Question

Let’s say you have a Person object and it has a method on it, promote(), that transforms it into a Captain object. What do you call this type of method/interaction?

It also feels like an inversion of:

myCaptain = new Captain(myPerson); 

Edit: Thanks to all the replies. The reason I’m coming across this pattern (in Perl, but relevant anywhere) is purely for convenience. Without knowing any implementation deals, you could say the Captain class ‘has a’ Person (I realize this may not be the best example, but be assured it isn’t a subclass).

Implementation I assumed:

// this definition only matches example A Person.promote() {   return new Captain(this) }  personable = new Person;  // A. this is what i'm actually coding myCaptain = personable.promote();  // B. this is what my original post was implying personable.promote(); // is magically now a captain? 

So, literally, it’s just a convenience method for the construction of a Captain. I was merely wondering if this pattern has been seen in the wild and if it had a name. And I guess yeah, it doesn’t really change the class so much as it returns a different one. But it theoretically could, since I don’t really care about the original.

Ken++, I like how you point out a use case. Sometimes it really would be awesome to change something in place, in say, a memory sensitive environment.

Answer 1

A method of an object shouldn’t change its class. You should either have a member which returns a new instance:

myCaptain = myPerson->ToCaptain(); 

Or use a constructor, as in your example:

myCaptain = new Captain(myPerson); 

I would call it a conversion, or even a cast, depending on how you use the object. If you have a value object:

Person person; 

You can use the constructor method to implicitly cast:

Captain captain = person; 

(This is assuming C++.)