Thread: Hiding Structure members

I want to include the header file linked_list.h into main and I want to have the structure LINKED_LIST available in main but not its members(NODE_PTR head, NODE_PTR tail). How do I achieve that?

linked_list.h

Code:

#ifndef LINKED_LIST__HH
#define LINKED_LIST__HH
#include "node.h"

typedef struct linked_list {
  NODE_PTR head;
  NODE_PTR tail;
} LINKED_LIST;

void add_head(LINKED_LIST*, int);
void add_tail(LINKED_LIST*, int);
void display_linked_list(LINKED_LIST);
void free_linked_list(LINKED_LIST*);
LINKED_LIST create_linked_list();

#endif

Basically I want to hide the details of the structure LINKED_LIST.

Another approach is to identify your "private" members with some special naming and never use them outside the functions which deals with the structure directly... Example:

Code:

struct list {
  struct node *head_, *tail_;
};

Here head_ and tail_ are pointers suitable to be used just inside list_*() functions, not outside them...

You're not 'hiding' anything, but using an 'hidding' approach to your naming convention.

Originally Posted by laserlight

The typical approach is to provide an opaque pointer. But this means that all the functions that make up the interface of your library must either accept a pointer parameter or return a pointer that is never dereferenced by the caller.

So for example, you could provide a header like this:

Code:

#ifndef LINKED_LIST__HH
#define LINKED_LIST__HH

typedef struct linked_list *LINKED_LIST;

void add_head(LINKED_LIST, int);
void add_tail(LINKED_LIST, int);
void display_linked_list(LINKED_LIST);
void free_linked_list(LINKED_LIST);
LINKED_LIST create_linked_list(void);

#endif

Then in your source file, you would have:

Code:

#include "linked_list.h"
#include "node.h"

struct linked_list {
    NODE_PTR head;
    NODE_PTR tail;
};

void add_head(LINKED_LIST list, int value)
{
    // ...
}

// ... etc ...

Just a follow up question.. We can defined a pointer to a structure(without defining the structure in the header file) because the compiler knows how big a pointer is right?

Exactly correct. (And a big hint about how everything in C works, under the hood: if the compiler knows how to allocate storage for it, you can probably get it to leave you alone. If the compiler doesn't know how to allocate storage for it, you have to do something different.)