I am working on reverse linked list on Windows 10 Pro (64 bit) with Visual Studio Community 2019. I would like to know how to resolve the error I get as below. I get below error in the while loop in the member function reverse() in the class List
(*Please assume the list is already prepared like contiguous integer e.g. 0,1,2,3,4,5) Could you please anyone give me some advice? Thank you in advance.
Unhandled exception thrown: read access violation. current was 0xDDDDDDDD.
#include <iostream>
#include <string.h>
#include <vector>
using std::cout;
using std::endl;
template <class Data>
class Node
{
public:
Node* next;
Data data;
};
template <class Data>
class List
{
private:
Node<Data>* head;
Node<Data>* tail;
int count;
public:
//constructor
List()
{
count = 0;
head = nullptr;
tail = nullptr;
}
int size()
{
return count;
}
void msg_empty_list()
{
cout << "The list is not modified since it is empty." << endl;
}
int push_back(Data data)
{
Node<Data>* node = new Node<Data>;
node->data = data;
node->next = nullptr;
if (head == nullptr)
{
head = node;
tail = node;
}
else if (head != nullptr)
{
tail->next = node;
tail = tail->next;
}
count ;
return count;
}
int push_front(Data data)
{
Node<Data>* node = new Node<Data>;
node->data = data;
node->next = nullptr;
if (head == nullptr)
head = node;
else if (head != nullptr)
{
node->next = head;
head = node;
}
count ;
return count;
}
int pop_front(void)
{
if (head == nullptr)
return -1;
else if (head != nullptr)
{
Node<Data>* temp;
temp = head;
head = head->next;
delete temp;
count--;
return count;
}
}
int pop_back(void)
{
if (head == nullptr)
return -1;
else if (head != nullptr)
{
Node<Data>* temp = head;
while (temp->next != tail)
temp = temp->next;
delete tail;
tail = temp;
count--;
return count;
}
}
int remove_at(int index)
{
if (head == nullptr)
return -1;
else if (head != nullptr)
{
cout << "Specified index = " << index << endl;
if (index == 0)
{
pop_front();
}
else if (index == -1)
{
pop_back();
}
Node<Data>* temp = head;
Node<Data>* rmv;
int countIndex = 0;
while (countIndex < index - 1)
{
temp = temp->next;
countIndex ;
}
rmv = temp->next;
temp->next = temp->next->next;
delete rmv;
count--;
return count;
}
}
void reverse()
{
Node<Data>* temp = nullptr;
Node<Data>* prev = nullptr;
Node<Data>* current = head;
while (current != nullptr)
{
temp = current->next; // where I get error "Unhandled exception thrown: read access violation. **current** was 0xDDDDDDDD."
current->next = prev;
prev = current;
current = temp;
}
head = prev;
}
void print()
{
Node<Data>* temp = head;
if (head == nullptr)
return;
for (int i = 0; i < count - 1; i )
{
cout << temp->data << ", ";
temp = temp->next;
}
cout << temp->data;
}
~List()
{
Node<Data>* temp = head;
while (temp->next != nullptr)
{
temp = temp->next;
delete head;
head = temp;
}
}
};
int main()
{
Node<int> x;
Node<bool> y;
Node<char> n;
List<int> list;
//insert items into list
for (int i = 0; i < 10; i )
{
list.push_back(i);
}
cout << "Original list[size=" << list.size() << "]: ";
//print the list
list.print();
cout << endl;
// push a node to the beginning of the list
cout << endl << "==> Push a node to the head" << endl;
list.push_front(-1);
cout << "Modified list[size=" << list.size() << "]: ";
list.print();
cout << endl;
// pop the head of the list
cout << endl << "==> Pop a node from the head" << endl;
if (list.size() == 0)
list.msg_empty_list();
else
{
list.pop_front();
cout << "Modified list[size=" << list.size() << "]: ";
list.print();
cout << endl;
}
/*
// pop the tail of the list
cout << endl << "==> Pop a node from the tail" << endl;
if (list.size() == 0)
list.msg_empty_list();
else
{
list.pop_back();
cout << "Modified list[size=" << list.size() << "]: ";
list.print();
cout << endl;
}
*/
// delete the node at the specified index
cout << endl << "==> Delete the node at the specified index" << endl;
if (list.size() == 0)
list.msg_empty_list();
else
{
list.remove_at(5);
cout << "Modified list[size=" << list.size() << "]: ";
list.print();
cout << endl;
}
// reverse the list
cout << endl << "==> Reverse the list" << endl;
if (list.size() == 0)
list.msg_empty_list();
else
{
list.reverse();
cout << "Modified list[size=" << list.size() << "]: ";
list.print();
cout << endl;
}
return 0;
}
CodePudding user response：
I can spot a few mistakes in your code, but let's concentrate on the pop_back() function:
int pop_back(void)
{
if (head == nullptr)
return -1;
else if (head != nullptr) // 1)
{
Node<Data>* temp = head;
while (temp->next != tail) // 2)
temp = temp->next;
delete tail;
tail = temp; // 3)
count--;
return count;
}
}
- Please don't do this:
if (a == nullptr) {...} else if (a != nullptr) {...}is redundant. Just leave the secondifaway. It lets a reader believe there could be a third case. - It may be luck that this usually works, but some of the other methods don't properly update the tail pointer, so this might never be true. Validate your
pop_frontmethod when there's only one element in the list. Other functions might have similar problems. - Here's the actual problem you're observing. You're not setting the
nextpointer of the new tail element to null, instead it points to the now deleted tail. Inserttail->next = nullptrafter this line.