没有为指针赋值分配被释放的指针

Question

I was trying to change a ListNode struct into a class format but am running into some issues when testing it.

Getting a.out(7016) malloc: * error for object 0x7fff65333b10: pointer being freed was not allocated * set a breakpoint in malloc_error_break to debug

chainLink.hpp 
#ifndef CHAINLINK_H
#define CHAINLINK_H

using namespace std;
#include <iostream>
#include <cstdlib>

template <typename Object>

class chainLink
{
    private:
        Object storedValue;
        chainLink *nextLink;
    public:
            //Constructor
        chainLink(const Object &value = Object()): storedValue(value)
        {
            nextLink = NULL;
        }
        /* Postcondition:   returns storedValue;
         */     
        Object getValue()
        {
            return storedValue;
        }

        /* Postcondition:   sets storedValue = value
         */
        void setValue(Object &value)
        {
            storedValue = value;
        }

        /* Postcondition:   sets nextLink to &value
         */
        void setNextLink(chainLink* next)
        {
            nextLink = next;
        }

        chainLink* getNext()
        {
            return nextLink;
        }
        ~chainLink()
        {
            delete nextLink;
        }
};
#endif

My test file, assume includes

int main()
{
    chainLink<int> x(1);
    cout << "X: " << x.getValue() << " "<< endl;
    chainLink<int> y(2);
    cout << "Y: " << y.getValue() << " "<< endl;
    chainLink<int>* z = &y;
    cout << &y << " " << z << endl;
    x.setNextLink(z);
}

OUTPUT: X: 1 Y: 2 0x7fff65333b10 0x7fff65333b10 a.out(7016) malloc: * error for object 0x7fff65333b10: pointer being freed was not allocated * set a breakpoint in malloc_error_break to debug Abort trap: 6

The error seems to be thrown by the setNextLink function.

Any help greatly appreciated.

Answer 1

In the last line of main you are calling setNextLink with a pointer to an object with automatic storage duration ( z holds the address of y ). Your list tries to delete that pointer when it is destroyed, hence the error ( y has not been allocated dynamically, and hence cannot be deleted dynamically).

Answer 2

You are giving setNextLink a pointer to an automatically allocated variable,

x.setNextLink(z); // z points to automatic object y

which you attempt to delete in the constructor.

~chainLink() {
    delete nextLink; // attempts to delete automatic object y
}

You need to pass a pointer to a dynamically allocated object, or make your own insde your chainLink class.

Note : In C++, struct s and class es are the same bar some differences . A equivalent types can be implemented using either of the two.

Answer 3

After the line x.setNextLink(z); x.nextLink points to z , which in turn is pointing to y . But y is a local object . It is allocated on stack and not the heap. So it is illegal to call delete on it.