Question

I created a double linked list class, and am trying to use it with a Vector class I created, in order to make a vector of linked lists, however at the end of the program it seems I am getting an error
malloc: *** error for object 0x100100be0: pointer being freed was not allocated
which I am assuming has to do with the destructor, also that is where Xcode is pointing me to. How do I circumvent this? I think my destructor works fine, however I guess I am wrong.

Test File:

#include <iostream>
#include <string>
#include "Vector.h"
#include "doubleLL.h"

using namespace std;

int main (int argc, const char * argv[])
{

    Vector<double_llist<string> > listWords(27);
    double_llist<string> numbers;
    numbers.push_back("one");
    numbers.push_back("two");
    numbers.push_back("three");
    listWords[0] = numbers;
    listWords[0].print();
}

doubleLL.h:

#ifndef DOUBLELL_H
#define DOUBLELL_H
#include <iostream>
using namespace std;

template <class T>
class double_llist {
private:
    struct node {
        T data;
        node* prev;
        node* next;
        node(T t, node* p, node* n) : data(t), prev(p), next(n) {}
        int count;
    };
    node* head;
    node* tail;

public:
    double_llist() : head( NULL ), tail ( NULL ) {}
    template<int N>
    double_llist( T (&arr) [N]) : head( NULL ), tail ( NULL )
    {
        for( int i(0); i != N; ++i)
            push_back(arr[i]);
    }
    bool empty() const { return ( !head || !tail ); }
    operator bool() const { return !empty(); } 
    void push_back(T);
    void push_front(T);
    T pop_back();
    void removeNode(node *);
    void print();

    node* search(T data) {
        node *tempNode;
        if (head == NULL) {
            // List is empty
            return NULL;
        } else {
            tempNode = head;
            while (tempNode != NULL) {
                if (tempNode->data == data) {
                    tempNode->count += 1;
                    if (tempNode->count >= 4) {
                        // Push tempNode to front of linked list
                        push_front(tempNode->data);
                        head->count = tempNode->count;
                        removeNode(tempNode);
                    }
                    return tempNode;
                } else {
                    tempNode = tempNode->next;
                }
            }
        }
        return NULL;
    }

    ~double_llist()
    {
        while(head)
        {
            node *temp(head);
            head = head->next;
            delete temp;
        }
    }

    double_llist& operator = ( const double_llist& other )
    {
        if (this == &other) {
            return *this;
        }
        while (!empty()) {
            pop_back();
        }
        for (node *itr = other.head->next; itr != other.tail; ++itr) {
            tail = new node(other.head->data, itr, NULL);
        }
        return *this;

    }


    double_llist(const double_llist& other)
    {
        head = new node;
        tail = new node;
        head->tail = tail;
        tail->prev = head;
        *this = other;
    }
};

template <class T>
void double_llist<T>::push_back(T data)
{
    tail = new node(data, tail, NULL);
    if( tail->prev )
        tail->prev->next = tail;

    if( empty() )
        head = tail;
}

template <class T>
void double_llist<T>::push_front(T data) {
    head = new node(data, NULL, head);
    if( head->next )
        head->next->prev = head;

    if( empty() )
        tail = head;
}


template <class T>
T double_llist<T>::pop_back()
{
    node* temp(tail);
    T data( tail->data );
    tail = tail->prev ;

    if( tail )
        tail->next = NULL;
    else
        head = NULL ;

    delete temp;
    return data;
}

template <class T>
void double_llist<T>::removeNode(node *n) {
    if(n == this->head) {
        this->head=this->head->next;
        this->head->prev = NULL;
    } else if (n==this->tail) {
        this->tail=this->tail->prev;
        this->tail->next = NULL ;
    } else {
        n->prev->next = n->next;
        n->next->prev = n->prev;
    }
}

template <class T>
void double_llist<T>::print() {
    node* temp;
    temp = this->head;
    int i = 0;
    while(temp != NULL)
    {
        if (i < 3) {
            cout << temp->data << endl;
            temp=temp->next;
            ++i;
        } else {
            return;
        }
    }
    cout << endl;
    return;
}

#endif

The error seems to be coming from doubleLL, so Vector.h was not included. If it is needed to help point me in the right direction, let me know.

Thanks!

Answer 1

You are not obeying the rule of 3: If you have either of a destructor, a copy constructor or an assignment operator implemented, you should implement all three. I stepped through your code and there appear to be a lot of object copies created and then destroyed, but since copying is not done properly, already destroyed memory is being deleted again.

Properly implement these and the problem will be no more.

EDIT:

I just finished a basic implementation of those:

double_llist& operator = ( const double_llist& other )
{
   head = NULL;
   tail = NULL;
   return *this;
}
double_llist(const double_llist& other)
{
   head = NULL;
   tail = NULL;
}

The code no longer crashes.

SECOND EDIT:

double_llist& operator = ( const double_llist& other )
{
   head = NULL;
   tail = NULL;
   node* otherNode = other.head;
   while ( otherNode )
   {
  push_back(otherNode->data);
  if ( otherNode == other.tail )
     break;
  otherNode = otherNode->next;
   }
   return *this;
}
double_llist(const double_llist& other)
{
   head = NULL;
   tail = NULL;
   node* otherNode = other.head;
   while ( otherNode )
   {
  push_back(otherNode->data);
  if ( otherNode == other.tail )
     break;
  otherNode = otherNode->next;
   }
}