NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/419

Text File | 1996-08-05 | 4.9 KB | 159 lines

Path: news.mel.aone.net.au!OzEmail!usenet
From: [e-mail] (Ted Jones)
Newsgroups: comp.lang.c++
Subject: Problem with polymorphism.
Date: Thu, 04 Jan 1996 14:10:43 GMT
Organization: Ted Jones Motor Co Ltd.
Message-ID: <4cgjkr$[e-mail]>
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Yo All!  (Email reply's please I don't read regularly enough)

I have a problem with polymorphisim. :(

Here's some code to chew on.

#ifndef _KEY_HPP_
#define _KEY_HPP_

class TKey {
private:
   // In derived classes there should be a "key1" variable defined
here.
public:
   // TKey's destructer is virtual as there may be several different
types of keys
   // with different allocation methods and hence different
de-allocation methods.
   virtual ~TKey(void) {};
   // Returns 0 if keys are equal, 1 if key2 is greater and -1 if key2
is l
   virtual int Compare(TKey *key2) = 0;
};

At this level TKey is abstract and knows nothing of it's descendents.
Here is it's decendent.

class iKey: public TKey {
private:
   int key;
public:
   iKey(int i) { key = i; }
//   virtual ~iTKey(void) {};
   virtual int Compare(TKey *key2);
   virtual int GetKey(void) { return key; }
};

And here is it's implemented pure virtual function Compare();

int iKey::Compare(TKey *key2)
{
   if(key == key2->GetKey()){
      return 0;
   } else {
      if(key < key2->GetKey()){
         return 1;
      } else {
         return -1;
      }
   }
}

This doesn't work tho because Compare is passed a TKey param and TKey
doesn't have the GetKey() member.   I don't think it should because it
doesn't have data to "get".  And if I do declare it there I run into
other problems.  Now if you understand what I'm trying to do and know
how to do it properly please share it with me.

Basically Compare is called by some other class.  Passing in a pointer
to another object derived from the TKey class.  Through polymorphisim
it should call the right GetKey().  Admittedly it would only be sane
to pass in a pointer to a iKey becuase only iKeys should be compared
with iKeys.  But I don't know what would happen if I was to declare
the original pure virtual function as:

virtual int Compare(void) = 0;

and then in all derived classes declare it as:

virtual int Compare(iKey *key2);

etc.  Can I overload virtual functions like that safely?  But then I
also run into the problem with the other class calling
TKey->Compare(tkey);  Because the other class only knows about TKey
and if I decalare it with (void) the other class wont compile.  I am
basically stuck as to what I can do. PLEASE HELP ME. Sob sob. :_(  I'm
loving C++ so far.  It's just when I do weird shit (which is just
plain not possible in C) I get stuck quite badly.

BTW here's the "other class" I was talking about.

class TBTree {                // An incomplete btree class.  Yeah
                    // yeah I know there are lots of class
private:                // libs out there that do this for me.
   typedef struct btree_node {        // But I'm learning and do it all the
      TKey *key;            // hard way to be a better programmer.
      TStorable *data;
      btree_node *left, *right;
   } TBTree_node;

   TBTree_node *btree_root;
   TBTree_node *current;

   int size;
   bool success;

   void RecurseInsert(TBTree_node *root, TBTree_node *node);
public:
   TBTree(void) { btree_root = NULL; size = 0; success = TRUE;}
   virtual ~TBTree(void);

   virtual void Insert(TKey *key, TStorable *data);

   int Size(void) { return size; }
   bool Success(void) { return success; }
};

And here's where it calls Compare();

void TBTree::RecurseInsert(TBTree_node *root, TBTree_node *node)
{
   if (root == NULL){
      root = node;
   } else {
      switch((root->key->Compare(node->key))){
         case 0:
            success = TRUE;
            root = node;                        // Overwrite existing
nodes.
            break;
         case 1:
            RecurseInsert(root->right, node);   // Insert on right.
            break;
         case -1:
            RecurseInsert(root->left, node);    // Insert on left.
            break;
         default:
            // Somehthing is amiss.  Abnormal Termination.
            assert(((root->key->Compare(node->key)) != 0) &&
                   ((root->key->Compare(node->key)) != 1) &&
                   ((root->key->Compare(node->key)) != -1)
                  );
            break;
      }
   }
}

- Dreamer/Reality
--- timEd-B10
 * Origin: æ The Digital Dream's lonely point. (3:774/1500.1)

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