27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.14 Class year_­month_­day [time.cal.ymd]

27.8.14.1 Overview [time.cal.ymd.overview]

namespace std::chrono { class year_month_day { chrono::year y_; // exposition only chrono::month m_; // exposition only chrono::day d_; // exposition only public: year_month_day() = default; constexpr year_month_day(const chrono::year& y, const chrono::month& m, const chrono::day& d) noexcept; constexpr year_month_day(const year_month_day_last& ymdl) noexcept; constexpr year_month_day(const sys_days& dp) noexcept; constexpr explicit year_month_day(const local_days& dp) noexcept; constexpr year_month_day& operator+=(const months& m) noexcept; constexpr year_month_day& operator-=(const months& m) noexcept; constexpr year_month_day& operator+=(const years& y) noexcept; constexpr year_month_day& operator-=(const years& y) noexcept; constexpr chrono::year year() const noexcept; constexpr chrono::month month() const noexcept; constexpr chrono::day day() const noexcept; constexpr operator sys_days() const noexcept; constexpr explicit operator local_days() const noexcept; constexpr bool ok() const noexcept; }; }
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year_­month_­day represents a specific year, month, and day.
year_­month_­day is a field-based time point with a resolution of days.
[Note 1:
year_­month_­day supports years- and months-oriented arithmetic, but not days-oriented arithmetic.
For the latter, there is a conversion to sys_­days, which efficiently supports days-oriented arithmetic.
— end note]
year_­month_­day meets the Cpp17EqualityComparable (Table 25) and Cpp17LessThanComparable (Table 26) requirements.
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year_­month_­day is a trivially copyable and standard-layout class type.

27.8.14.2 Member functions [time.cal.ymd.members]

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constexpr year_month_day(const chrono::year& y, const chrono::month& m, const chrono::day& d) noexcept;
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Effects: Initializes y_­ with y, m_­ with m, and d_­ with d.
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constexpr year_month_day(const year_month_day_last& ymdl) noexcept;
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Effects: Initializes y_­ with ymdl.year(), m_­ with ymdl.month(), and d_­ with ymdl.day().
[Note 1:
This conversion from year_­month_­day_­last to year_­month_­day might be more efficient than converting a year_­month_­day_­last to a sys_­days, and then converting that sys_­days to a year_­month_­day.
— end note]
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constexpr year_month_day(const sys_days& dp) noexcept;
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Effects: Constructs an object of type year_­month_­day that corresponds to the date represented by dp.
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Remarks: For any value ymd of type year_­month_­day for which ymd.ok() is true, ymd == year_­month_­day{sys_­days{ymd}} is true.
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constexpr explicit year_month_day(const local_days& dp) noexcept;
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Effects: Equivalent to constructing with sys_­days{dp.time_­since_­epoch()}.
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constexpr year_month_day& operator+=(const months& m) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Effects: *this = *this + m.
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Returns: *this.
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constexpr year_month_day& operator-=(const months& m) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Effects: *this = *this - m.
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Returns: *this.
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constexpr year_month_day& year_month_day::operator+=(const years& y) noexcept;
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Effects: *this = *this + y.
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Returns: *this.
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constexpr year_month_day& year_month_day::operator-=(const years& y) noexcept;
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Effects: *this = *this - y.
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Returns: *this.
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constexpr chrono::year year() const noexcept;
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Returns: y_­.
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constexpr chrono::month month() const noexcept;
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Returns: m_­.
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constexpr chrono::day day() const noexcept;
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Returns: d_­.
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constexpr operator sys_days() const noexcept;
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Returns: If ok(), returns a sys_­days holding a count of days from the sys_­days epoch to *this (a negative value if *this represents a date prior to the sys_­days epoch).
Otherwise, if y_­.ok() && m_­.ok() is true, returns sys_­days{y_­/m_­/1d} + (d_­ - 1d).
Otherwise the value returned is unspecified.
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Remarks: A sys_­days in the range [days{-12687428}, days{11248737}] which is converted to a year_­month_­day has the same value when converted back to a sys_­days.
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[Example 1: static_assert(year_month_day{sys_days{2017y/January/0}} == 2016y/December/31); static_assert(year_month_day{sys_days{2017y/January/31}} == 2017y/January/31); static_assert(year_month_day{sys_days{2017y/January/32}} == 2017y/February/1); — end example]
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constexpr explicit operator local_days() const noexcept;
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Returns: local_­days{sys_­days{*this}.time_­since_­epoch()}.
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constexpr bool ok() const noexcept;
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Returns: If y_­.ok() is true, and m_­.ok() is true, and d_­ is in the range [1d, (y_­/m_­/last).day()], then returns true; otherwise returns false.

27.8.14.3 Non-member functions [time.cal.ymd.nonmembers]

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constexpr bool operator==(const year_month_day& x, const year_month_day& y) noexcept;
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Returns: x.year() == y.year() && x.month() == y.month() && x.day() == y.day().
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constexpr strong_ordering operator<=>(const year_month_day& x, const year_month_day& y) noexcept;
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Effects: Equivalent to: if (auto c = x.year() <=> y.year(); c != 0) return c; if (auto c = x.month() <=> y.month(); c != 0) return c; return x.day() <=> y.day();
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constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Returns: (ymd.year() / ymd.month() + dm) / ymd.day().
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[Note 1:
If ymd.day() is in the range [1d, 28d], ok() will return true for the resultant year_­month_­day.
— end note]
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constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Returns: ymd + dm.
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constexpr year_month_day operator-(const year_month_day& ymd, const months& dm) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Returns: ymd + (-dm).
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constexpr year_month_day operator+(const year_month_day& ymd, const years& dy) noexcept;
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Returns: (ymd.year() + dy) / ymd.month() / ymd.day().
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[Note 2:
If ymd.month() is February and ymd.day() is not in the range [1d, 28d], ok() can return false for the resultant year_­month_­day.
— end note]
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constexpr year_month_day operator+(const years& dy, const year_month_day& ymd) noexcept;
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Returns: ymd + dy.
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constexpr year_month_day operator-(const year_month_day& ymd, const years& dy) noexcept;
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Returns: ymd + (-dy).
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const year_month_day& ymd);
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Effects: Equivalent to: return os << (ymd.ok() ? format(STATICALLY-WIDEN<charT>("{:%F}"), ymd) : format(STATICALLY-WIDEN<charT>("{:%F} is not a valid date"), ymd));
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template<class charT, class traits, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, year_month_day& ymd, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
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Effects: Attempts to parse the input stream is into the year_­month_­day ymd using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid year_­month_­day, is.setstate(ios_­base​::​failbit) is called and ymd is not modified.
If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.
If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.
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Returns: is.