[fpc-devel] Class field reordering

[Martin Schreiber]

On Monday 16 July 2012 17:25:58 michael.vancanneyt at wisa.be wrote:
> On Mon, 16 Jul 2012, Martin Schreiber wrote:
> > On Monday 16 July 2012 16:50:06 michael.vancanneyt at wisa.be wrote:
> >> Well, from your code adding the following to the protected section:
> >>
> >>    Property ValueBuffer : Pointer Read FValueBuffer;
> >>    Property Validating : Boolean Read FValidating ;
> >>
> >> Should be enough to remove the need for the TFieldCracker ?
> >
> > Unfortunately no:
> > "
> > procedure tmsebufdataset.checkfreebuffer(const afield: tfield);
> > begin
> > {$ifdef FPC}{$warnings off}{$endif}
> > with tfieldcracker(afield) do begin
> > {$ifdef FPC}{$warnings on}{$endif}
> >  if foffset and 2 <> 0 then begin
> >   freemem(fvaluebuffer);
> >   fvaluebuffer:= nil;
> >  end;
> > end;
> > end;
> > "
>
> If I understand correctly, you have in the record buffer just the pointer
> of the (wide)string instead of the actual string data ?
>
Correct. The UnicodeString pointer. That has not directly to do with the 
procedure above BTW. This one is to allow changing field values in OnValidate 
IIRC.

> If so, why then didn't you implement the string fields as blobs are
> implemented in SQLDB (it is basically the same problem), in that case you
> would not need the internal fields of TField at all ?
>
Maybe, but what about performance? Another complication is the "updatebuffer" 
with the "oldvalues".

> If that is not possible, it means you need a mechanism to 'finalize' the
> record buffer whenever it is re-filled or goes out of scope ?
>
That mechanism is implemented in tmsebufdataset and does not need 
tfieldcracker IIRC. tmsebufdatset holds a list of record offsets which must 
be finalized.

Thanks, Martin