Forms astd::basic_string<CharT>s consisting of the day value stored ind 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 intoos.

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));

whereSTATICALLY_WIDEN<CharT>("...") is"..." ifCharT ischar, andL"..." ifCharT iswchar_t.

[edit]Return value

os

[edit]Example

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

310742 is not a valid day00 is not a valid day

[edit]See also

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