std::chrono::weekday::operator[] - cppreference.com

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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 std::chrono::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 [07] or if !ok() the values (an underlying weekday and an index) held in the result are unspecified.

2) Constructs a std::chrono::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

#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: