Not understanding this forward declaration that I'm getting

Question

So, I have the following .h files: StudentRosterDef.h and StudentRoster.h

StudentRosterDef.h:

typedef struct Node *NodeP;
typedef struct Student *StudentP;

StudentRoster.h:

typedef struct StudentRoster *StudentRosterP;

//Below prototype creates empty studentroster and returns a NULL pointer
StudentRosterP newStudentRoster();

Now, I have the following .c file to accompany : StudentRoster.c

StudentRoster.c:

#include "StudentRosterDef.h"
#include "StudentRoster.h"

struct StudentRosterP
{
    NodeP root;
};

struct NodeP
{
    StudentP left;
    StudentP right;
};

StudentRosterP newStudentRoster()
{
    StudentRosterP thisRoster = (StudentRosterP) malloc(sizeof(StudentRosterP));
    thisRoster->root = 0x00;
    thisRoster = 0x00;
    return thisRoster;
};

Here is the message I get after running the gcc command on the terminal:

StudentRoster.c:27:12 : error: incomplete definition type of 'struct StudentRoster'
        thisRoster->root = 0x00;

        ~~~~~~~~^

./StudentRoster.h:14:16: note: forward declaration of 'struct StudentRoster'
    typedef struct StudentRoster *StudentRosterP;
                   ^

1 error generated.

The StudentRoster.h file can not be changed or modified in anyway as it is a supplied file and the .c and other accompanying .h files must be built to fit the description of StudentRoster.h exactly. Thanks for any help in advance!

Answer 1

You have:

typedef struct StudentRoster *StudentRosterP;

You also have:

struct StudentRosterP
{
    NodeP root;
};

These types are unrelated; you need:

struct StudentRoster // No P at the end here!
{
    NodeP root;
};

Rinse and repeat.

(Note that brunocodutra was saying much the same as this in his answer, but it perhaps wasn't stated quite so precisely or concisely, at least not at first.)

---

Could you explain why NOT having the Pointer as the struct definition would give me an error.

The P suffix is a human convention; the compiler doesn't know that the convention exists. When you define struct StudentRosterP, you still have not defined the struct StudentRoster that is required to access the internals of StudentRosterP thisRoster;. This defines a variable that is a pointer to an incomplete type struct StudentRoster. You can have and use such pointers as long as you don't need to access anything within the structure. They are 'opaque types' and very useful. But using thisRoster->root tries to access the internals of an opaque type, and that doesn't work.

I guess this is all just one more reason not to like using typedef with pointers, even though I understand your hands are tied about the type names.

Answer 2

You need to define the types struct Node and struct StudentRoster instead of using the name the pointer typedefs (struct NodeP and struct StudentRosterP), so the following piece of code is probably what you actually mean:

struct StudentRoster  // No P
{
    NodeP root;
};

struct Node  // No P
{
    StudentP left;
    StudentP right;
};