Document number: P1466r2


Howard E. Hinnant
2019-06-17

Miscellaneous minor fixes for chrono

Contents

Revision History

Introduction

This is a collection of minor fixes and upgrades to the <chrono> library that have come to my attention since the acceptance of P0355r7. This paper is written with respect to N4810.

Wording

  1. Update the value of the __cpp_lib_chrono in table "Standard library feature-test macros" of [support.limits.general] to reflect date of approval of this proposal.

  2. Feedback from the field asks for a way to identify if a utc_time 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 utc_time and sys_time, and in the parsing and formatting of utc_time, 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 <chrono>.

    Insert into the synopsis 27.2 Header <chrono> synopsis [time.syn]: 

    struct leap_second_info;

template <class Duration>
   leap_second_info get_leap_second_info(utc_time<Duration> const& ut);

    Insert new paragraphs in 27.7.2.3 Non-member functions [time.clock.utc.nonmembers]: 

    struct leap_second_info
{
    bool    is_leap_second;
    seconds elapsed;
};

    The type leap_second_info has data members and special members specified above. It has no base classes or members other than those specified. 

    template <class Duration>
leap_second_info
get_leap_second_info(utc_time<Duration> const& ut);

    Returns: A leap_second_info where is_leap_second is true if ut is during a leap second insertion, and otherwise false. elapsed is the number of leap seconds between 1970-01-01 and ut. If is_leap_second is true, the leap second referred to by ut is included in the count.

  3. 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 %Q to represent the durations's value and %q to represent the duration's unit, for formatting only. Example: format("%Q %q", 45ms) == "45 ms".

    Add two new rows to Table 87 — Meaning of format conversion specifiers:

    %Q The duration's numeric value (as if extracted via .count()).
    %q The duration's unit suffix as specified in [time.duration.io].

  4. About half of the clients are upset that the currently specified encoding for weekday implies that Sunday is the first day of the week (consistent with the current C and C++ specifications for tm.tm_wday), and the other half of the clients will be upset if the weekday encoding follows the ISO specification of [1, 7] maps to [Monday, Sunday].

    This change strikes a compromise in an attempt to please everyone (a nearly impossible task).

    • The weekday{unsigned} constructor accepts both mappings, which means that [0, 6] maps to [Sunday, Saturday] and [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.

    • The explicit conversion to unsigned is removed from weekday and named conversions are inserted in its place: c_encoding() and iso_encoding(). The client can choose which mapping from weekday to unsigned he desires by choosing one of these member functions.

    Modify 27.8.6.2 [time.cal.wd.members] as indicated: 

    constexpr explicit weekday(unsigned wd) noexcept;

    Effects: Constructs an object of type weekday by initializing wd_ with wd, except if wd_ == 7, stores 0. The value held is unspecified if wd is not in the range [0, 255]. 

    constexpr explicit operator unsigned() const noexcept;

    Returns: wd_. 

    constexpr unsigned c_encoding() const noexcept;

    Returns: wd_. 

    constexpr unsigned iso_encoding() const noexcept;

    Returns: wd_ == 0u ? 7u : wd_.

    Modify 27.8.6.3 [time.cal.wd.nonmembers] as indicated: 

    constexpr bool operator==(const weekday& x, const weekday& y) noexcept;

    Returns: unsigned{x} == unsigned{y} x.wd_ == y.wd_. 

    constexpr weekday operator+(const weekday& x, const days& y) noexcept;

    Returns: weekday{modulo(static_cast<long long>(unsigned{x.wd_}) + y.count(), 7)}. 

    template<class charT, class traits>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday& wd);

    Effects: If wd.ok() == true inserts format(os.getloc(), fmt, wd) where fmt is "%a" widened to charT. Otherwise inserts unsigned{wd.wd_} << " is not a valid weekday".

  5. time_of_day 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 time_of_day and its internal state that changes mode between 24h and 12h time. This note changes the name of time_of_day to hh_mm_ss to bring the semantics away from holding just a time duration since midnight, and more towards holding any duration as an {hours, minutes, seconds, subseconds} 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: hh_mm_ss is an {hours, minutes, seconds, subseconds} structure which easily converts to and from a duration and provides easy formatting.

    Modify 27.1 General [time.general]

    Table 85 — Time library summary
    Subclause Header(s)
    ...
    27.8 Civil calendar
    27.9 Class template time_of_dayhh_mm_ss
    27.10 12/24 hour functions
    27.1027.11 Time zones
    ...

    Modify 27.2 Header <chrono> synopsis [time.syn] 

    ...
// 27.9, class template time_of_dayhh_mm_ss
template <class Duration> class time_of_dayhh_mm_ss;
template<> class time_of_day<hours>;
template<> class time_of_day<minutes>;
template<> class time_of_day<seconds>;
template<class Rep, class Period> class time_of_day<duration<Rep, Period>>;

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);
template<class charT, class traits, class Rep, class PeriodDuration>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os,
               const time_of_dayhh_mm_ss<Dduration<Rep, Period>>& t);

// 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;
...

    Modify 27.7.1.3 Non-member functions [time.clock.system.nonmembers]

    Effects:

    ...

    os << year_month_day{dp} << ' ' << time_of_dayhh_mm_ss{tp-dp};

    Replace [time.tod] with [time.hms]:

    27.9 Class template hh_mm_ss [time.hms]

    27.9.1 Overview [time.hms.overview] 

    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 = see below;
    using precision                            = see below;

    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);

    The hh_mm_ss class template splits a duration into a multi-field time structure hours:minutes:seconds and possibly subseconds, where subseconds will be a duration unit based on a non-positive power of 10. The Duration template parameter dictates the precision to which the time is split. A hh_mm_ss models negative durations with a distinct is_negative getter that returns true when the input duration is negative. The individual duration fields always return non-negative durations even when is_negative() indicates the structure is representing a negative duration.

    If Duration is not an instance of duration, the program is ill-formed.

    27.9.2 Members [time.hms.members] 

    static unsigned constexpr fractional_width = see below;

    fractional_width is the number of fractional decimal digits represented by precision. fractional_width has the value of the smallest possible integer in the range [0, 18] such that precision will exactly represent all values of Duration. If no such value of fractional_width exists, then fractional_width is 6.

    [Example:

    Duration fractional_width Formatted fractional second
    output of Duration{1}
    hours, minutes, and seconds 0
    milliseconds 3 .001
    microseconds 6 .000001
    nanoseconds 9 .000000001
    duration<int, ratio<1, 2>> 1 .5
    duration<int, ratio<1, 3>> 6 .333333
    duration<int, ratio<1, 4>> 2 .25
    duration<int, ratio<1, 5>> 1 .2
    duration<int, ratio<1, 6>> 6 .166666
    duration<int, ratio<1, 7>> 6 .142857
    duration<int, ratio<1, 8>> 3 .125
    duration<int, ratio<1, 9>> 6 .111111
    duration<int, ratio<1, 10>> 1 .1
    duration<int, ratio<756, 625>> // microfortnights 4 .2096

    — end example] 

    using precision = see below;

    precision is duration<common_type_t<Duration::rep, seconds::rep>, ratio<1, 10fractional_width>>. 

    constexpr explicit hh_mm_ss(Duration d);

    Effects: constructs an object of type hh_mm_ss which represents the Duration d with precision precision.

    Stores d < Duration::zero() in is_neg.

    Stores duration_cast<chrono::hours>(abs(d)) in h.

    Stores duration_cast<chrono::minutes>(abs(d) - hours()) in m.

    Stores duration_cast<chrono::seconds>(abs(d) - hours() - minutes()) in s.

    If treat_as_floating_point_v<precision::rep> is true, stores abs(d) - hours() - minutes() - seconds() in ss. Else stores duration_cast<precision>(abs(d) - hours() - minutes() - seconds()) in ss. [Note: When precision is seconds with integral representation, subseconds() always returns 0s — end note]

    Ensures: If treat_as_floating_point_v<precision::rep> is true, to_duration() returns d, else to_duration() returns duration_cast<precision>(d). 

    constexpr bool is_negative() const noexcept;

    Returns: is_neg. 

    constexpr chrono::hours hours() const noexcept;

    Returns: h. 

    constexpr chrono::minutes minutes() const noexcept;

    Returns: m. 

    constexpr chrono::seconds seconds() const noexcept;

    Returns: s. 

    constexpr precision subseconds() const noexcept;

    Returns: ss. 

    constexpr precision to_duration() const noexcept;

    Returns: If is_neg, returns -(h + m + s + ss), else returns h + m + s + ss. 

    constexpr explicit operator precision() const noexcept;

    Returns: to_duration().

    27.9.3 Non-members [time.hms.nonmembers] 

    template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, hh_mm_ss<Duration> const& hms);

    Effects: Outputs to os according to the format "%T" ([time.format]). If hms.is_negative(), the output is preceded with '-'.

    Returns: os.

    [Example: 

    for (auto ms : {-4083007ms, 4083007ms, 65745123ms})
{
     hh_mm_ss hms{ms};
     cout << hms << '\n';
}
cout << hh_mm_ss{65745s} << '\n';

    Produces the output (assuming the "C" locale): 

    -01:08:03.007
01:08:03.007
18:15:45.123
18:15:45

    — end example]

    Insert a new section, 27.10 12/24 hour functions [time.12]:

    27.10 12/24 hour functions [time.12]

    These functions aid in translating between a 12h format time of day, and a 24h format time of day. 

    constexpr bool is_am(const hours& h) noexcept;

    Returns: 0h <= h && h <= 11h. 

    constexpr bool is_pm(const hours& h) noexcept;

    Returns: 12h <= h && h <= 23h. 

    constexpr hours make12(const hours& h) noexcept;

    Returns: The 12-hour equivalent of h in the range [1h, 12h]. If h is not in the range [0h, 23h], the value returned is unspecified. 

    constexpr hours make24(const hours& h, bool is_pm) noexcept;

    Returns: If is_pm is false, returns the 24-hour equivalent of h in the range [0h, 11h], assuming h represents an ante meridiem hour. Else returns the 24-hour equivalent of h in the range [12h, 23h], assuming h represents a post meridiem hour. If h is not in the range [1h, 12h], the value returned is unspecified.