Miscellaneous minor fixes for chrono

Contents

• Introduction
• Wording

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.

Wording

1. 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 new paragraphs in 26.7.2.3 Non-member functions [time.clock.utc.nonmembers]:

   template <class Duration>
   pair<bool, seconds>
   is_leap_second(utc_time<Duration> const& ut)
   

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

2. 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].

3. 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 [Monday, 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 26.8.6.2 [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: unsigned{((wd_ == 0u) ? 7u : wd_)}.

4. link is missing some comparison operator definitions. The declarations are correctly in the synopsis. The missing definitions should be the "usual" formula based on == and <.

   Add paragraphs to 26.10.9.3 Non-member functions [time.zone.link.nonmembers]:

   bool operator!=(const link& x, const link& y) noexcept;
   

   Returns: !(x == y).

   bool operator> (const link& x, const link& y) noexcept;
   

   Returns: y < x.

   bool operator<=(const link& x, const link& y) noexcept;
   

   Returns: !(y < x).

   bool operator>=(const link& x, const link& y) noexcept;
   

   Returns: !(x < y).