Better still, I don't think we need m_typesFixedLengthInBytes or getNumericTypeFixedLengthInBytes.
If you have a VoltType in the caller (as you do), it seems silly to get its name string and pass that into a function that maps the name back to an attribute of the object you had to start with. Why can't you just use an accessor that returns the int member and then treat the -1 value like you are now treating the null return. So all you ned is something like getLengthInBytesForFixedTypes below -- except getLengthInBytesForFixedTypes throws instead of returning -1. Maybe it really doesn't have to do that -- maybe its callers don't really care? So, either simplify it or make another simpler member accessor function.