Algebraic data type equivalent in C

Question

I'm writing a program that reads a stream of data and parses it into some values: either integers, floats, characters, or a compound value that contains a set of values (can be nested). How could I represent that in C? I was thinking of an union of int, float, char, then having an array of pointers to such unions for the compound value, but that can't be nested.

Answer 1

(I'm imagining that you are parsing an Xml file)

We'll assume that you have a bunch of nodes. Each node can have a value, it can be one of a set of sibling, and it could have children. That would give you a struct like:

 struct Node
 {
       DATA Value;
       DATATYPE  Type;
       Node* nextSibling;
       Node* firstChild;
 };

DATA could be a union like you described, or separate variables. However, since you will reading values from it in the same form as you stored them, a union should be fine. DATATYPE should be an enum.

Answer 2

You mean char and not char[]? All char values can be stored in an int. For that matter, it's a safe bet that all the int values you want (and all possible int values on your machine) can be represented exactly by double.

So, I recommend a tree structure with double payloads in the nodes. Use an enum to discriminate the type, if necessary. You can represent an n-ary tree using a single child pointer and a single linked-list "next" pointer… Wikipedia has a diagram somewhere but I can't find it :v( .