Question

I’ve got two header files, each requiring a type defined in the other. When I try to compile, I get an error regarding an unknown type name. (If I provide only struct declarations and not definitions, I get an incomplete-types error.) What’s a solution that will let me share these structs properly?

Right now, my code looks rather like the following (just imagine the #ifndef preprocessor directives etc.):

<headerA.h>

#include "headerB.h"
typedef struct { 
  mytypeB myB;
} mytypeA;

<headerB.h>

#include "headerA.h"
typedef struct {} mytypeB;
void foo( mytypeA * myA);

Answer 1

You should forward-declare the struct mytypeA instead of including headerA.h:

Inside headerB.h:

struct mytypeA; // <<<--- Forward declaration

void foo(struct mytypeA* myA);

This works because you are not using the actual mytypeA, only a pointer to it. You cannot pull the same trick with headerA, because mytypeA includes the actual mytypeB.