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Question

I want to have two structs that can contain each other. Here is an example:

struct a {
  struct b bb;
};

struct b {
  struct a aa;
};

But this code doesn't compile. gcc says:

test.c:3: error: field ‘bb’ has incomplete type

Is there a way to achieve this?

Answer 1

How is that supposed to work? a would contain b , which would contain a , which would contain b , etc...

I suppose you want to use a pointer instead?

struct b;

struct a {
  struct b *bb;
};

struct b {
  struct a *aa;
};

Even that though is bad coding style - circular dependencies should be avoided if possible.

Answer 2

struct a;
struct b;

struct a{
   struct b *bb;
};

struct b{
   struct a *aa;
};

Most of the header file declare the structure before defining its members. Structure definition will be defined in somewhere else.

Answer 3

The usual way of dealing with this is to make them pointers and then dynamically allocate them or even just assign the pointer from the address of a static instance of the other struct.

struct a {
  struct b *bb;
};

struct b {
  struct a *aa;
};

struct a a0;
struct b b0;

void f(void) {
  a0.bb = &b0;
  b0.aa = &a0;
}

I would suggest, however, that you look for a tree-structured organization. Perhaps both objects could point to a common third type.

Answer 4

This is nonsensical.

Imagine if you say that every X contains a Y and every Y contains an X , then inside each X is a Y which in turn contains an X , which in turn contains a Y , which in turn contains an X , ad infinitum .

Instead, you can have an X contain a reference to or (or pointer to ) a Y and vice-versa.