[fpc-devel] Creating new classes at runtime

[Michael.VanCanneyt at Wisa.be]

On Sat, 13 Nov 2004, Mattias Gaertner wrote:

> On Fri, 12 Nov 2004 22:32:48 +0100 (CET)
> Michael.VanCanneyt at Wisa.be wrote:
> 
> > 
> > 
> > On Fri, 12 Nov 2004, Mattias Gaertner wrote:
> > 
> > > 
> > > I don't know, if this is the right list, but the topic is quite fpc specific and needs some compiler gurus:
> > > 
> > > The Lazarus IDE needs to create new classes at runtime. For example when loading/creating a descendent of TDataModule named TMyDataModule, 
> > > it needs a unique TMyDataModule = class(TDataModule) complete with vmt, method table (empty), field table (empty), typeinfo 
> > > (no properties), and .. ?
> > > At the moment lazarus only supports TDataModule and TForm, so I was able to just define the two and copy. 
> > > But for inheriting I need 'real' custom classes.
> > > 
> > > Creating the empty method table, field table and new typeinfo seems to be easy. But I found some fields, which I don't know:
> > > In objpash.inc there is
> > >     const
> > >        vmtInstanceSize         = 0;
> > >        vmtParent               = sizeof(ptrint)*2;
> > > 
> > > The fields behind vmtParent are easy (at least I think so). But what about the 8 bytes (i386) behind vmtInstanceSize and in front of vmtParent?
> > > They seem to be a negative number and a pointer? What are they are good for?
> > > 
> > > And: Is this solution portable or will I get into trouble?
> > 
> > What are you trying to do ? 
> 
> Well, that's a long story ...
> 
>  
> > As soon as you have the class, you know everything there is to know or that
> > you need to know. You don't need to have more than the TMyClass class pointer.
> 
> Wow. That would be great.
> 
>  
> > In the case of TMyDatamodule, you have the TMyDataModule class pointer, so
> > You can create an instance and 'show' that on screen ?
> 
> Where do I get this TMyDataModule class pointer? 

>From the registration routine, of course. You must register it somewhere...

> I also want TMyDataModule1, TMyOwnDataModule, TAnotherMyDataModule, ... . The classname should be different. 
> And I want a TMyGrandChildDataModule = class(TAnotherMyDataModule) = class(TMyDataModule) = class(TDataModule). 
> I only have a TDataModule class pointer. The rest should be created by the IDE just from class names. 
> But maybe I think too complicated. Fact is, I have class names and I need class pointers.

You must require a class pointer, there is no other way. It is no different
from registering components in the IDE.

> 
>  
> > For forms you must only assume it is a descendant of TCustomForm.
> > 
> > The Delphi IDE also can only handle things that descend from TCustomForm and
> > TDatamodule. 
> 
> That's not sufficient. I want at least TComponent.

That is not practical, since you cannot 'show' a TComponent on screen ?
TDataModule and TCustomform can be 'shown'. An arbitrary TComponent can only
be shown as an icon on a form or datamodule. If this is your intention, then
you already have it in the IDE.

How will you show e.g. TDBManager ? (just picking an arbitrary component I
have at work)

> > When registering custom forms or datamodules in the IDE, you 
> > need to supply Delphi with the actual class pointer, and then it knows what 
> > to do...
> > If you want I can show you code that registers custom forms in the Delphi
> > IDE, I use it myself at work.
> 
> Yes, please enlighten me. :)

I'll send the code to you in private.

Michael.