29 Input/output library [input.output]

29.7 Formatting and manipulators [iostream.format]

29.7.5 Output streams [output.streams]

29.7.5.5 Rvalue stream insertion [ostream.rvalue]

template<class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);
Constraints: The expression os << x is well-formed when treated as an unevaluated operand.
Effects: As if by: os << x;
Returns: os.