C - Field has incomplete type

Question

In the below representation,

struct Cat{
  char *name;
  struct Cat mother;
  struct Cat *children;
};

Compiler gives below error for second field, but not third field,

 error: field ‘mother’ has incomplete type
   struct Cat mother;
              ^

How to understand this error?

[Problem in defining object of a structure inside itself](http://stackoverflow.com/questions/5329570/problem-in-defining-object-of-a-structure-inside-itself) — BLUEPIXY, Jan 28 '17 at 21:38

score 22 · Accepted Answer · edited Jun 06 '19 at 13:19

22

The error means that you try and add a member to the struct of a type that isn't fully defined yet, so the compiler cannot know its size in order to determine the objects layout.

In you particular case, you try and have struct Cat hold a complete object of itself as a member (the mother field). That sort of infinite recursion in type definition is of course impossible.

Nevertheless, structures can contain pointers to other instances of themselves. So if you change your definition as follows, it will be a valid struct:

struct Cat{
  char *name;
  struct Cat *mother;
  struct Cat *children;
};

edited Jun 06 '19 at 13:19

SolarBear

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answered Jan 28 '17 at 21:36

StoryTeller - Unslander Monica

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so that means, `struct Cat children[10]` will also be not allowed. What is recursive in type definition here? – user1787812 Jan 28 '17 at 21:38
1

@user1787812 - Correct. However `struct Cat *children[10]` (not the `*`) is allowed. An array of *pointers* is perfectly okay. – StoryTeller - Unslander Monica Jan 28 '17 at 21:38
@user1787812 - The recursion is in the type containing itself **completely** as a member. – StoryTeller - Unslander Monica Jan 28 '17 at 21:39
Why `struct Cat *children` member does not make the type recursive? – user1787812 Jan 28 '17 at 21:40
@user1787812 - because it's a pointer. You don't need the complete type definition to declare a pointer to it. – StoryTeller - Unslander Monica Jan 28 '17 at 21:41
So, Do you mean infinite recursive type definition, in the sense, that type checker(in compiler) tries to know the type of `mother` and gets into infinite recursive calls? – user1787812 Jan 28 '17 at 21:46
1

@user1787812 - It doesn't go into recursive calls. It stops and emits an error because it recognizes the type contains itself as member. – StoryTeller - Unslander Monica Jan 28 '17 at 21:47

C - Field has incomplete type

1 Answers1