><ins>12/24 hour functions</ins></td>
<td></td>
</tr>
<tr>
<td><del>27.10</del><ins>27.11</ins></td>
<td>Time zones</td>
<td></td>
</tr>
<tr>
<td colspan=3>...</td>
</tr>
</table>
</blockquote>
<p>
Modify 27.2 Header <code><chrono></code> synopsis [time.syn]
</p>
<blockquote>
<pre>
...
// 27.9, class template <del>time_of_day</del><ins>hh_mm_ss</ins>
template <class Duration> class <del>time_of_day</del><ins>hh_mm_ss</ins>;
<del>template<> class time_of_day<hours>;</del>
<del>template<> class time_of_day<minutes>;</del>
<del>template<> class time_of_day<seconds>;</del>
<del>template<class Rep, class Period> class time_of_day<duration<Rep, Period>>;</del>
<del>template<class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<hours>& t);
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<minutes>& t);
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<seconds>& t);</del>
template<class charT, class traits, class <del>Rep, class Period</del><ins>Duration</ins>>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os,
const <del>time_of_day</del><ins>hh_mm_ss</ins><<ins>D</ins><del>d</del>uration<del><Rep, Period></del>>& t);
<ins>// 27.10, 12/24 hour functions
constexpr bool is_am(const hours& h) noexcept;
constexpr bool is_pm(const hours& h) noexcept;
constexpr hours make12(const hours& h) noexcept;
constexpr hours make24(const hours& h, bool is_pm) noexcept;</ins>
...
</pre>
</blockquote>
<p>
Modify 27.7.1.3 Non-member functions [time.clock.system.nonmembers]
</p>
<blockquote>
<p>
<i>Effects:</i>
</p>
<blockquote>
<p>...</p>
<pre>
os << year_month_day{dp} << ' ' << <del>time_of_day</del><ins>hh_mm_ss</ins>{tp-dp};
</pre>
</blockquote>
</blockquote>
<p>
Replace [time.tod] with [time.hms]:
</p>
<blockquote>
<p>
27.9 Class template <code>hh_mm_ss</code> [time.hms]
</p>
<p>
27.9.1 Overview [time.hms.overview]
</p>
<blockquote><pre>
template <class Duration>
class hh_mm_ss
{
bool is_neg; // exposition only
chrono::hours h; // exposition only
chrono::minutes m; // exposition only
chrono::seconds s; // exposition only
precision ss; // exposition only
public:
static unsigned constexpr fractional_width = <i>see below</i>;
using precision = <i>see below</i>;
constexpr hh_mm_ss() noexcept : hh_mm_ss{Duration::zero()} {}
constexpr explicit hh_mm_ss(Duration d);
constexpr bool is_negative() const noexcept;
constexpr chrono::hours hours() const noexcept;
constexpr chrono::minutes minutes() const noexcept;
constexpr chrono::seconds seconds() const noexcept;
constexpr precision subseconds() const noexcept;
constexpr explicit operator precision() const noexcept;
constexpr precision to_duration() const noexcept;
};
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, hh_mm_ss<Duration> const& hms);
</pre></blockquote>
<p>
The <code>hh_mm_ss</code> class template splits a duration into a multi-field time
structure <i>hours:minutes:seconds</i> and possibly <i>subseconds</i>, where <i>subseconds</i>
will be a <code>duration</code> unit based on a non-positive power of 10.
The <code>Duration</code> template parameter dictates the precision to which the time is
split. A <code>hh_mm_ss</code> models negative durations with a distinct
<code>is_negative</code> getter that returns <code>true</code> when the input duration
is negative. The individual duration fields always return non-negative durations even
when <code>is_negative()</code> indicates the structure is representing a negative
duration.
</p>
<p>
If <code>Duration</code> is not an instance of <code>duration</code>, the program
is ill-formed.
</p>
<p>
27.9.2 Members [time.hms.members]
</p>
<pre>
static unsigned constexpr fractional_width = <i>see below</i>;
</pre>
<blockquote>
<p>
<code>fractional_width</code> is the number of fractional decimal digits
represented by <code>precision</code>. <code>fractional_width</code> has the
value of the smallest possible integer in the range [0, 18] such that
<code>precision</code> will exactly represent all values of
<code>Duration</code>. If no such value of <code>fractional_width</code>
exists, then <code>fractional_width</code> is 6.
</p>
<p>
[<i>Example:</i>
</p>
<table>
<tr>
<th><code>Duration</code></th>
<th><code>fractional_width</code></th>
<th>Formatted fractional second<br>output of <code>Duration{1}</code></th>
</tr>
<tr>
<td><code>hours</code>, <code>minutes</code>, and <code>seconds</code></td>
<td><code>0</code></td>
<td></td>
</tr>
<tr>
<td><code>milliseconds</code></td>
<td><code>3</code></td>
<td>.001</td>
</tr>
<tr>
<td><code>microseconds</code></td>
<td><code>6</code></td>
<td>.000001</td>
</tr>
<tr>
<td><code>nanoseconds</code></td>
<td><code>9</code></td>
<td>.000000001</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 2>></code></td>
<td><code>1</code></td>
<td>.5</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 3>></code></td>
<td><code>6</code></td>
<td>.333333</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 4>></code></td>
<td><code>2</code></td>
<td>.25</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 5>></code></td>
<td><code>1</code></td>
<td>.2</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 6>></code></td>
<td><code>6</code></td>
<td>.166666</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 7>></code></td>
<td><code>6</code></td>
<td>.142857</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 8>></code></td>
<td><code>3</code></td>
<td>.125</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 9>></code></td>
<td><code>6</code></td>
<td>.111111</td>
</tr>
<tr>
<td><code>duration<int, ratio<1, 10>></code></td>
<td><code>1</code></td>
<td>.1</td>
</tr>
<tr>
<td><code>duration<int, ratio<756, 625>> //</code> <i>microfortnights</i></td>
<td><code>4</code></td>
<td>.2096</td>
</tr>
</table>
<p>
<i>— end example</i>]
</p>
</blockquote>
<pre>
using precision = <i>see below</i>;
</pre>
<blockquote>
<p>
<code>precision</code> is <code>duration<common_type_t<Duration::rep, seconds::rep>, ratio<1, 10<sup>fractional_width</sup>>></code>.
</p>
</blockquote>
<pre>
constexpr explicit hh_mm_ss(Duration d);
</pre>
<blockquote>
<p>
<i>Effects:</i> constructs an object of type <code>hh_mm_ss</code> which represents
the <code>Duration d</code> with precision <code>precision</code>.
</p>
<blockquote>
<p>
Initializes <code>is_neg</code> with <code>d < Duration::zero()</code>.
</p>
<p>
Initializes <code>h</code> with <code>duration_cast<chrono::hours>(abs(d))</code>.
</p>
<p>
Initializes <code>m</code> with <code>duration_cast<chrono::minutes>(abs(d) - hours())</code>.
</p>
<p>
Initializes <code>s</code> with <code>duration_cast<chrono::seconds>(abs(d) - hours() - minutes())</code>.
</p>
<p>
If
<code>treat_as_floating_point_v<precision::rep></code> is <code>true</code>,
initializes <code>ss</code> with <code>abs(d) - hours() - minutes() - seconds()</code>.
Else initializes <code>ss</code> with <code>duration_cast<precision>(abs(d) - hours() - minutes() - seconds())</code>.
[<i>Note:</i> When <code>precision</code> is <code>seconds</code> with integral
representation, <code>subseconds()</code> always returns <code>0s</code> <i>— end note</i>]
</p>
</blockquote>
<p>
<i>Ensures:</i> If <code>treat_as_floating_point_v<precision::rep></code> is <code>true</code>,
<code>to_duration()</code> returns <code>d</code>, else <code>to_duration()</code>
returns <code>duration_cast<precision>(d)</code>.
</p>
</blockquote>
<pre>
constexpr bool is_negative() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>is_neg</code>.
</p>
</blockquote>
<pre>
constexpr chrono::hours hours() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>h</code>.
</p>
</blockquote>
<pre>
constexpr chrono::minutes minutes() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>m</code>.
</p>
</blockquote>
<pre>
constexpr chrono::seconds seconds() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>s</code>.
</p>
</blockquote>
<pre>
constexpr precision subseconds() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>ss</code>.
</p>
</blockquote>
<pre>
constexpr precision to_duration() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> If <code>is_neg</code>, returns <code>-(h + m + s + ss)</code>,
else returns <code>h + m + s + ss</code>.
</p>
</blockquote>
<pre>
constexpr explicit operator precision() const noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>to_duration()</code>.
</p>
</blockquote>
<p>
27.9.3 Non-members [time.hms.nonmembers]
</p>
<pre>
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, hh_mm_ss<Duration> const& hms);
</pre>
<blockquote>
<p>
<i>Effects:</i> Outputs to <code>os</code>
according to the format
<code>"%T"</code> ([time.format]). If <code>hms.is_negative()</code>,
the output is preceded with <code>'-'</code>.
</p>
<p>
<i>Returns:</i> <code>os</code>.
</p>
<p>
[<i>Example:</i>
</p>
<blockquote><pre>
for (auto ms : {-4083007ms, 4083007ms, 65745123ms})
{
hh_mm_ss hms{ms};
cout << hms << '\n';
}
cout << hh_mm_ss{65745s} << '\n';
</pre>
<p>
Produces the output (assuming the "C" locale):
</p>
<pre>
-01:08:03.007
01:08:03.007
18:15:45.123
18:15:45
</pre></blockquote>
<p>
<i>— end example</i>]
</p>
</blockquote>
</blockquote>
<p>
Insert a new section, 27.10 12/24 hour functions [time.12]:
</p>
<blockquote>
<p>
27.10 12/24 hour functions [time.12]
</p>
<p>
These functions aid in translating between a 12h format time of day, and a 24h format
time of day.
</p>
<pre>
constexpr bool is_am(const hours& h) noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>0h <= h && h <= 11h</code>.
</p>
</blockquote>
<pre>
constexpr bool is_pm(const hours& h) noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> <code>12h <= h && h <= 23h</code>.
</p>
</blockquote>
<pre>
constexpr hours make12(const hours& h) noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> The 12-hour equivalent of <code>h</code> in the range
<code>[1h, 12h]</code>. If <code>h</code> is not in the range
<code>[0h, 23h]</code>, the value returned is unspecified.
</p>
</blockquote>
<pre>
constexpr hours make24(const hours& h, bool is_pm) noexcept;
</pre>
<blockquote>
<p>
<i>Returns:</i> If <code>is_pm</code> is <code>false</code>, returns the 24-hour
equivalent of <code>h</code> in the range <code>[0h, 11h]</code>, assuming <code>h</code>
represents an ante meridiem hour. Else returns the 24-hour equivalent of <code>h</code>
in the range <code>[12h, 23h]</code>, assuming <code>h</code> represents a post
meridiem hour. If <code>h</code> is not in the range
<code>[1h, 12h]</code>, the value returned is unspecified.
</p>
</blockquote>
</blockquote>
</blockquote>
</li>
</ol>
</body>
</html>