C++ deleting pointer to interface vector memory leak - c++

Question

Using: CodeBlocks 13.12, GNU GCC mingw32-g++, Dr.Memory
So I have an assignment to make a vector queue (first in first out).
I made the program and everything worked fine, but in the assignment we need to use an interface IQueue. This interface can not be changed.
#ifndef IQUEUE_H
#define IQUEUE_H
template <typename T>
class IQueue {
public:
virtual void enqueue(const T& element) = 0;
virtual T dequeue() = 0;
virtual T front() const = 0;
virtual bool isEmpty() const = 0;
};
#endif
This is (some) of my Queue.h just so you get the picture.
#ifndef QUEUE_H
#define QUEUE_H
#include <iostream>
#include <string>
#include <stdexcept>
#include "iqueue.h"
using namespace std;
template <typename T>
class Queue : virtual public IQueue<T> {
public:
Queue();
Queue(int capacity);
Queue(int capacity, int capacity_increment);
~Queue();
Queue(const Queue<T> &original);
void operator=(const Queue<T> &original);
void enqueue(const T& element);
T dequeue();
T front() const;
bool isEmpty() const;
private:
T *items;
int nr_of_items;
int capacity;
void expand(); //expands the array if the nr_of_items is bigger than capacity
void freeMemory();
};
/*stuff*/
template<typename T>
Queue<T>::~Queue() {
this->freeMemory();
}
template<typename T>
void Queue<T>::freeMemory() {
delete[] this->items;
}
I'm using freeMemory() in the operator= that's why it's a separate function.
So now to the main
#include "iqueue.h"
#include "queue.h"
int main() {
IQueue<string> *sq = new Queue<string>();
/*Do stuff with the queue*/
IQueue<string> *sq2 = new Queue<string>();
sq2 = sq;
IQueue<int> *iq = new Queue<int>();
/*Do stuff with the queue*/
IQueue<int> *iq2 = new Queue<int>();
iq2 = iq;
/*how to delete?*/
return 0;
}
Things I've tested:
delete sq;delete sq2;delete iq; delete iq2; with cout in freeMemory() it doesn't run at all.
Same as before but I tested making a virtual deconstructor in the IQueue. with cout in freeMemory() it runs once and then crashes. I get 4 unaddressable accesses, 2 invalid heap argument, 2 memory leaks.
Don't really get what happens here.
We haven't used uniqe pointers yet but when I googled around that was suggested as a good method. But I would need a good explanation for my program to know how to do that.
Tried vector::erase cplusplus.com link.
Feels like this is the right way to go, however I just get errors about "vector erase used without template class". I've included < vector >.
Any answer that can point me in the right direction is appreciated. General info about why the things I tried didn't work would be nice and give me better understanding.
I'll edit more code in if needed.
Sidenote: We were told you put throw(...) in the .h file if there should be a try catch exception in the function like this:
virtual T dequeue()throw(…) = 0;
But I just got errors, is this a standard way of doing it?

Answer 1

IQueue<string> *sq = new Queue<string>();
IQueue<string> *sq2 = new Queue<string>();
sq2 = sq;
First memory leak. You lose pointer to sq2 object.
IQueue<int> *iq = new Queue<int>();
IQueue<int> *iq2 = new Queue<int>();
iq2 = iq;
Second memory leak. Same as above.
Now if you delete pointer to iq1 or iq2 you will have third leak because your interface has no virtual destructor so destructor in your implementation will not be called.

Answer 2

Things that I have noticed fron your code and points are:
1) You are working with pointers. That is, you are actually assigning one pointer to the other. So, operator=() will not be invoked. And memory will leak there.
2) As you said in your first point, you are deleting sq and then sq2, both of which were pointing to the same memory. Thus heap corruption was detected. Same is happenning with iq and iq2.
3) Also use virtual destructor.