User-defined literal --C ++ 11 (ten)

No.	head File	Namespace	Literals	Explanation
1	<chrono>	std::literals::chrono_literals	h, min, s, ms, us, ns	123s seconds
2	<string>	std::literals::string_literals	s	"Abc" s is a std :: string
3	<string_view>	std::literals::string_literals	en
4	<complex>	std::literals::complex_literals	i, the, ow	12.7i imaginary part, 1 + 2i is a complex number

Complex constexpr < Double > operator "" I ( Long  Double Arg)   // literal definition of the imaginary part 
{
     return { 0 , Arg}; 
} 
Complex < Double > Z = 2.7 + 2 .3i;