27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.7 Class weekday_­indexed [time.cal.wdidx]

27.8.7.1 Overview [time.cal.wdidx.overview]

namespace std::chrono {
  class weekday_indexed {
    chrono::weekday  wd_;       // exposition only
    unsigned char    index_;    // exposition only

  public:
    weekday_indexed() = default;
    constexpr weekday_indexed(const chrono::weekday& wd, unsigned index) noexcept;

    constexpr chrono::weekday weekday() const noexcept;
    constexpr unsigned        index()   const noexcept;
    constexpr bool ok() const noexcept;
  };
}
1
#
weekday_­indexed represents a weekday and a small index in the range 1 to 5.
This class is used to represent the first, second, third, fourth, or fifth weekday of a month.
2
#
[Note
:
A weekday_­indexed object can be constructed by indexing a weekday with an unsigned.
— end note
]
[Example
: 
constexpr auto wdi = Sunday[2]; // wdi is the second Sunday of an as yet unspecified month
static_assert(wdi.weekday() == Sunday);
static_assert(wdi.index() == 2);
— end example
]
3
#
weekday_­indexed is a trivially copyable and standard-layout class type.