25 Time library [time]

25.8 The civil calendar [time.cal]

25.8.4 Class month [time.cal.month]

25.8.4.1 Overview [time.cal.month.overview]

namespace std::chrono {
  class month {
    unsigned char m_;           // exposition only
  public:
    month() = default;
    explicit constexpr month(unsigned m) noexcept;

    constexpr month& operator++()    noexcept;
    constexpr month  operator++(int) noexcept;
    constexpr month& operator--()    noexcept;
    constexpr month  operator--(int) noexcept;

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

    explicit constexpr operator unsigned() const noexcept;
    constexpr bool ok() const noexcept;
  };
}
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month represents a month of a year.
It normally holds values in the range 1 to 12, but may hold non-negative values outside this range.
It can be constructed with any unsigned value, which will be subsequently truncated to fit into month's unspecified internal storage.
month is Cpp17EqualityComparable (Table 22) and Cpp17LessThanComparable (Table 23), and participates in basic arithmetic with months objects, which represent a difference between two month objects.
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month is a trivially copyable and standard-layout class type.

25.8.4.2 Member functions [time.cal.month.members]

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explicit constexpr month(unsigned m) noexcept;
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Effects: Constructs an object of type month by initializing m_­ with m.
The value held is unspecified if m is not in the range [0, 255].
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constexpr month& month::operator++() noexcept;
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Effects: *this += months{1}.
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Returns: *this.
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constexpr month operator++(int) noexcept;
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Effects: ++(*this).
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Returns: A copy of *this as it existed on entry to this member function.
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constexpr month& operator--() noexcept;
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Effects: *this -= months{1}.
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Returns: *this.
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constexpr month operator--(int) noexcept;
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Effects: --(*this).
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Returns: A copy of *this as it existed on entry to this member function.
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constexpr month& operator+=(const months& m) noexcept;
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Effects: *this = *this + m.
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Returns: *this.
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constexpr month& operator-=(const months& m) noexcept;
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Effects: *this = *this - m.
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Returns: *this.
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explicit constexpr month::operator unsigned() const noexcept;
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Returns: m_­.
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constexpr bool month::ok() const noexcept;
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Returns: 1 <= m_­ && m_­ <= 12.

25.8.4.3 Non-member functions [time.cal.month.nonmembers]

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constexpr bool operator==(const month& x, const month& y) noexcept;
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Returns: unsigned{x} == unsigned{y}.
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constexpr bool operator<(const month& x, const month& y) noexcept;
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Returns: unsigned{x} < unsigned{y}.
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constexpr month operator+(const month& x, const months& y) noexcept;
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Returns: 
month{modulo(static_cast<long long>(unsigned{x}) + (y.count() - 1), 12) + 1}
where modulo(n, 12) computes the remainder of n divided by 12 using Euclidean division.
[ Note
:
Given a divisor of 12, Euclidean division truncates towards negative infinity and always produces a remainder in the range of [0, 11].
Assuming no overflow in the signed summation, this operation results in a month holding a value in the range [1, 12] even if !x.ok().
— end note
 ]
[ Example
:
February + months{11} == January.
— end example
 ]
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constexpr month operator+(const months& x, const month& y) noexcept;
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Returns: y + x.
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constexpr month operator-(const month& x, const months& y) noexcept;
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Returns: x + -y.
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constexpr months operator-(const month& x, const month& y) noexcept;
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Returns: If x.ok() == true and y.ok() == true, returns a value m in the range [months{0}, months{11}] satisfying y + m == x.
Otherwise the value returned is unspecified.
[ Example
:
January - February == months{11}.
— end example
 ]
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const month& m);
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Effects: If m.ok() == true inserts format(os.getloc(), fmt, m) where fmt is "%b" widened to charT.
Otherwise inserts unsigned{m} << " is not a valid month".
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Returns: os.
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template<class charT, class traits> basic_ostream<charT, traits>& to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month& m);
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Effects: Streams m 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, month& m, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
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Effects: Attempts to parse the input stream is into the month m using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid month, is.setstate(ios_­base​::​failbit) shall be called and m 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.