Re: How to "extend" a widget?

[Michael Torrie <torriem gmail com>]

On 12/16/2011 02:52 PM, jjacky wrote:
> Thanks. I will have a look into Vala, although for projects I have 
> planned, I really want/need to be using C.

Precisely.  Vala makes the C Gobjects.  You could use Vala to construct
a class with a bunch of empty methods and then use the generated C code
and fill in your code, abandoning the vala as a scaffold.

> And while GTK is oriented-object, it is written in C and, AFAIK, there 
> are no such things as classes in C?

"Classes" and other Object-oriented things are merely syntactic sugar in
any language.  GTK is certainly object-oriented through and through with
all the classic aspects of Polymorphism, inheritance, and encapsulation.

> I believe the way to create a widget based on/extending another one is 
> how I have done so far (based on the docs /tutorials I read), only I'm 
> not sure how to properly do things like "overriding" functions to change 
> behaviors or add features.

Overriding methods is done through the GOBject's vtable.  Virtual
methods (as in C++) are dispatched through a function table.  In the raw
C code of GTK objects, this is a special struct.  At least the way it
used to work in GTK (my knowledge is a bit rusty) was that one struct
contained the object's data, and this other stuct (sometimes called a
'klass' struct), contained the vtable.  Any call to a virtual method
would, using the object struct pointer itself, look up the klass
structure, and look up the method to call (classic polymorphism).

> If Vala allows to do that easily and does produce C code though, I might 
> try and see how it does/what code it produces, but I'm not sure learning 
> a new language is something I wanna get into right now...

Sure.  as I said in my e-mail GTK developers used to use a tool called
gob to create GObject-based classes, but that also has a
pseudo-language.  That said, working with GOBjects in C requires a lot
of boilerplate code (to implement the object-orientedness), so using
Vala or GOB (whatever the latest incarnation is), seems like a good idea
to me, specially if you want to make your own classes and inherit, etc.