Hi, is it possible to have a virtual class variable? I want to have a pointer available per class that can be a different value. Something like this:

  TA = class
    class var Foo: TObject; virtual;
  end;
  TB = class(TA)
    class var Foo: TSpecialObject; override;
  end;

I have in mind multiple levels of inheritance. I am storing a vmt like record/object in the variable, so the variable the child class overrides will be compatible with the ancestor class. It's possible there may be many instances of the objects so I do not want to create an variable per instance if I can help it.

Regards,

Andrew

My current idea for a solution is here:

http://pastebin.com/P3JsDQ03

Can anyone think of something with less code? I guess I could save a little if I skip the property and directly use GetVMT and SetVMT.

Regards,

Andrew

Class variable is stored the same way as a regular variable and has the only difference is that it can be accessible with the class name prefix. The thing you need requires different implementation - something like storing a virtual class variable in VMT. There is no implementation for that.

Best regards,
Paul Ishenin

What is the current implementation?

I would guess that class vars are stored in the vmt already...

Please correct me if I've got this wrong.

Current classes have a structure containing partially the following:

TClassInstance = record
vmt: Pointer;
{Space for parent class variables are stored here followed by variables
declared for this object}
variable1: SomeType;
variable2: SomeType;
etc.
end;
TClassVMT = record
  parent_class_vmt: Pointer;
  virtual_procs: array[0..n] of pointer;
  class_var_1: Sometype;
  class_var_2: Sometype;
  etc...
end;

Is this right?

Thanks,

Andrew

Andrew Haines wrote

What is the current implementation? I would guess that class vars are stored in the vmt already...

No, class var and regular variable has no difference except the scope. It is a static variable which is shared between all instances and descendants - so why should it be stored in vmt or near it?

Best regards,
Paul Ishenin

Ahhh ok I understand now. , Thank you.