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std::chrono::operator<<(std::chrono::day) - cppreference.com

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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

os

Example

#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: 

31
07
42 is not a valid day
00 is not a valid day

See also