Debug Assertion Failed error - c++

Question

I'm trying to overload the + operator for a project that I'm doing but this keeps on happening. I think the reason is that the object I created is getting deleted when the operator is getting invoked. Not sure how to fix it though. Here's part of my code:
Matrix Matrix::operator+ (const Matrix& m) const
{
//something wrong here
Matrix sum(rows, cols);
for (int i = 0; i < cols; i++)
{
for (int j = 0; j < rows; j++)
{
sum.element[i][j] = this->element[i][j] + m.element[i][j];
}
}
return sum;
}
Additional Information
Matrix::Matrix(int r, int c)
{
rows = r;
cols = c;
element = new int *[c];
for (int i = 0; i < c; i++)
{
element[i] = new int[r];
}
for (int i = 0; i < cols; i++)
{
for (int j = 0; j < rows; j++)
{
element[i][j] = 0;
}
}
}
Matrix::Matrix(const Matrix& m)
{
this->element = m.element;
}
Matrix::~Matrix()
{
//freeing up the arrays
for (int i = 0; i < cols; i++)
{
delete[] element[i];
element[i] = NULL;
}
delete[] element;
element = NULL;
}
Matrix& Matrix::operator= (const Matrix& m)
{
//problem if rows and cols are longer in the new matrix
//need to create a temp matrix to expand to new one
for (int i = 0; i < cols; i++)
{
for (int j = 0; j < rows; j++)
{
this->element[i][j] = m.element[i][j];
}
}
return *this;
}
int* Matrix::operator[] (int n)
{
return element[n];
}
The specific error I am getting is:
Debug Assertion Failed!
Expression: _BLOCK_TYPE_IS_VALID(pHead->nBlockUse)
In line 52 which I did this:
Matrix total = mat + m;
Where mat and m are both object matrices

Answer 1

I'm guessing the problem is caused by this:
Matrix::Matrix(const Matrix& m)
{
this->element = m.element;
}
That is an invalid copy constructor for two reasons. First, you're not initializing rows or cols. Second, you'll now have two Matrix objects pointing to the same memory - both of which will delete it on destruction:
{
Matrix m1(3, 5);
Matrix m2 = m1;
} // BOOM!
You need to do a deep copy here:
Matrix::Matrix(const Matrix& m)
: rows(m.rows), cols(m.cols)
{
element = new int *[cols];
for (int i = 0; i < cols; ++i) {
element[i] = new int[rows];
for (int j = 0; j < rows; ++j) {
element[i][j] = m.element[i][j];
}
}
}