variable or field declared void

This is not actually a problem with the function being "void", but a problem with the function parameters. I think it's just g++ giving an unhelpful error message.

EDIT: As in the accepted answer, the fix is to use std::string instead of just string.

It for example happens in this case here:

void initializeJSP(unknownType Experiment);

Try using std::string instead of just string (and include the <string> header). C++ Standard library classes are within the namespace std::.