Overloaded typecasts don't work

I've built a little class representing a decimal number, called Complex. I want to be able to cast it to double, so here's my code

Complex.h

public:
    operator double();

Complext.cpp

Complex::operator double()
{
return _num;
}

_num is a field of type double of course

That seems to be okay. The problem is in another class where I actually try to cast a complex object into double. That's my f开发者_JAVA技巧unction:

const RegMatrix& RegMatrix::operator *= (double num)
{
    Complex factor(num);
    for(int i=0; i<_numRow; i++)
    {
        for(int j=0; j<_numCol; j++)
        {
            Complex temp = _matrix[i][j];
            _matrix[i][j] = (double) (temp * factor); //<---ERROR HERE
        }
    }
    return *this;
}

That results in invalid cast from type ‘const Complex’ to type ‘double’

I have no clue why it happens. Any ideas? Thanks!

You need to make your operator double a const function:

operator double() const;

and

Complex::operator double() const {
    return _num;
}

As the error clearly states, the is no conversion to double for a const Complex. Simply define your conversion operator like this:

operator double() const;

Complex::operator double() const
{
    return _num;
}

Note the const modifier at the end. You should really look for information on const-correctness in C++.