30 Time library [time]

30.8 The civil calendar [time.cal]

30.8.14 Class year_month_day [time.cal.ymd]

30.8.14.1 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; }; }

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 28) and Cpp17LessThanComparable (Table 29) requirements.

year_month_day is a trivially copyable and standard-layout class type.

30.8.14.2 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;

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;

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 can 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;

Effects: Constructs an object of type year_month_day that corresponds to the date represented by dp.

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;

Effects: Equivalent to constructing with sys_days{dp.time_since_epoch()}.

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constexpr year_month_day& operator+=(const months& m) noexcept;

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]).

Effects: *this = *this + m.

Returns: *this.

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constexpr year_month_day& operator-=(const months& m) noexcept;

Effects: *this = *this - m.

Returns: *this.

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constexpr year_month_day& year_month_day::operator+=(const years& y) noexcept;

Effects: *this = *this + y.

Returns: *this.

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constexpr year_month_day& year_month_day::operator-=(const years& y) noexcept;

Effects: *this = *this - y.

Returns: *this.

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constexpr chrono::year year() const noexcept;

Returns: y_.

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constexpr chrono::month month() const noexcept;

Returns: m_.

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constexpr chrono::day day() const noexcept;

Returns: d_.

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constexpr operator sys_days() const noexcept;

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.

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.

[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;

Returns: local_days{sys_days{*this}.time_since_epoch()}.

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constexpr bool ok() const noexcept;

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.

30.8.14.3 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;

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;

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;

Returns: (ymd.year() / ymd.month() + dm) / ymd.day().

[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;

Returns: ymd + dm.

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constexpr year_month_day operator-(const year_month_day& ymd, const months& dm) noexcept;

Returns: ymd + (-dm).

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constexpr year_month_day operator+(const year_month_day& ymd, const years& dy) noexcept;

Returns: (ymd.year() + dy) / ymd.month() / ymd.day().

[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;

Returns: ymd + dy.

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constexpr year_month_day operator-(const year_month_day& ymd, const years& dy) noexcept;

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);

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);

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.

Returns: is.