VirtualConstructor

Used to create an object with the exact type given at runtime. The general type may be assumed.

See:

http://www1.bell-labs.com/user/cope/Patterns/C%2b%2bIdioms/EuroPLoP98.html#VirtualConstructor

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OK, that's interesting. In ObjectPascal, I found this:

The most notable feature of ObjectPascal as in Delphi is the use of type variable, and through type variables the usage of VirtualConstructor (yes you can declare a constructor virtual in Delphi). As far as I know there's no other OO typed language having such a useful feature.

If I have this right, it seems that the VirtualConstructor is a feature of DelphiLanguage, but has to be implemented using some sort of support class in other languages.

So we might have

 TFoo = class
 public
  constructor Create; virtual;
 end;

 TBar = class(TFoo);
 public
  constructor Create; override;
 end;

 TFooClass = class of TFoo; //declares a class reference type

 ...

 var
  fc: TFooClass;
  afoo: TFoo;
 begin
  fc := TFoo;
  afoo := fc.Create; // returns a TFoo

 ...

  fc := TBar;
  afoo := fc.Create; // returns a TBar

 {contrived example - we could just call TFoo.Create or TBar.Create in these cases}

 end;

There must be similar features in other typed languages surely?

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If you think you need a VirtualConstructor, you may be better off with a ClassFactory.

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VirtualConstructor is also a nickname for the FactoryMethodPattern in the DesignPatternsBook

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ClassFactory is needed when there's no support for strong typed MetaClassVariable with inheritance support and VirtualMetaClassMethod

MetaClassVariable are especially useful in InitializationSequence, allowing to build sets of ClassRepository