spivak | A cross-platform Karaoke player | Music Player library

In Python, objects cannot contain other objects. An object in Python can only contain a reference to another object.

Thus, in Python, an object of class BinOp contains three references - self.left, self,right, and self.op. But it does not actually contain three objects.

In C, a struct can actually contain other structs. So an object (in C, the word "object" just means a region of memory interpreted to be of a particular type - not much like Python objects) of type struct binop will contain two objects of type struct node as well as one of type token.

In C, all types must have a size. Let's find some lower bounds on size (these are only lower bounds because we must take padding into account).

sizeof(struct binop) >= sizeof(struct node) + sizeof(token) + sizeof(struct node)

sizeof(struct node) >= sizeof(union nvalue) + sizeof(enum ntype)

sizeof(union nvalue) >= min(sizeof(struct binop), sizeof(struct numnode)) >= sizeof(struct binop)

So we know that sizeof(struct binop) >= sizeof(struct node) + sizeof(token) + sizeof(struct node) > sizeof(struct node) >= sizeof(union nvalue) >= sizeof(struct binop).

Therefore, sizeof(struct binop) > sizeof(struct binop).

This is a problem, as a number cannot be greater than itself. So C cannot compile this code for you because it doesn't know how big a struct binop should be.

In fact, any time a struct contains itself in this way, with no pointers, this is going to cause a serious issue.

The good news is that we can break the cycle by redefining