[fpc-devel] Creating new classes at runtime

[Mattias Gaertner]

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? 
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.

 
> 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.


> 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. :)


Mattias