Re: Best practise inheritance

[Joël Krähemann <jkraehemann gmail com>]

Hi

No, you can talk in C of direct inheritance. Or implementing an interface.

As Tristan told you. The struct contains the other struct as not using
a pointer.

struct MyCompositeWidget
{
  GtkAlignment alignment;

  GtkBox *box;
};

The properties are inherited, too. As long you use
g_type_register_static() or alike
passing the parent type. You can peek the parent class.

It might hard to understand for you what has the meaning class. A
class has always
the very same methods. Else you have to subtype and override the class.

The class methods can chained up by using the peeked class.

GTK_WIDGET_CLASS(my_composite_widget_parent_class)->show(my_instance)

Note GLib-Object interfaces are limited to one nesting level of GTypeInterface.

So there is no interface inheritance.

I think it's a good point to do object orientated programming with C
because you have to
know what's going on.

Bests,
Joël


On Mon, Mar 20, 2017 at 5:15 PM, Tristan Van Berkom
<tristan vanberkom codethink co uk> wrote:
> On Mon, 2017-03-20 at 16:36 +0100, S. Jacobi wrote:
> > First of all, inheritance may be the wrong word here in plain c, but
> > I
> > don't know how else to name it.
>
> Sorry replied to this from my phone and missed some things...
>
> > In different projects I see different approaches how to derive custom
> > widgets from existing ones. I can roughly group them into 2 to 3.
> >
> > 1) The header only has a typedef to make the struct opaque. All
> > variables needed are put into the struct in the .c file.
> >
> > myType.h
> > typedef struct _MyType        MyType;
> >
> > myType.c
> > struct _MyType
> > {
> >   GtkWidget   *parent;
> >   /* additions */
> >   guint                i;
> >   ...
> > };
>
> Note also that this approach imposes that absolutely nothing can derive
> from MyType later, because MyType becomes anonymous.
>
> > 2a) The header defines a private struct, and all variables needed are
> > put into this private struct.
> >
> > myType.h
> > typedef struct _MyTypePriv MyTypePriv;
> > typedef struct _MyType           MyType;
> >
> > myType.c
> > struct _MyTypePriv
> > {
> >   GtkWidget   *parent;
> >   /* additions */
> >   guint                i;
> > };
> >
> > struct _MyType
> > {
> >   MyTypePriv  *priv;
> > };
>
> Actually this approach (2a) I've never seen before, not a good idea,
> but possibly works (except for the other detail below)...
>
>
> > 2b) Similar to 2a, but the parent is put in the "main" struct, not
> > the
> > private part.
> >
> > myType.h
> > typedef struct _MyTypePriv MyTypePriv;
> > typedef struct _MyType           MyType;
> >
> > myType.c
> > struct _MyTypePriv
> > {
> >   /* additions */
> >   guint                i;
> > };
> >
> > struct _MyType
> > {
> >   GtkWidget   *parent
> >   MyTypePriv  *priv;
> > };
>
> So in all of these cases, you have missed an important detail:
>
> struct _MyType {
>     GtkWidget *parent_instance;
>
>     ... anything else...
> }
>
> Is wrong, as it will only reserve one pointer size for the parent
> structure, what you have been seeing is always in fact:
>
> struct _MyType {
>     GtkWidget parent_instance
>
>     ... anything else ...
> }
>
> The parent instance must be known and not anonymous, you need to
> include the struct itself as the first member of your derived type, not
> merely a pointer to that.
>
> Cheers,
>     -Tristan
>
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