如何正确释放2D数组C ++

Question

we just received the results of our final exam in C++. One of the questions was to write a simple representation of 2-dimensional matrix of double . I had some points deduced at the destructor implementation with the note reading invalid implementation and no further information. Could you please tell me where I went wrong (if I did indeed go wrong).

This is the code I wrote:

~Matrix() {
   for (int i=0; i<_rows; i++) {
       for (int j=0; j<_cols; j++) {
          delete _arr[i][j];
        }
        delete _arr[i];
    }
    delete [] _arr;
}

The code for allocating the memory is pretty straight forward, I'll paste it to make things clearer, but the problematic part is in the code above.

Matrix(int rows, int cols) :
_arr(new double*[rows]), _rows(rows), _cols(cols)
{
    for (int i=0; i<_rows; i++) {
        _arr[i] = new double[_cols];
    }
}

Did they rightfully take away the points? Was my dtor implementation indeed invalid?

Answer 1

The delete _arr[i][j] is wrong and shouldn't be there; delete _arr[i]; is also wrong - since this is an array that you allocated with new [] , you need to use delete [] _arr[i]; .

The rule of thumb is: a delete for each new ; and a delete[] for each new[] .

Also the delete s of composites are to be executed in reverse order compared to the new s (which you did already).

Thus we get

~Matrix() {
    for (int i = 0; i < _rows; i++) {
        delete[] _arr[i];
    }
    delete[] _arr;
}

Answer 2

You should call delete[] for your every new[] , they should be pairing.

~Matrix() {
   for (int i=0; i<_rows; i++) {
        delete[] _arr[i];
    }
    delete [] _arr;
}

Answer 3

You have two new then three delete . What's wrong should be obvious.

Besides, you have to use delete [] for both dimensions, since you have an array of double-array and then an array of double (double*[] and double[]), that lead you to :

~Matrix() {
    for (int i = 0; i < _rows; i++) {
        delete[] _arr[i];
    }
    delete[] _arr;
}

It's not necessary to destroy each value of the array of double, since they have'nt been created by a new

Moreover, a simplest implementation uses only one allocation :

Matrix(int rows, int cols) : _arr(new double[rows*cols]), _rows(rows), _cols(cols)
{
}

~Matrix() {
    delete[] _arr;
}

In this implementation, you should provide a correct accessor, since matrix[i,j] = _arr[i+j*row] or matrix[i,j] = _arr[i*cols+j] (depends on the way you arrange the values in the array).