Delete Memory allocated in another function?

Question

So I just finished my implementation of merge sort but it occurred to me that I didn't delete the memory returned from recursive calls that I was discarding, so I added delete statements for array1 and array2, and then suddenly my merge sort doesn't work.....Why does adding the delete statements near the end of my function screw everything up? Do I need to free the memory?

The code is below:

/**
 * Runs merge sort on this ArrayList<T>. Interface function to the central,
 * recursive, merge sort function.
 */
template<class T>
void ArrayList<T>::mergeSort() {

    T* temp = mergeSort(array, size);
    delete [] array;
    array = temp;
}

/**
 * Runs merge sort on the passed in array. Recursive.
 *
 * @param array the array to sort.
 * @param arraySize the size of the array that is to be sorted.
 * @return the sorted array.
 */
template<class T>
T* ArrayList<T>::mergeSort(T* array, int arraySize) {

    T* returnArray = array;

    //If the array is more than one element.
    if (arraySize > 1) {

        int size1 = arraySize / 2;
        int size2 = arraySize - size1;

        T* array1;
        T* array2;

        //Recurse.
        array1 = mergeSort(array, size1);
        array2 = mergeSort(array + size1, size2);

        returnArray = new T[arraySize];

        //Loop through all elements in returnArray.
        int i = 0, j = 0, k = 0;
        while (i < arraySize) {

            //Place the lesser of two elements in returnArray.
            if ((array1[j] <= array2[k] && j < size1)
                    || k == size2) {

                returnArray[i] = array1[j];
                j++;
            }
            else {

            returnArray[i] = array2[k];
                k++;
            }

            i++;
        }

/---THESE ARE THE DELETES IN QUESTION!!-----/

        delete [] array1;
        delete [] array2;
    }

    return returnArray;
}

Answer 1

i see a problem when array size is equal to 2 since you call

array1 = mergeSort(array, size1);
array2 = mergeSort(array + size1, size2);

and size1=1,size2=1 then both of these calls will return and you will have the following values in the variables

array1 = array;
array2 = array+1;

and array2 isn't an allocated memory address so deleting it should fail with an error or do something undefined so i would suggest fixing that before you continue

Answer 2

I haven't looked in full detail but one thing springs to mind: when mergeSort is called with an array whose size is <= 1 it returns the passed array without allocating a new one but upon returning from the recursive call you will attempt to delete it anyway. You could improve your code by adding:

if (size1 > 1)
   delete [] array1;
if (size2 > 1)
   delete [] array2;