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.vanber...@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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