Delphi: writing to the private ancestor's field in descendant class

Question

I need to fix a third-party component. This component's class has private variable which is actively used by its descendants:

TThirdPartyComponentBase = class
private
  FSomeVar: Integer;
public
  ...
end;

TThirdPartyComponent = class (TThirdPartyComponentBase)
protected
   procedure Foo; virtual;
end;

procedure TThirdPartyComponent.Foo;
begin
  FSomeVar := 1; // ACCESSING PRIVATE FIELD!
end; 

This works because both classes are in the same unit, so they're kinda "friends".

But if I'll try to create a new class in a new unit

TMyFixedComponent = class (TThirdPartyComponent)
  procedure Foo; override; 
end;

I can't access FSomeVar anymore, but I need to use it for my fix. And I really don't want to reproduce in my code all that tree of base classes.

Can you advise some quick hack to access that private field without changing the original component's unit if it's possible at all?

Answer 1

Expose the value of the private variable by a protected property in TThirdPartyComponent.

TThirdPartyComponent = class (TThirdPartyComponentBase)
private
   Procedure SetValue(Value: Integer);
   Function GetValue: Integer;
protected
   Property MyVar: Integer read GetValue write Setvalue; 
   procedure Foo; virtual;
end;

Procedure TThirdPartyComponent.SetValue(Value: Integer);
begin
  FSomeVar := Value ;
end;

Function GetValue: Integer;
begin
  result := FSomeVar;
end;

In TMyFixedComponent class use the MyVar Property in the procedure which you would like to override.

Answer 2

You have to use a hack to access a private field in any class (including a base class) in a different unit. In your case define in your unit:

type
  __TThirdPartyComponentBase = class 
  private 
    FSomeVar: Integer;
  end;

Then get the access:

__TThirdPartyComponentBase(Self).FSomeVar := 123;

Of course, that is dangerous, because you will need to control changes in the base class. Because if the fields layout will be changed and you will miss this fact, then the above approach will lead to failures, AV's, etc.

Answer 3

Don't know if this will help, but I seem to recall there is a way to "crack" a private variable into visibility.

I know, for example, I've encountered warnings from the compiler when I've moved a property from lower visibility (in the base class) to a more visible level (in my descendant). The warning stated that it's being declared at a different level of visibility...

It's been some time and I'm not certain, but I believe what you can do is in your descendant declare the same variable as protected. (You may have to use the Redeclare keyword for this to compile.)

Sorry I don't have more specific information on how to do this (if it's indeed possible.) Perhaps this posting will prompt one of the wizards here into correcting me! :-)