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std::chrono::weekday:: operator[]

From cppreference.net
C++
Date and time library
std::chrono::weekday
constexpr std:: chrono :: weekday_indexed
operator [ ] ( unsigned index ) const noexcept ;
(1) (since C++20)
constexpr std:: chrono :: weekday_last
operator [ ] ( std:: chrono :: last_spec ) const noexcept ;
(2) (since C++20)
1) Constructs a weekday_indexed from * this and index . The result represents the index -th weekday in some yet-to-be-specified month. If index is not in the range [ ​ 0 ​ , 7 ] or if ! ok ( ) the values (an underlying weekday and an index) held in the result are unspecified.
2) Constructs a weekday_last from * this . The result represents the last weekday in some yet-to-be-specified month.

Return value

1) std:: chrono :: weekday_indexed ( * this, index )
2) std:: chrono :: weekday_last ( * this )

Example

Run this code 
#include <chrono>
#include <iostream>
using namespace std::chrono;
int main()
{
    constexpr auto second_tuesday_in_October_2019 =
        year_month_day{Tuesday[2] / October / 2019y};
    constexpr auto last_tuesday_in_October_2019 =
        year_month_day{Tuesday[last] / October / 2019y};
    std::cout << second_tuesday_in_October_2019 << '\n'
              << last_tuesday_in_October_2019 << '\n'; 
}

Possible output: 

2019-10-08
2019-10-29