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Question

I'm trying to write some functions to work on a balanced binary tree.

First I wrote a typical binary tree interface. This encapsulates the general functionality associated with binary trees.

The tree has nodes

typedef struct Node
{
  Node* left;
  Node* right;
  Node* parent;

  int key;

  void* value;

} Node;

and some functions that do insert , remove and search .

Now I want to extend that interface to work on a different types of binary trees, which inherits Node .

typedef enum Color
{
  RED,
  BLACK

} Color;

typedef struct RBTreeNode
{
  Node* genericNode;
  Color color;

} RBTreeNode;

RBTree refers to Red-Black Trees

The trouble ensues when I try to write a "tree repair" function.

void repairRBTree(Node* nodeInserted)
{

  // If nodeInserted's parent is NULL, nodeInserted is the root of the tree.
  // Red-Black tree properties suggest root node's color be black.
  if (nodeInserted->parent == NULL)
    {
      RBTreeNode* nodeInsertedTC = (RBTreeNode*)nodeInserted;
      nodeInsertedTC->color      = BLACK;
    }

  // If nodeInserted's parent's color is BLACK, nodeInserted has replaced a RED NULL node.
  // Red-Black tree properties suggest RED node's parent be BLACK,
  // which is the case currently, so there's nothing to be done.
  else if (nodeInserted->parent->(COLOR??))
    {
      return;
    }
}

In this if statement,

  if (nodeInserted->parent == NULL)
    {
      RBTreeNode* nodeInsertedTC = (RBTreeNode*)nodeInserted;
      nodeInsertedTC->color      = BLACK;
    }

if I had previously cast nodeInserted as Node* , that means the pointer itself is a RBTreeNode* , so if what I'm thinking is correct, casting it back to RBTreeNode* should do what I think it should.

But here

  // If nodeInserted's parent's color is BLACK, nodeInserted has replaced a RED NULL node.
  // Red-Black tree properties suggest RED node's parent be BLACK,
  // which is the case currently, so there's nothing to be done.
  else if (nodeInserted->parent->(COLOR??))
    {
      return;
    }
}

I don't have access to nodeInserted->parent 's Color enum. And I don't think casting it to RBTreeNode will do much good.

The only solution I know will work is if I rewrite all of my generalized functions to take in RBTreeNode as param instead of Node , but I really don't want to do that.

Is there a better solution?

Answer 1

You should not use a pointer to implement inheritance. Use a Node field instead of a pointer:

typedef struct RBTreeNode
{
  Node genericNode;
  Color color;

} RBTreeNode;

This way, when you cast Node* to RBTreeNode* , it will have access to all the fields of RBTreeNode .

Since you are probably using a c++ compiler, a c++ analogy might help. Having a first field of type Node is like having inheritance in c++, ie struct RBTreeNode: Node . Having a first field of a pointer type is like having virtual inheritance, ie struct RBTreeNode: virtual Node . Both ways work, until you need a downcast. Virtual inheritance in c++ alerts the reader that you have something fishy about your inheritance hierarchy ("diamond inheritance"), so you should only use it when normal inheritance won't do.