Why can't a struct have a member that is of the same type as itself?

Question

I\'m stuck on understanding what\'s happening with this struct (from C for Programmers a Deitel book).

The book says, \"A structure cannot contain an

Answer 1

The magic here becomes clear when you understand the C concept of an incomplete type. A struct can only contain completed types, i.e. those that the compiler knows the size of. When the compiler sees

struct foo {

it knows that there will be a struct with the tag foo; this type (struct foo) is incomplete at this very moment. It becomes complete not until the matching } is seen.

However, and this is the magic, a pointer to an incomplete type is a complete type, because the size of any pointer is known--no matter what type it points to. So after the three tokens above, it is okay to declare a struct member

  struct foo *ptr_to_struct_foo;

inside a struct foo and before the matching }.

Another often used incomplete type is void--this type cannot even be completed, which is why C doesn't allow you to declare

void foo;

However, it is perfectly fine to declare pointers to such a type:

void *foo;

But indirecting through a ptr-to-void of course is not allowed. Why? Now you know the answer: because that would yield an incomplete type.