Question

Given the following code:

typedef struct elementT {
  int data;
  struct elementT *next;
} element;

Why is it necessary to do struct elementT *next and I can’t do element *next inside the struct declaration itself? Is it because it is not declared yet?

Answer 1

The typedef only takes place after the struct has been defined. Consider the following code; the syntax is invalid, but it hopes it shows the order/precedence of things:

typedef (struct {
  int field1, field2;
}) item;

I.e., the struct{...} is an "expression" that denotes a type. The typedef operates on that type by giving it a name.

By contrast,

struct foo {
    struct foo *next;
};

works because of two special rules of C: struct <name> not only defines a type, but gives it a struct tag as well, and that tag is immediately visible inside the rest of the struct declaration (for the obvious reason: linked lists, trees, etc. would be very painful to implement without these rules).