25 Time library [time]

25.8 The civil calendar [time.cal]

25.8.6 Class weekday [time.cal.wd]

25.8.6.1 Overview [time.cal.wd.overview]

namespace std::chrono {
  class weekday {
    unsigned char wd_;          // exposition only
  public:
    weekday() = default;
    explicit constexpr weekday(unsigned wd) noexcept;
    constexpr weekday(const sys_days& dp) noexcept;
    explicit constexpr weekday(const local_days& dp) noexcept;

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

    constexpr weekday& operator+=(const days& d) noexcept;
    constexpr weekday& operator-=(const days& d) noexcept;

    explicit constexpr operator unsigned() const noexcept;
    constexpr bool ok() const noexcept;

    constexpr weekday_indexed operator[](unsigned index) const noexcept;
    constexpr weekday_last    operator[](last_spec) const noexcept;
  };
}
weekday represents a day of the week in the civil calendar.
It normally holds values in the range 0 to 6, corresponding to Sunday through Saturday, but it may hold non-negative values outside this range.
It can be constructed with any unsigned value, which will be subsequently truncated to fit into weekday's unspecified internal storage.
weekday is Cpp17EqualityComparable (Table 22).
[Note
:
weekday is not Cpp17LessThanComparable because there is no universal consensus on which day is the first day of the week.
weekday's arithmetic operations treat the days of the week as a circular range, with no beginning and no end.
end note
]
weekday is a trivially copyable and standard-layout class type.

25.8.6.2 Member functions [time.cal.wd.members]

explicit constexpr weekday(unsigned wd) noexcept;
Effects: Constructs an object of type weekday by initializing wd_­ with wd.
The value held is unspecified if wd is not in the range [0, 255].
constexpr weekday(const sys_days& dp) noexcept;
Effects: Constructs an object of type weekday by computing what day of the week corresponds to the sys_­days dp, and representing that day of the week in wd_­.
[Example
:
If dp represents 1970-01-01, the constructed weekday represents Thursday by storing 4 in wd_­.
end example
]
explicit constexpr weekday(const local_days& dp) noexcept;
Effects: Constructs an object of type weekday by computing what day of the week corresponds to the local_­days dp, and representing that day of the week in wd_­.
Remarks: The value after construction is identical to that constructed from sys_­days{dp.time_­since_­epoch()}.
constexpr weekday& operator++() noexcept;
Effects: *this += days{1}.
Returns: *this.
constexpr weekday operator++(int) noexcept;
Effects: ++(*this).
Returns: A copy of *this as it existed on entry to this member function.
constexpr weekday& operator--() noexcept;
Effects: *this -= days{1}.
Returns: *this.
constexpr weekday operator--(int) noexcept;
Effects: --(*this).
Returns: A copy of *this as it existed on entry to this member function.
constexpr weekday& operator+=(const days& d) noexcept;
Effects: *this = *this + d.
Returns: *this.
constexpr weekday& operator-=(const days& d) noexcept;
Effects: *this = *this - d.
Returns: *this.
explicit constexpr operator unsigned() const noexcept;
Returns: wd_­.
constexpr bool ok() const noexcept;
Returns: wd_­ <= 6.
constexpr weekday_indexed operator[](unsigned index) const noexcept;
Returns: {*this, index}.
constexpr weekday_last operator[](last_spec) const noexcept;
Returns: weekday_­last{*this}.

25.8.6.3 Non-member functions [time.cal.wd.nonmembers]

constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
Returns: unsigned{x} == unsigned{y}.
constexpr weekday operator+(const weekday& x, const days& y) noexcept;
Returns:
weekday{modulo(static_cast<long long>(unsigned{x}) + y.count(), 7)}
where modulo(n, 7) computes the remainder of n divided by 7 using Euclidean division.
[Note
:
Given a divisor of 7, Euclidean division truncates towards negative infinity and always produces a remainder in the range of [0, 6].
Assuming no overflow in the signed summation, this operation results in a weekday holding a value in the range [0, 6] even if !x.ok().
end note
]
[Example
:
Monday + days{6} == Sunday.
end example
]
constexpr weekday operator+(const days& x, const weekday& y) noexcept;
Returns: y + x.
constexpr weekday operator-(const weekday& x, const days& y) noexcept;
Returns: x + -y.
constexpr days operator-(const weekday& x, const weekday& y) noexcept;
Returns: If x.ok() == true and y.ok() == true, returns a value d in the range [days{0}, days{6}] satisfying y + d == x.
Otherwise the value returned is unspecified.
[Example
:
Sunday - Monday == days{6}.
end example
]
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const weekday& wd);
Effects: If wd.ok() == true inserts format(os.getloc(), fmt, wd) where fmt is "%a" widened to charT.
Otherwise inserts unsigned{wd} << " is not a valid weekday".
Returns: os.
template<class charT, class traits> basic_ostream<charT, traits>& to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const weekday& wd);
Effects: Streams wd into os using the format specified by the NTCTS fmt.
fmt encoding follows the rules specified in [time.format].
Returns: os.
template<class charT, class traits, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, weekday& wd, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the weekday wd using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid weekday, is.setstate(ios_­base::failbit) shall be called and wd 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.
Returns: is.