Question

Sorry if this question has been asked before. On my search through SO I didn’t find one that asked what I wanted to know.

Basically, when I have this:

typedef struct node
{
    int data;
    node *node;
} *head;

and do node *newItem = new node;

I am under the impression that I am declaring and reserving space, but not defining, a pointer to struct node, is that correct?

So when I do

newItem->data = 100 and newItem->next = 0

I get confused. newItem = 0would declare what exactly? Both data and next? The object as a whole?

I’m especially confused when I use typedef. Which part is the macro? I assume node because that’s how I call it, but why do I need it?

Finally, what happens when I do:

node *temp;
temp = new node;

temp = head->next;
head->next = newItem;
newItem->next = temp;

I mean, head->next is a pointer pointing to object newItem, so I assume not to newItem.data or next themselves. So how can I use an uninitialized pointer that I described above safely like here? is head now not pointing to an uninitialized pointer?

Answer 1

I am under the impression that I am
declaring and reserving space, but not
defining, a pointer to struct node, is
that correct?

No. You are declaring a pointer, allocating space on the stack for the pointer, and dynamically allocating storage for a node to it it.

Don’t confuse yourself by writing stuff like this:

typedef struct node
{
    int data;
    node * next;
} *head;

The way to write the struct in C++ is:

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

You can now create a pointer:

node * pnode;

which allocates storage for the pointer.

and you can dynamically allocate storage for a node, and make the pointer point to it:

pnode =  new node;

or do it all in one:

node * pnode = new node;

Now when you say:

pnode->data = 10;

you are not allocating anything. You are assigning 10 to the member called data of the node instance pointed to by pnode. Of course, if you had given your node a constructor (which you should normally do), you could do it all in one:

struct node
{
    int data;
    node * next;

    node( int n, node * np ) : data( n ), next( np ) {}
};

node * pnode = new node( 10, 0 );