Will I ever need "pointer of type pointer to int?"

**dat** · 10-29-2002

Will I ever use "Pointer of type pointer to int?" Or any other type like double, char or whatever? And if so, when and where?

Thanks,

Dat

**moi** · 10-29-2002

when you need it, be ready to use it. but they are not the most common pointers. i've used char **foo a bunch with text parsing stuff.

**Magos** · 10-29-2002

It is sometimes useful in linked lists. If you want to modify the previous node's NextNode pointer from the current node, you need to get the adress of that pointer to change it from within the previous node (Did that make sense? Well, I tried...

).

Code:

//Not good, only the local pointer is changed
void SetNextNode(NODE* Node)
{
   Node = this->NextNode;
}

//Works better
void SetNextNode(NODE** Node)
{
   *Node = this->NextNode;
}

**dat** · 10-29-2002

Originally posted by Magos
It is sometimes useful in linked lists. If you want to modify the previous node's NextNode pointer from the current node, you need to get the adress of that pointer to change it from within the previous node (Did that make sense? Well, I tried... ).

Code:

//Not good, only the local pointer is changed void SetNextNode(NODE* Node) { Node = this->NextNode; } //Works better void SetNextNode(NODE** Node) { *Node = this->NextNode; }

Huh?

**The Dog** · 10-29-2002

You'll obviously need a pointer if you want to pass a pointer by reference as in Magos' example.

Here's the same thing, but you might grasp it better :

Code:

int OpenFile(FILE** fp, char* fileName, char* mode)
{
      if ( (*fp = fopen(fileName, mode)) == NULL )
      {
            return -1;
      }
      return 0;
}

If the parameter fp was just a FILE*, then the stream would only be accessible within the function OpenFile(), and not outside it.

**biosninja** · 10-29-2002

Huh?

it's part of a linked list.

Study linked lists, very cool and usefull

**FCF** · 10-30-2002

Study linked lists, very cool and usefull

I agree.

Currently i am studying linked list, there are two different implementations-- array-based and use malloc()'s linked list(sorry dunno the name). My question is:
When i use the array-based linked list, i usually define a constant first like #define MAX_NODE 999. u know, the constant is fixed so if the data exceeds the MAX_NODE, the linked list can't handle the data. The conclusion is: array-based linked list is not so good. Am i right?

**Magos** · 10-31-2002

Originally posted by FCF
The conclusion is: array-based linked list is not so good. Am i right?

Depends. Everything has its good and bad sides. Having a static array as the list consumes unused memory if you have little data, and might not be large enough if you have a lot of data.
On the other hand, you can't directly access a specific element using a dynamic linked list, though you can theoretically store an unfinite amount of data.

If you know exactly how many elements you need to store, use arrays. If you don't know (you might add or delete during run-time) use a dynamic linked list.

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