cedlemo cedlemo - 3 months ago 11
C Question

GObject subclassing in C unable to create a subclass instance

I wanted to try the GObject API. The idea is to create a Vehicule class which is a

DERIVABLE
type and create a subclass of Vehicule called Car which is a
FINAL
type. My problem is that when I try in a little program to create a class Car, my program block and doesn't return at the instance creation.

Here is the minimum code in order to reproduce my problem. I use the convention tuto-vehicule /tuto-car.

tuto-vehicule.h

#ifndef __TUTO_VEHICULE_H__
#define __TUTO_VEHICULE_H__
#include <glib-object.h>

G_BEGIN_DECLS

typedef struct _TutoVehiculePrivate {
GObject parent_instance;
GString *name;
guint nb_wheels;
} TutoVehiculePrivate;

typedef struct _TutoVehiculeClass {
GObjectClass parent_class;
} TutoVehiculeClass;

#define TUTO_TYPE_VEHICULE tuto_vehicule_get_type()
G_DECLARE_DERIVABLE_TYPE(TutoVehicule, tuto_vehicule, TUTO, VEHICULE, GObject);


TutoVehicule *tuto_vehicule_new (void);

void tuto_vehicule_set_name(TutoVehicule *self, gchar *name);
void tuto_vehicule_set_nb_wheels(TutoVehicule *self, gint nb_wheels);
void tuto_vehicule_print_name(TutoVehicule *self);
void tuto_vehicule_print_nb_wheels(TutoVehicule *self);

G_END_DECLS

#endif


tuto-vehicule.c

#include "tuto-vehicule.h"
#include <stdio.h>

G_DEFINE_TYPE_WITH_PRIVATE(TutoVehicule, tuto_vehicule, G_TYPE_OBJECT);

static void
tuto_vehicule_dispose(GObject *object)
{
TutoVehiculePrivate *priv = tuto_vehicule_get_instance_private(TUTO_VEHICULE(object));
printf("tuto_vehicule_dispose\n");
g_string_free(priv->name, TRUE);
G_OBJECT_CLASS(tuto_vehicule_parent_class)->dispose(object);
}

static void
tuto_vehicule_class_init(TutoVehiculeClass *klass)
{
GObjectClass *g_object_class = G_OBJECT_CLASS(klass);
/* instance destructor*/
g_object_class->dispose = tuto_vehicule_dispose;
printf("tuto_vehicule_class_init for TutoVehiculeClass created by G_DECLARE_DERIVABLE_TYPE\n");
}

static void
tuto_vehicule_init(TutoVehicule *self)
{
TutoVehiculePrivate *priv = tuto_vehicule_get_instance_private(self);
priv->name = g_string_new(NULL);
priv->nb_wheels = 0;
printf("tuto_vehicule_init\n");
}

TutoVehicule *
tuto_vehicule_new(void)
{
TutoVehicule *vehicule_instance = g_object_new(TUTO_TYPE_VEHICULE, NULL);
return vehicule_instance;
}

void tuto_vehicule_set_name(TutoVehicule *self, gchar *name)
{
TutoVehiculePrivate *priv = tuto_vehicule_get_instance_private(self);
g_string_assign(priv->name, name);
}

void tuto_vehicule_set_nb_wheels(TutoVehicule *self, gint nb_wheels)
{
TutoVehiculePrivate *priv = tuto_vehicule_get_instance_private(self);
priv->nb_wheels = nb_wheels;
}

void tuto_vehicule_print_name(TutoVehicule *self)
{
TutoVehiculePrivate *priv = tuto_vehicule_get_instance_private(self);
printf("Vehicule name: %s\n", priv->name->str);
}

void tuto_vehicule_print_nb_wheels(TutoVehicule *self)
{
TutoVehiculePrivate *priv = tuto_vehicule_get_instance_private(self);
printf("Vehicule number of wheels: %d\n", priv->nb_wheels);
}


tuto-car.h

#ifndef __TUTO_CAR_H
#define __TUTO_CAR_H
#include "tuto-vehicule.h"
#include <glib-object.h>

G_BEGIN_DECLS

#define TUTO_TYPE_CAR tuto_car_get_type ()
G_DECLARE_FINAL_TYPE(TutoCar, tuto_car, TUTO, CAR, TutoVehicule);

typedef struct _TutoCar {
TutoVehicule parent_instance;
} TutoCar;

TutoCar *tuto_car_new(void);
void tuto_car_print_name(TutoCar *self);
void tuto_car_print_nb_wheels(TutoCar *self);
G_END_DECLS
#endif


tuto-car.c

#include "tuto-car.h"
#include <stdio.h>

G_DEFINE_TYPE(TutoCar, tuto_car, TUTO_TYPE_CAR);

static void
tuto_car_class_init(TutoCarClass *klass)
{
printf("tuto_car_class_init\n");
}

static void
tuto_car_init(TutoCar *self)
{
printf("tuto_car_init\n");
}

TutoCar *
tuto_car_new(void)
{
TutoCar *car = g_object_new(TUTO_TYPE_CAR, NULL);
tuto_vehicule_set_name(TUTO_VEHICULE(car), "car");
tuto_vehicule_set_nb_wheels(TUTO_VEHICULE(car), 4);
return car;
}

void
tuto_car_print_name(TutoCar *self)
{
tuto_vehicule_print_name(TUTO_VEHICULE(self));
}

void
tuto_car_print_nb_wheels(TutoCar *self)
{
tuto_vehicule_print_nb_wheels(TUTO_VEHICULE(self));
}


I use those two classes in a simple main file :

#include <stdio.h>
#include "tuto-vehicule.h"
#include "tuto-car.h"

int main(int argc, char **argv)
{
printf("Create a TutoVehicule instance\n");

TutoVehicule *moto = tuto_vehicule_new();
printf("Puts its name to moto\n");
tuto_vehicule_set_name(moto, "moto");
printf("Puts its number of wheels to 2\n");
tuto_vehicule_set_nb_wheels(moto, 2);

printf("Create a TutoCar instance\n");
TutoCar *car = tuto_car_new();

printf("Use methods instance\n");

tuto_vehicule_print_name(moto);
tuto_vehicule_print_nb_wheels(moto);

tuto_vehicule_print_name(TUTO_VEHICULE(car));
tuto_vehicule_print_nb_wheels(TUTO_VEHICULE(car));

tuto_car_print_name(car);
tuto_car_print_nb_wheels(car);

printf("Destroy the instances created previously\n");
g_object_unref(car);
g_object_unref(moto);

return 0;
}


The compilation works with :

gcc -Wall -o main_vehicule `pkg-config --libs --cflags gobject-2.0` main_vehicule.c tuto-car.c tuto-vehicule.c


But when I run the program, as I said below, I have this ouput and the program block without returning nor emitting error messages:

Create a TutoVehicule instance
tuto_vehicule_class_init for TutoVehiculeClass created by G_DECLARE_DERIVABLE_TYPE
tuto_vehicule_init
Puts its name to moto
Puts its number of wheels to 2
Create a TutoCar instance


I know that my program block at this statement:

TutoCar *car = g_object_new(TUTO_TYPE_CAR, NULL);


What have I missed? The official docs are not really clear.

edit

Here is a gdb session:

(gdb) break tuto-car.c:tuto_car_new
Breakpoint 1 at 0x400dcb: file tuto-car.c, line 21.
(gdb) run
Starting program: /home/cedlemo/Projets/C/GObject/derivable_type/main_vehicule
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
Create a TutoVehicule instance
tuto_vehicule_class_init for TutoVehiculeClass created by G_DECLARE_DERIVABLE_TYPE
tuto_vehicule_init
Puts its name to moto
Puts its number of wheels to 2
Create a TutoCar instance

Breakpoint 1, tuto_car_new () at tuto-car.c:21
21 TutoCar *car = g_object_new(TUTO_TYPE_CAR, NULL);
(gdb) backtrace
#0 tuto_car_new () at tuto-car.c:21
#1 0x0000000000400c02 in main (argc=1, argv=0x7fffffffded8) at main_vehicule.c:16
(gdb) next


The program block in gdb too so I had to hit ctrl+c and inspect the stack:

(gdb) backtrace
#0 0x00007ffff75ba269 in syscall () from /usr/lib/libc.so.6
#1 0x00007ffff790618f in g_cond_wait () from /usr/lib/libglib-2.0.so.0
#2 0x00007ffff78e8437 in g_once_init_enter () from /usr/lib/libglib-2.0.so.0
#3 0x0000000000400d1e in tuto_car_get_type () at tuto-car.c:4
#4 0x0000000000400d44 in tuto_car_get_type () at tuto-car.c:4
#5 0x0000000000400dd0 in tuto_car_new () at tuto-car.c:21
#6 0x0000000000400c02 in main (argc=1, argv=0x7fffffffded8) at main_vehicule.c:16

Answer

Here is the answer :

in the file tuto-car.c, I a the line :

G_DEFINE_TYPE(TutoCar, tuto_car, TUTO_TYPE_CAR);

for the two macros :

  • G_DEFINE_TYPE
  • G_DEFINE_TYPE_WITH_PRIVATE

the last parameter is the GType of the parent. In my code I used the GType of the class it self. I should have used TUTO_TYPE_VEHICULE

G_DEFINE_TYPE(TutoCar, tuto_car, TUTO_TYPE_VEHICULE);

With this the previous code is fully working.