comp.lang.c FAQ list · Question 1.15

Q: How can I define a pair of mutually referential structures? I tried

	typedef struct {
		int afield;
		BPTR bpointer;
	} *APTR;

	typedef struct {
		int bfield;
		APTR apointer;
	} *BPTR;

but the compiler doesn't know about BPTR when it is used in the first structure declaration.

A: As in question 1.14, the problem lies not in the structures or the pointers but the typedefs. First, give the two structures tags, and define the link pointers without using typedefs:

	struct a {
		int afield;
		struct b *bpointer;
	};

	struct b {
		int bfield;
		struct a *apointer;
	};

The compiler can accept the field declaration struct b *bpointer within struct a, even though it has not yet heard of struct b. (struct b is ``incomplete'' at that point.) Occasionally it is necessary to precede this couplet with the empty declaration

	struct b;

to mask the declarations (if in an inner scope) from a different struct b in an outer scope.

After declaring the two structures using struct tags, you can then declare the typedefs separately:

	typedef struct a *APTR;
	typedef struct b *BPTR;

Alternatively, you can define the typedefs before the struct definitions[footnote] , in which case you can use them when declaring the link pointer fields:

	typedef struct a *APTR;
	typedef struct b *BPTR;

	struct a {
		int afield;
		BPTR bpointer;
	};

	struct b {
		int bfield;
		APTR apointer;
	};