How to convert UID to C++ String

I need to convert a UID to a more generic C++ string in order to concatenate onto a SQL statement. So far I have tried the following to no avail:

1.
UID PreviousUID;
std::string sUID = (std::string)PreviousUID;

produces error: C2440: 'type cast' : cannot convert from 'UID' to 'std::string'

2. Also looked into going through PMString but nothing looked very promising.

Environment: Windows

This seems like something that should be very easy. Anybody know how to do this?