问题
I've been working on an assignment and now I'm stuck with buggy destructors. I have to create a generic binary tree with all the usual member functions and some special operators. There's also a restriction: everything must work iteratively so no nasty recursive hacks this time.
There is obviously something very wrong with the destructor of BinTreeNode class because if I delete the node like this:
BinTreeNode<int> * node = new BinTreeNode<int>();
delete node;
I can still access its data:
node->getData(); //should fail miserably
so deletion has no effect but I have no usable idea how I should correct the destructor. It seems to me that the algorithm should be about right so I suspect there's something wrong with how I use pointers but at this point I'm so confused that I don't even understand my own code.
Code I have this far:
BinTree.h
#ifndef BINTREE_H_
#define BINTREE_H_
#ifndef NULL
#define NULL 0
#endif
#include "BinTreeNode.h"
template <class T>
class BinTree
{
private:
BinTreeNode<T> * root;
public:
//constructors and destructor
BinTree():
root(NULL){}
BinTree(T data):
root(new BinTreeNode<T>(data)){}
~BinTree();
//search
BinTreeNode<T> * search(T data);
//insert
bool insert(T data);
//remove
bool remove(T data);
};
template <class T>
BinTree<T>::~BinTree()
{
delete root;
}
template <class T>
BinTreeNode<T> * BinTree<T>::search(T data)
{
BinTreeNode<T> * node = new BinTreeNode<T>(data);
BinTreeNode<T> * current = root;
while (current != NULL)
{
if (*current == *node)
{
delete node;
return root;
}
else if (*node < *current)
{
current = current->getLeft();
}
else
{
current = current->getRight();
}
}
delete node;
return NULL;
}
template <class T>
bool BinTree<T>::insert(T data)
{
BinTreeNode<T> * node = new BinTreeNode<T>(data);
BinTreeNode<T> * current = root;
while (current != NULL)
{
if (*current == *node)
{
delete node;
return false;
}
else if (*node < *current)
{
if (current->getLeft() == NULL)
{
current->setLeft(node);
return true;
}
else
{
current = current->getLeft();
}
}
else
{
if (current->getRight() == NULL)
{
current->setRight(node);
return true;
}
else
{
current = current->getRight();
}
}
}
return false;
}
#endif
BinTreeNode.h
#ifndef BINTREENODE_H_
#define BINTREENODE_H_
#ifndef NULL
#define NULL 0
#endif
template <class T>
class BinTreeNode
{
private:
T data;
BinTreeNode<T> *left, *right, *parent;
public:
//constructors and destructor
BinTreeNode():
data(NULL), left(NULL), right(NULL), parent(NULL){}
BinTreeNode(T data):
data(data), left(NULL), right(NULL), parent(NULL){}
~BinTreeNode();
//set and get data member
T getData() const;
void setData(T data);
//set and get left and right branches
BinTreeNode<T> * getLeft() const;
BinTreeNode<T> * getRight() const;
void setLeft(BinTreeNode<T> * node);
void setRight(BinTreeNode<T> * node);
//set and get parent
BinTreeNode<T> * getParent() const;
void setParent(BinTreeNode<T> * node);
//comparison operators
bool operator<(const BinTreeNode<T>& node) const;
bool operator>(const BinTreeNode<T>& node) const;
bool operator==(const BinTreeNode<T>& node) const;
};
template <class T>
BinTreeNode<T>::~BinTreeNode()
{
BinTreeNode<T> * current = this;
BinTreeNode<T> * parent = NULL;
while (current != NULL)
{
parent = current->getParent();
if (current->getLeft() == NULL)
current = current->getLeft();
else if (current->getRight() == NULL)
current = current->getRight();
else
{
if (parent->getRight() == current)
parent->setRight(NULL);
else
parent->setLeft(NULL);
current = NULL; // this line (among others) is very suspicious
}
current = parent;
}
}
template <class T>
T BinTreeNode<T>::getData() const
{
return data;
}
template <class T>
void BinTreeNode<T>::setData(T data)
{
this->data = data;
}
template <class T>
BinTreeNode<T> * BinTreeNode<T>::getLeft() const
{
return left;
}
template <class T>
BinTreeNode<T> * BinTreeNode<T>::getRight() const
{
return right;
}
template <class T>
void BinTreeNode<T>::setLeft(BinTreeNode<T> * node)
{
node->setParent(this);
left = node;
}
template <class T>
void BinTreeNode<T>::setRight(BinTreeNode<T> * node)
{
node->setParent(this);
right = node;
}
template <class T>
BinTreeNode<T> * BinTreeNode<T>::getParent() const
{
return parent;
}
template <class T>
void BinTreeNode<T>::setParent(BinTreeNode<T> * node)
{
parent = node;
}
template <class T>
bool BinTreeNode<T>::operator<(const BinTreeNode<T>& node) const
{
return this->data < node.data;
}
template <class T>
bool BinTreeNode<T>::operator>(const BinTreeNode<T>& node) const
{
return this->data > node.data;
}
template <class T>
bool BinTreeNode<T>::operator==(const BinTreeNode<T>& node) const
{
return this->data == node.data;
}
#endif /* BINTREENODE_H_ */
回答1:
Your BinTreeNode destructor should simply be:
template <class T>
BinTreeNode<T>::~BinTreeNode() {
delete left;
delete right;
}
That will call left and right's destructors recursively, freeing the memory allocated for those nodes and their child nodes. This will as a consequence free the entire tree.
Assigning NULL to a pointer does not free the memory pointed by it.
On the other hand, what you mention, that after deletion, this line:
node->getData();
Still returns data, is perfectly normal. Deletion frees the memory, but the data stored in it might still be available for a while, until something new is written in that memory address. Accessing an already free'd memory address implies undefined behaviour.
BTW, you should use "0"(without quotes) in C++ instead of NULL. Therefore, there it's not necessary to use the #ifndef NULL(...).
EDIT: I hadn't seen the "no recursion" comment. Here's a non-recursive algorithm:
#include <deque>
/* ... */
template <class T>
BinTreeNode<T>::~BinTreeNode() {
std::deque deq;
// we're going to delete our children
deq.push_back(this);
while(deq.size()) {
BinTreeNode<T> *ptr = deq.front();
deq.pop_front();
if(ptr) {
deq.push_back(ptr->left);
deq.push_back(ptr->right);
// we don't want the child nodes
// to double delete the children
ptr->left = 0;
ptr->right = 0;
// avoid deleteing ourselves
if(ptr != this)
delete ptr;
}
}
}
I haven't tested it, but it should work.
来源：https://stackoverflow.com/questions/9745736/generic-binary-tree-node-destructor-issue