Re: Chaining Up (GLib Tutorial Error)

[Stefan Kost <ensonic hora-obscura de>]

Yevgen Muntyan schrieb:
> Phil Lello wrote:
>> According to the GObject tutorial, chaining-up in 
>> constructor/dispose/finalize code should be handled as follows:
>>
>> static void
>> b_method_to_call (B *obj, int a)
>> {
>>   BClass *klass;
>>   AClass *parent_class;
>>   klass = B_GET_CLASS (obj);
>>   parent_class = g_type_class_peek_parent (klass);
>>
>>   /* do stuff before chain up */
>>   parent_class->method_to_call (obj, a);
>>   /* do stuff after chain up */
>> }
>>   
> 
> Yes, a tutorial mistake.

Why? The major difference is that app do this:

static GObjectClass *parent_class = NULL;

xxx_class_init (GstElementClass * klass)
{
  parent_class = g_type_class_peek_parent (klass);
  ...
}

and later e.g.
G_OBJECT_CLASS (parent_class)->finalize (object);
to chainup.



>> (Unless there is something horribly wrong with my implementation of 
>> _GET_CLASS...)
>>
>> This works fine until we create CClass, a direct descendant of BClass. 
>> When CClass is using the default 'method_to_call' code, e.g. in 
>> finalize, B_GET_CLASS actually returns CClass. parent_class is now 
>> BClass, and 'method_to_call' invokes itself, leading eventually to stack 
>> overflow.
>>   

B_GET_CLASS should always give you BClass and never CClass imho.

> 
> Yes.
> 
>> The solution I've used, and that I think should be in the tutorial is:
>>
>> klass = g_type_class_peek (MY_CLASS_TYPE);
>>
>> Should I submit sample code to reproduce / raise a bug, or is this email 
>> sufficient? I can provide a patch to tut_howto.xml if wanted, but this 
>> seems excessive for a 1 line change.
>>   
> 
> It'd be better to use G_DEFINE_TYPE in the tutorial, IMO. A nice
> macro, saves you from many troubles.
>
You're very welcome to do it. Its planned for a long time. IMHO the current
details explanation should stay there as well. What should be added is more
something like : "no that you've read about the internals, this is how you can
do it in practise and save typing".

Stefan

> 
> Best regards,
> Yevgen
> 
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