Defined in header <chrono>
template< class CharT, class Traits >
std::basic_ostream<CharT, Traits>&
operator<<( std::basic_ostream<CharT, Traits>& os, const std::chrono::day& d );
(since C++20)

Forms a std::basic_string<CharT> s consisting of the day value stored in d formatted as a decimal number, with a leading zero if the result would otherwise be a single decimal digit. Then, if !d.ok(), append " is not a valid day" to the formatted string. Inserts that string into os.

Equivalent to.

return os << (d.ok() ?
std::format(STATICALLY_WIDEN<CharT>("{:%d}"), d) :
std::format(STATICALLY_WIDEN<CharT>("{:%d} is not a valid day"), d));

where STATICALLY_WIDEN<CharT>("...") is "..." if CharT is char, and L"..." if CharT is wchar_t.

Return value



#include <chrono>
#include <iostream>
int main()
    constexpr std::chrono::day d1 {31}, d2 {7}, d3 {42}, d4 {};
    std::cout << d1 << '\n'
              << d2 << '\n'
              << d3 << '\n'
              << d4 << '\n';

Possible output:

42 is not a valid day
00 is not a valid day

See also

stores formatted representation of the arguments in a new string
(function template)
specialization of std::formatter that formats a day according to the provided format
(class template specialization)

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