A recursive type has infinite size because it doesn't have an indirection.
Erroneous code example:
struct ListNode { head: u8, tail: Option<ListNode>, // error: no indirection here so impossible to // compute the type's size }
When defining a recursive struct or enum, any use of the type being defined from inside the definition must occur behind a pointer (like Box
, &
or Rc
). This is because structs and enums must have a well-defined size, and without the pointer, the size of the type would need to be unbounded.
In the example, the type cannot have a well-defined size, because it needs to be arbitrarily large (since we would be able to nest ListNode
s to any depth). Specifically,
size of `ListNode` = 1 byte for `head` + 1 byte for the discriminant of the `Option` + size of `ListNode`
One way to fix this is by wrapping ListNode
in a Box
, like so:
struct ListNode { head: u8, tail: Option<Box<ListNode>>, }
This works because Box
is a pointer, so its size is well-known.