About Union and it's uses in FB

[na_th_an]

The thing is useful, among other things, and as mentioned, to save memory. It is used for example in the structure which is used to give attributes to a grammar using flex/bison, when symbols can have information attached. As this information can be of different types, you could have used a normal type structure, but that wastes memory as only one field would be used. Using a union, you get the same functionality but you take only the memory which is needed to store the biggest element.