typedef structures

typedef Struct node {
char elem;
struct node * next;
} StackType;

typedef StackType * StackPtr;
StackPtr top;

is now StackPtr a type of variable?? and top is a variable of StackPtr type??

i get confused going from

struct node {
int val;
struct node *next;
};

to

typedef struct node{
char elem;
struct node *next;
}StackType;

these two structures are essentially the same thing, right???????


if that is the case, then is the following correct

Code:

---


correcting errors in this fragment:

void push (StackPtr *top, char p)
{
StackPtr newptr;
newptr = malloc();
newptr.elem = p;
newptr->next = top;
top = newptr;
}

---

should be changed to

Code:

---

void push(StackPtr *top, char p)
{
StackPtr newptr;
newptr=malloc(sizeof(StackPtr));
newptr->elem = p;
newptr->next = top;
top = newptr;
}

---

Code:

---

void push (StackPtr *top, char p)
{
StackPtr newptr;
newptr = malloc();
newptr.elem = p;
newptr->next = top;
top = newptr;
}

---

This is incorrect. Close, but incorrect. I'll assume you're using pseudo code, in which, this would be correct. It, correctly, would be this:

Code:

---

void push (StackPtr *top, char p)
{
    StackPtr newptr;
    newptr = malloc( sizeof( StackType ) );
    newPtr->elem = p; /* you're using a pointer, remember */
    newptr->next = top;
    top = newptr;
}

---

Yeah. Your second example is correct.

using 'typedef' means you're defining a new variable type. I actually prefer to do this when making my nodes:

Code:

---

typedef struct node Node;
Node {
    Node *n;
    int myVar;
};

---

Nice and clean.

Quzah.

Quzah, your sizeof was of StackType, mine was sizeof StackPtr.
Does it make a difference??? Seems it would, now that I think about it, cuz StackType is a structure type and StackPtr is just a variable of that type. Right??

Yes, there is a difference.
The difference is like:

x = malloc( sizeof( struct ThisStruct ) );
x = malloc( sizeof( struct ThisStruct* ) );

The first, you're allocating space for the whole structure.
The second, you're only allocating space for a _pointer_ to a structure. Big difference.

Quzah.

Instead of using the old, P-style declarations

Code:

---

//  ----p-style----

typedef Struct node {
char elem;
struct node * next;
} StackType;

typedef StackType * StackPtr;
StackPtr top;

---

Let's do it this way instead-- much more readable--

Code:

---


typedef Struct node
  {
  char elem;
  struct node *next;
  }StackType,*StackPtr;

StackPtr top;

---

enjoy.