[Gambas-devel] need a bit of explanation - please

[Benoit Minisini]

On vendredi 15 août 2008, Kari Laine wrote:
> On Fri, Aug 15, 2008 at 1:38 PM, Benoit Minisini <
>
> gambas at ...1...> wrote:
> > On vendredi 15 août 2008, Kari Laine wrote:
> > > Hi,
> > >
> > > I try to figure out Gambas and because I am not C-guru I have used two
> > > hours now to understand possibly a basic thing...
> > > Could someone help?
> > >
> > > In function
> > >
> > > CLASS *CLASS_register_class(GB_DESC *ptr, CLASS *class)
> > >
> > > There is a loop
> > >
> > >   desc = (CLASS_DESC *)&gambas[1];
> > >
> > >   for (start = NULL; start == NULL; desc++)
> > >
> > > The problem I have is to understand this bit
> > >     switch ((intptr_t)desc->gambas.name)
> > > Where gambas.name is initialized to have the values tested in the case
> > > clauses? Somehow name's pointer is usefull to give out information.
> > >
> > > Also in the same switch there is following
> > >       case (intptr_t)GB_INHERITS_ID:
> > >         CLASS_inheritance(class, CLASS_find((const char
> > > *)desc->gambas.type));
> > >         break;
> > > Why is CLASS_find called with a generall name like Control ?
> > >
> > > Hopefully someone understands my question.
> > >
> > > Best Regards
> > > Kari Laine
> >
> > Sorry, I don't understand what you are asking for... Can you formulate
> > your question differently?
>
> I am not sure if I can ask this in a right way.
>
> Could you explain this line.
> switch ((intptr_t)desc->gambas.name)
>
> How gambas.name pointer can be cast to intptr_t?
>
> And how it takes one of following values
> GB_INHERITS_ID
> GB_AUTO_CREATABLE_ID:
> GB_NOT_CREATABLE_ID:
> GB_VIRTUAL_CLASS_ID:
> GB_HOOK_CHECK_ID
>
> Best Regards
> Kari Laine

'desc' is a pointer to a 'GB_DESC' structure declared in 'gambas.h'. 

This structure describes one symbol of a native class. You can see these 
structures used in each class file of the interpreter ('gbx_c_*.c' files) or 
any component.

It is accessed in 'CLASS_register_class' via the 'GB_CLASS_DESC' union and 
its 'gambas' field.

'desc->gambas.name' is the name of the symbol, with one letter added to its 
beginning. It describes the type of the symbol, and if it is static: for 
example, "MLoad" is a dynamic method named 'Load', or "rLength" is a dynamic 
read-only property named 'Length'.

But this symbol can takes special values to specify some class global 
parameters: GB_INHERITS_ID for the parent class, GB_VIRTUAL_CLASS_ID for a 
virtual class, and so on.

These special values are false pointers, whose value should never be taken by 
real pointers: 1, 2, 3, and so on.

So the 'intptr_t' cast is just to make the compiler understand that we are 
dealing with integer values that are stored inside pointers.

This is sort of dirty programming. Do not do that in your own code. :-) 
The 'GB_DESC' structure should have been declared as an union of different 
structures.

I hope it is clearer now.

Regards,

-- 
Benoit Minisini