Linked list implementation in classes c++, showing Segmentation fault [closed] - c++

Question

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 5 years ago.
Improve this question
I know the segmentation fault is occurring in this while loop: (while(temp != NULL){temp = temp->next;}), but I have no idea why.
#include<iostream>
using namespace std;
class zDepthList {
typedef struct node {
int data;
node* next;
node* prev;
} Node;
public:
zDepthList() {
head = NULL;
}
zDepthList(int array[], int length) {
Node *temp, *ptr;
int i = 0;
while(i != length - 1) {
temp = head;
ptr = new Node;
ptr->data = array[i];
i++;
ptr->next = NULL;
if(head == NULL) {
head = ptr;
ptr->prev = NULL;
}
else {
while(temp != NULL) {
temp = temp->next;
}
}
temp->next = ptr;
ptr->prev = temp;
}
}
void out(const char order) {
cout << head->data << endl;
return;
}
private:
Node *head;
};

Answer 1

For starters you have to initialize the head to NULL.
And after this while loop
else {
while(temp != NULL) {
temp = temp->next;
}
}
temp->next = ptr;
ptr->prev = temp;
the pointer temp is equal to NULL because it is the condition to interrupt the loop. Thus this statement
temp->next = ptr;
results in undefined behavior.
If you have a double-linked list it is natural to introduce also data member tail that it could be easy to append new nodes.
So you should include
class zDepthList {
//...
private:
Node *head, *tail;
};
In this case the constructors can look the following way
zDepthList() : head( nullptr ), tail( nullptr )
{
}
zDepthList( const int a[], size_t n ) : head( nullptr ), tail( nullptr )
{
for ( size_t i = 0; i < n; i++ )
{
Node *tmp = new Node { a[i], nullptr, tail };
tail == nullptr ? head = tmp : tail->next = tmp;
tail = tmp;
}
}
Here is a demonstrative program
#include <iostream>
class zDepthList {
typedef struct node {
int data;
node* next;
node* prev;
} Node;
public:
zDepthList() : head(nullptr), tail(nullptr)
{
}
zDepthList(const int a[], size_t n) : head(nullptr), tail(nullptr)
{
for (size_t i = 0; i < n; i++)
{
Node *tmp = new Node{ a[i], nullptr, tail };
tail == nullptr ? head = tmp : tail->next = tmp;
tail = tmp;
}
}
std::ostream & out( std::ostream &os = std::cout ) const
{
for (Node *current = head; current; current = current->next)
{
os << current->data << ' ';
}
return os;
}
private:
Node *head, *tail;
};
int main()
{
int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
zDepthList l(a, sizeof(a) / sizeof(*a));
l.out() << std::endl;
}
The program output is
0 1 2 3 4 5 6 7 8 9

Answer 2

You never set head but you access it. This means it is uninitialized and this is an UB.
You have 2 ctors and you initialize head only then, when it is called without any parameter.