Writing operator functions

Yes, hi

How to write "operator +" and "operator =" functions in a class with
a defined constructor? The following code demonstrates that I don't
really understand how to do it... I think it has something to do with
the compiler calling the destructor twice. Could someone point out
where I go wrong?
P.S.: The error it gives is "Debug Assertion Failure ....." (at run
time)
P.P.S: Everything else works just fine (without the use of "operator
+"
and "operator =" functions)

//code "tested" on Microsoft Visual C++ 6.0
#include <iostream>

using namespace std;

class DatArray {
public:
    int rows, columns;
    double **a;
    DatArray(int r, int c);
    ~DatArray();
    void operator = (double d1);
    void operator = (DatArray da1); //problematic
    void operator ++ ();
    //DatArray operator + (double d1); //does not work
};

DatArray::DatArray(int r, int c) {
    int c1;
    rows = r;
    columns = c;
    a = new double* [rows];
    for (c1 = 0; c1 < rows; c1++) a[c1] = new double [columns];
}

DatArray::~DatArray() {
    int c1;
    for (c1 = 0; c1 < rows; c1++) delete [] a[c1];
    delete [] a;
}

void DatArray::operator = (double d1) {
    int c1, c2;
    for (c1 = 0; c1 < rows; c1++)
        for (c2 = 0; c2 < columns; c2++) a[c1][c2] = d1;
}

void DatArray::operator = (DatArray da1) {
    int c1, c2;
    for (c1 = 0; c1 < rows; c1++)
        for (c2 = 0; c2 < columns; c2++) a[c1][c2] = da1.a[c1][c2];
}

void DatArray::operator ++ () {
    int c1, c2;
    for (c1 = 0; c1 < rows; c1++)
        for (c2 = 0; c2 < columns; c2++) a[c1][c2]++;
}

/*DatArray DatArray::operator + (double d1) {
    int c1, c2;
    for (c1 = 0; c1 < rows; c1++)
        for (c2 = 0; c2 < columns; c2++) a[c1][c2] = a[c1][c2] + d1;
    return *this;
}*/

int main() {
    int c1, c2;
    DatArray myarray(3, 4), myarray2(3, 4);
    myarray = 10.1;
    for (c1 = 0; c1 < myarray.rows; c1++) {
        for (c2 = 0; c2 < myarray.columns; c2++) cout << myarray.a[c1][c2]
<< " ";
        cout << endl;
    }
    cout << endl;
    for (c1 = 0; c1 < 100; c1++) myarray++;
    //myarray2 = myarray + 100; //this should be simpler, but does not
work ?!?!?!?!?!
    myarray2 = myarray;
    for (c1 = 0; c1 < myarray.rows; c1++) {
        for (c2 = 0; c2 < myarray.columns; c2++) cout << myarray2.a[c1][c2]
<< " ";
        cout << endl;
    }
    cin.get(); //currently, after this, ERROR ... WHY?!?!?!?!?! (how to
fix this?!?!?!?!?)
    return 0;
}
//end of code "tested" on Microsoft Visual C++ 6.0

Thanks,
Valerij