OO polymorphism design - oop

Question

What is the best way to do the following:
Suppose I have a class called Person and many derived classes for specialized persons.
Suppose at the beginning of my app, I know I have to deal with a person but I won't know what kind of person it is until much later (something beyond my control so I cannot determine the Person type at the beginning).
So at the beginning I will create a Person and fill in attributes for it. Later, when I know what kind of Person it is, I would instantiate a specialized person and copy over the any saved attributes for her.
Is there a more elegant way to do this without creating two objects?

Answer 1

If you don't know the type of person up front, you won't be able to avoid instantiating two objects. There has to be something to contain the base Person attributes before you know the specialized person, but you can't take advantage of polymorphism without instantiating the specialized object later.
One option is to use a composition pattern, in which each specialized person contains a Person instance rather than inheriting from it. You still have to instantiate two objects, but you don't have to rewrite the code to copy over the saved attributes every time. Here's an example (C# syntax):
public interface IPerson
{
string Name { get; }
int Age { get; }
}
public class Person : IPerson
{
public string Name { get; private set; }
public int Age { get; private set; }
public Person(string name, int age)
{
Name = name;
Age = age;
}
}
public abstract class SpecialPersonBase : IPerson
{
private IPerson myPerson;
protected SpecialPersonBase(IPerson person)
{
myPerson = person;
}
public string Name { get { return myPerson.Name; } }
public int Age { get { return myPerson.Age; } }
public abstract string Greet();
}
public class Doctor : SpecialPersonBase
{
public Doctor(IPerson person) : base(person) { }
public override string Greet()
{
return "How are you feeling?";
}
}
public class Accountant : SpecialPersonBase
{
public Accountant(IPerson person) : base(person) { }
public override string Greet()
{
return "How are your finances?";
}
}
You could use the classes like this:
IPerson bob = new Person("Bob", "25");
// Do things with the generic object
// until you can determine the specific type
SpecialPerson specialBob;
if (bobIsDoctor)
{
specialBob = new Doctor(bob);
}
else if (bobisAccountant)
{
specialBob = new Accountant(bob);
}
specialBob.Greet();