Re: g_object_new(G_TYPE_OBJECT,NULL) failed

[Tristan Van Berkom <tvb gnome org>]

Yueyu Lin wrote:
> Hi,
>     After reading the G-lib system carefully,I think I understand the GLib system. So I writer the following codes:
[...]

Here are a few comments:

> gint pet_get_ages(Pet *pet){
>   if((pet)&&(pet->priv)){
>     return pet->priv->ages;
>   }
>   return 0;
> }
>
> void pet_set_ages(Pet *pet,int ages){
>   if(pet){
>     g_print("set ages, the address is %x",pet);
>     if(!(pet->priv)){
>       pet->priv = g_newa(PrivStruct,1);
>     }
>     pet->priv->ages = ages;
>   }
> }

You might want to put `pet->priv = g_new0 (PrivStruct, 1);` in
`pet_instance_init ();' this way, if `IS_PET ()' is true, you always
know that the data is already there.

> static void pet_finalize(GObject *object) {
>   Pet *self = (Pet*)object;
>   GObjectClass *parent;
>   g_free(self->priv);
>   parent->finalize(object);
> }

This code will crash; you need to just use
 `parent_class->finalize (object)' instead of using
a wild pointer ;-p

(Note that parent_class was obtained with g_object_class_peek_parent()
in class_init; so if there is an instance; you know that `parent_class'
is assigned and you dont need to check it).

> static GObject *pet_constructor(GType type,
> 				guint n_construct_properties,
> 				GObjectConstructParam *construct_properties){
>   GObject *obj;
>   {
>     PetClass *klass;
>     GObjectClass *parent_class;
>     klass = PET_CLASS(g_type_class_peek(TYPE_PET));
>     parent_class = G_OBJECT_CLASS(g_type_class_peek_parent(klass));
>     obj = parent_class->constructor(type,
> 				    n_construct_properties,
> 				    construct_properties);
>   }
>  
>   return obj;
> }

Hmmm,
    I didn't read up on this new "constructor" idea; but I'm sure
its not nescisary for your code, also; the common practice is to
simply use a global `static GObjectClass *parent_class' and assign
it once class-wide in your pet_class_init function (I guess this is
just a slight optimization though).

> static void pet_class_init(gpointer g_class,
> 		     gpointer g_class_data){
>   g_print("init class \n");
>   GObjectClass *gobject = G_OBJECT_CLASS(g_class);
>   PetClass *klass = PET_CLASS(g_class);  
>   gobject->constructor = pet_constructor;
>   gobject->finalize = pet_finalize;
> }


> static void pet_instance_init(GTypeInstance *instance,
> 			      gpointer      g_class){
>   g_print("hello init\r\n");
> }


> GType pet_get_type(){
>   static GType type = 0;
>   if(type == 0){
>     g_print("regist pet type\r\n");
>     static const GTypeInfo info = {
>       sizeof(PetClass),
>       NULL,/*base_init*/
>       NULL,/*base_finalize*/
>       pet_class_init,/*class_init*/
>       NULL,/*class_finalize*/
>       NULL,
>       sizeof(Pet),
>       0,/*n_preallocs*/
>       pet_instance_init         /*instance_init*/
>     };
>     type = g_type_register_static(G_TYPE_OBJECT,
> 				  "LinyyPetType",
> 				  &info,0);
>     g_print("the type id is %d",type);
>     
>   }
>   return type;
> }
>
>
>
> int main(int argc,char *argv[]) {
>   g_print("hello\r\n");
>   GObject *object = g_object_new(G_TYPE_OBJECT,NULL);
>   g_free(object);
>   Pet *pet = g_object_new(TYPE_PET,NULL);
>   pet_set_ages(pet,3);
>   int age = pet_get_ages(pet);
>   g_print("it's age is %d",age);
>   g_free(pet);
>   return 0;
> }

Hmmm,
   I guess this is compiling because you are using a c++ compiler ?
Anyway; I guess it should work anyway.

One thing you'll have to do is make a call to `g_type_init()' before
using the type system.

Cheers,
                             -Tristan