Huffman Encoding C++ code is throwing a fatal error - c++

Question

I am writing a code for famous algorithm Huffman Encoding. I am getting a fatal error which turn system into blue screen and then restart. This error occurs in display_Codes which have recursive calls. The error occurs on the following lines:
display_Codes(root->l, s + "0");
display_Codes(root->r, s + "1" );
Following is the complete code.
#include <iostream>
#include <bits/stdc++.h>
using namespace std;
class HeapNode_Min {
public:
char d;
unsigned f;
HeapNode_Min *l, *r;
HeapNode_Min(char d, unsigned f)
{
this->d = d;
this->f = f;
}
~HeapNode_Min()
{
delete l;
delete r;
}
};
class Analyze {
public:
bool operator()(HeapNode_Min* l, HeapNode_Min* r)
{
return (l->f > r->f);
}
};
void display_Codes(HeapNode_Min* root, string s)
{
if(!root)
return;
if (root->d != '$')
cout << root->d << " : " << s << "\n";
display_Codes(root->l, s + "0");
display_Codes(root->r, s + "1" );
}
void HCodes(char data[], int freq[], int s)
{
HeapNode_Min *t,*r, *l ;
priority_queue<HeapNode_Min*, vector<HeapNode_Min*>, Analyze> H_min;
int a=0;
while (a<s){H_min.push(new HeapNode_Min(data[a], freq[a])); ++a;}
while (H_min.size() != 1) {
l = H_min.top(); H_min.pop();
r = H_min.top(); H_min.pop();
t = new HeapNode_Min('$', r->f + l->f);
t->r = r; t->l = l;
H_min.push(t);
}
display_Codes(H_min.top(), "");
}
int main()
{
try
{
int frequency[] = { 3, 6, 11, 14, 18, 25 }; char alphabet[] = { 'A', 'L', 'O', 'R', 'T', 'Y' };
int size_of = sizeof(alphabet) / sizeof(alphabet[0]);
cout<<"Alphabet"<<":"<<"Huffman Code\n";
cout<<"--------------------------------\n";
HCodes(alphabet, frequency, size_of);
}
catch(...)
{
}
return 0;
}

Answer 1

You never set l or r to nullptr, but your code relies on the pointers being either valid or nullptr:
void display_Codes(HeapNode_Min* root, string s)
{
if(!root)
return;
if (root->d != '$')
cout << root->d << " : " << s << "\n";
display_Codes(root->l, s + "0");
display_Codes(root->r, s + "1" );
}
Pass a root with no left and no right node, then neither root->l nor root->r have a value that you could use for anything. Passing them to the next recursion lets you dereference them which invokes undefined behavior.
To fix that you need to initialize the pointers, eg in the constructor:
HeapNode_Min(char d, unsigned f) : d(d),f(f),l(nullptr),r(nullptr) { }
Also your class does not follow the rule of 3/5/0.