error: dereferencing pointer to incomplete type

i have the following code parts:

typedef struct Object* ObjectP;

ObjectP CreateObject(void *key) {

    printf("GOT %d\n",(*(int*) key));
    ObjectP objP = (ObjectP开发者_运维问答) malloc(sizeof(Object));
    if (objP == NULL) {
        //TODO ReportError(MEM_OUT);
        return NULL;
    }
    objP->_key = key;
    objP->_next = NULL;
    objP->_numInChain = 1;

    return objP;

}

typedef struct Object {
  ObjectP _next;
  void* _key;
  int _numInChain;
} Object;

and in another file:

void IntPrint(const void *key) {
    assert(key != NULL);
    printf("%d",*((const int*)key));
}

and in another file I have the main() :

int main(int argc, char* argv[]) {
 int* key = (int*) malloc(sizeof(int));
 *key = 20;
 ObjectP a = CreateObject(key);
 IntPrint(a->_key);  //THIS DOESN'T COMPILE
 return 0;
}

IntPrint doesn't compile. it writes:

error: dereferencing pointer to incomplete type

and i can't understand why, because IntPrint recieves void* and a->_key is also void*.

Thanks!

---

You posted a bunch of virtually irrelevant pieces of code, but omitted the most important one: what is ObjectP and how it is defined? The compiler tells you that it is defined as a pointer to incomplete type. That's your problem. What exactly is wrong with the definition is impossible to say without actually seeing it.

After the edit: Your problem is that the definition of struct Object is not visible at the point where you are trying to access a->key (in.e. in main). You either forgot to include it or something like that. Is the definition of struct Object located in the same header file as definition of ObjectP?

---

Where do you have ObjectP defined? I suggest not declaring it before Object is fully defined:

typedef struct Object {
  struct Object*  _next;
  void* _key;
  int _numInChain;
} Object;

typedef Object* ObjectP;

This way it should be always safe to use ObjectP. However, without you posting more code, this is just a blind guess.

---

Try the following in your main function:

IntPrint((const void *) a->_key);

The definition of _key in the structure Object doesn't have the modifier constant and that could cause a warning or an error depending the flags you're using when compiling.