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std::chrono::operator<<(std::chrono::day)

From cppreference.com
< cpp‎ | chrono‎ | day
 
 
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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.

[edit] Return value

os

[edit] 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

[edit] See also

(C++20)
stores formatted representation of the arguments in a new string
(function template) [edit]
formatting support for day
(class template specialization) [edit]