Cast object to parent

Question

I have a problem with inheritance in C++. Here is what I would like to do:

class parent
{
   public:
   int a;
};

class son : public parent
{
   public:
   int b;
   operator parent()
   {
      parent p; p.a = a + b;
      return p;
   }
}


void funct(parent a)
{
    cout<<a.a;
}

son s; s.a = 3; s.b = 4;
funct(s);

And I would like it to print out 7. Instead, the casting is implicitly done without affecting anything, and 3 is printed out. Could anyone give me an advice on how to do this? Is this even possible?

Answer 1

According to the C++ Standard

A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.

You could use a virtual function that outputs data members or you could define operator << as pseudo-virtual (that is that calls a virtual function) for the classes.

For example

class parent
{
   public:
   virtual std::ostream & out( std::ostream &os ) const { return os << a; }
   int a;
};

class son : public parent
{
   public:
   int b;
   std::ostream & out( std::ostream &os ) const { return os << a + b; }
};

std::ostream & operator <<( std::ostream &os, const parent &obj )
{
   return obj.out( os );
}

Answer 2

This is how 7 will be printed:

funct(s.operator parent());

And one point to remember:

funct(s) //would result in object slicing as the 'funct' takes base class object by value.

Answer 3

Object is automatically casting to parent class by method like this:

operator base_type&() {return (base_type&)*this;}

You can't change this method.

You can only change inheritance to protected or private. The compiler never do a implicit casting then, even if you define casting operator.

You can only write a ordinary method which can do that casting and call it, when you need.