[fpc-pascal] virtual class var?

[Andrew Haines]

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
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Re: [fpc-pascal] virtual class var?

[Andrew Haines]

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
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Re: [fpc-pascal] virtual class var?

[Paul Ishenin]

07.08.12, 8:18, Andrew Haines wrote:

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;


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
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Re: [fpc-pascal] virtual class var?

[Andrew Haines]

On 08/06/12 23:02, Paul Ishenin wrote:

 
 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
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Re: [fpc-pascal] virtual class var?

[Paul Ishenin]

07.08.12, 11:24, 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

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Re: [fpc-pascal] virtual class var?

[Andrew Haines]

Ahhh ok I understand now. , Thank you.
-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.

Paul Ishenin paul.ishe...@gmail.com wrote:

07.08.12, 11:24, 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

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