Dereferencing pointer to incomplete type - c

Question

I am new to C programming and as a mini project I decided to try to implement a stack in C using OOP style structure in a file GenericStack.h as shown below:
void _GENERICSTACK0001(void *,void *);
void *_GENERICSTACK0002(void *);
int _GENERICSTACK0003(void *);
typedef struct
{
struct GenericStackNode *next;
void *data;
int type;
}GenericStackNode;
typedef struct
{
struct GenericStackNode *top;
int count;
void (*add)(void *,void *);
void *(*pop)(void *);
int (*hasNext)(void *);
int (*getCount)(void *);
}GenericStack;
GenericStack newGenericStack()
{
GenericStack *genStack = malloc(sizeof(GenericStack));
genStack->add = _GENERICSTACK0001;
genStack->pop = _GENERICSTACK0002;
genStack->hasNext = _GENERICSTACK0003;
genStack->getCount = _GENERICSTACK0003;
genStack->top=NULL;
genStack->count = 0;
return *genStack;
}
void _GENERICSTACK0001(void *self,void *data)//add
{
GenericStack *genStack = self;
if(genStack->top == NULL)
{
genStack->top = malloc(sizeof(GenericStackNode));
genStack->top->next = NULL;
genStack->top->type = 0;
genStack->top->data = data;
}
else
{
GenericStackNode *temp = malloc(sizeof(GenericStackNode));
temp->next = genStack->top;
temp->type = 0;
temp->data = data;
genStack->top = temp;
genStack->count++;
}
}
void *_GENERICSTACK0002(void *self)//pop
{
GenericStack *genStack = self;
void *data = NULL;
if(genStack->top == NULL)
{
return data;
}
else
{
GenericStackNode *temp = genStack->top;
genStack->top = genStack->top->next;
data = temp->data;
free(temp);
genStack->count--;
return data;
}
}
int _GENERICSTACK0003(void *self)
{
GenericStack *genStack = self;
return genStack->count;
}
All I need to know is why (among many others) I get the specific error:
GenericStack.h:41:16: error: dereferencing pointer to incomplete type
genStack->top->type = 0;
I have checked the other answers on stackoverflow concerning "dereferencing pointer to incomplete type" but I cant seem to understand.

Answer 1

You're getting an error from GenericStack, but you have a problem in both GenericStack and GenericStackNode.
In C, struct X and X are different types. When you write:
struct GenericStackNode *next;
it declares a type called struct GenericStackNode (and a member which is a pointer to that type). This type is incomplete because you have not provided the struct definition.
The type could be completed by providing a struct definition later, but you never do that. Instead, you define an unnamed struct and typedef GenericStackNode to it , but that has no effect on struct GenericStackNode.
Then, struct GenericStackNode *top; still uses this same incomplete type, not the struct you defined above.
Assuming you meant for this pointer to be a pointer to the same type of struct it's contained in, you could use this pattern for both of your structs:
typedef struct X X;
struct X
{
X *ptr;
};
Often people combine the typedef with the struct definition but I find it clearer to have them separate.

Answer 2

You already type-defined GenericStackNode as a type, there is no need for struct GenericStackNode anymore, just GenericStackNode :
typedef struct
{
struct GenericStackNode *top;
...
}
should be only
typedef struct
{
GenericStackNode *top;
...
}
also , you can't use GenericStackNode when you still havn't defined it yet :
typedef struct
{
struct GenericStackNode *next;
void *data;
int type;
} GenericStackNode ;
you can write :
typedef struct GenericStackNode
{
struct GenericStackNode *next;
void *data;
int type;
} GenericStackNode ;