Best way to create child class from a abstract Base class

Question

Best way to create child class from a abstract Base class. In my project I have a base class abstract and two child class in this way. ``` TbaseClass=class public procedure Doit;virtual;abstract; procedure Calculate;virtual;abstract; Tchild1class=class(TbaseClass) .. protected procedure CalcArea;virtual; public procedure Doit;override; procedure Calculate;override; Tchild2class=class(Tchild1class) protected procedure CalcArea;override; ``` In my buttonclick code I create the needed childclass in this way, and is working. ``` ..... var T:TBaseClass; begin case OP of 1: T:=Tchild1class.Create; 2: T:=TChild2Class.Create; T.Doit; T.Calculate; end; ``` My question is the next. I can make a constructor create procedure in my base abstract class , that depending of int parameter generate the needed child class??? For Example: ``` constructor TBaseClass.Create(OP:integer); begin inherited; case OP of 1: Tchild1class.Create; ///>>>??? 2: TChild2Class.Create; //>>>>??? end; ``` It's this possib…

Accepted Answer

No, that is not possible. Once a constructor has begun running, the class of object being created is fixed. An object cannot change its class, not even during construction.

You can write a factory function that takes the integer parameter and chooses which descendant class to instantiate. It would be a lot like your proposed invalid constructor, just as a standalone function or class method instead of a constructor.

You could also write a function that just returns the class, and then you can instantiate it afterward, like this:

```
type
  TBaseClassClass = class of TBaseClass;

function GetDescendantClass(op: Integer): TBaseClassClass;
begun
  case op of
    1: Result := TChild1Class;
    2: Result := TChild2Class;
    else Assert(False);
  end;
end;

var
  DescendantClass: TBaseClassClass;

DescendantClass := GetDescendantClass (op);
T := DescendantClass.Create;
T.DoIt;
```

Using that technique, it's sometimes useful to make the constructor be virtual, but that's not a requirement.

Best way to create child class from a abstract Base class

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