c++11 how to convert a hex string into unicode string

Seems that even with C++11 this is not straightforward to do, e.g.,

string instring = "0x1234";  // hex string
string outstring = "ሴ"       // equivalent to "\u1234"

Assuming I only have this hexadecimal number as an input string, what's the best way to achieve this?

Looks like this '\u' prefix only works when immediately followed by the hex number.