How to convert a user defined type to primitive type?

Question

Let's say we have a class called Complex which represents a complex number. I want to convert this object to a double object.

The other way around i can do by implementing a copy ctor in Complex:
Complex(const double &d);

However, i can't implement i copy ctor in double which will receive a Complex.

How do i do this? I know there is a way with operator overloading, but i couldn't find how.
Eventually i want the this line will compile:
Complex c;
(double)c;

Thanks!!

Answer 1

Implement a conversion operator on your Complex class:

class Complex
{
 // ...
  operator double() const
  { 
    double ret = // magic happens here
    return ret;
  }
};

If for whatever reason you don't want to muck about with this, you can provide a global conversion function:

double convert_to_double(const Complex& rhs)
{ 
  double ret = // magic happens
  return ret;
}

Answer 2

The proper way of doing this is adding a conversion operator to your class.

class myclass {    
public:    
    operator double() const
    {
        return _mydouble;
    }
...
};

and used like this:

myclass c;
double d = c; // invokes operator double