25 Time library [time]

25.8 The civil calendar [time.cal]

25.8.13 Class year_­month [time.cal.ym]

25.8.13.1 Overview [time.cal.ym.overview]

namespace std::chrono {
  class year_month {
    chrono::year  y_;           // exposition only
    chrono::month m_;           // exposition only

  public:
    year_month() = default;
    constexpr year_month(const chrono::year& y, const chrono::month& m) noexcept;

    constexpr chrono::year  year()  const noexcept;
    constexpr chrono::month month() const noexcept;

    constexpr year_month& operator+=(const months& dm) noexcept;
    constexpr year_month& operator-=(const months& dm) noexcept;
    constexpr year_month& operator+=(const years& dy)  noexcept;
    constexpr year_month& operator-=(const years& dy)  noexcept;

    constexpr bool ok() const noexcept;
  };
}
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year_­month represents a specific month of a specific year, but with an unspecified day.
year_­month is a field-based time point with a resolution of months.
year_­month is Cpp17EqualityComparable (Table 22) and Cpp17LessThanComparable (Table 23).
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year_­month is a trivially copyable and standard-layout class type.

25.8.13.2 Member functions [time.cal.ym.members]

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constexpr year_month(const chrono::year& y, const chrono::month& m) noexcept;
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Effects: Constructs an object of type year_­month by initializing y_­ with y, and m_­ with m.
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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 year_month& operator+=(const months& dm) noexcept;
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Effects: *this = *this + dm.
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Returns: *this.
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constexpr year_month& operator-=(const months& dm) noexcept;
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Effects: *this = *this - dm.
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Returns: *this.
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constexpr year_month& operator+=(const years& dy) noexcept;
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Effects: *this = *this + dy.
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Returns: *this.
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constexpr year_month& operator-=(const years& dy) noexcept;
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Effects: *this = *this - dy.
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Returns: *this.
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constexpr bool ok() const noexcept;
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Returns: y_­.ok() && m_­.ok().

25.8.13.3 Non-member functions [time.cal.ym.nonmembers]

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constexpr bool operator==(const year_month& x, const year_month& y) noexcept;
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Returns: x.year() == y.year() && x.month() == y.month().
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constexpr bool operator<(const year_month& x, const year_month& y) noexcept;
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Returns: If x.year() < y.year() returns true.
Otherwise, if x.year() > y.year() returns false.
Otherwise, returns x.month() < y.month().
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constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
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Returns: A year_­month value z such that z - ym == dm.
Complexity: with respect to the value of dm.
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constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
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Returns: ym + dm.
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constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
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Returns: ym + -dm.
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constexpr months operator-(const year_month& x, const year_month& y) noexcept;
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Returns: 
x.year() - y.year() + months{static_cast<int>(unsigned{x.month()}) -
                             static_cast<int>(unsigned{y.month()})}
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constexpr year_month operator+(const year_month& ym, const years& dy) noexcept;
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Returns: (ym.year() + dy) / ym.month().
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constexpr year_month operator+(const years& dy, const year_month& ym) noexcept;
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Returns: ym + dy.
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constexpr year_month operator-(const year_month& ym, const years& dy) noexcept;
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Returns: ym + -dy.
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const year_month& ym);
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Returns: os << ym.year() << '/' << ym.month().
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template<class charT, class traits> basic_ostream<charT, traits>& to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month& ym);
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Effects: Streams ym into os using the format specified by the NTCTS fmt.
fmt encoding follows the rules specified in [time.format].
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Returns: os.
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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& ym, 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 ym using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid year_­month, is.setstate(ios_­base​::​failbit) shall be called and ym shall not be 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.