The Missing GObject Tutorial Sample

Well, perhaps not exactly, but still -- I think it should work.

I've recently started poking around the GObject library, which is part of GLib. GObject is a C library aimed at providing OOP programmability that easily integrates with (usually) dynamic third-party languages. Basically, it allows you to write "glue" code between $your_dynamic_language and the GObject library just once, and then hook into any libraries created with GObject without writing further glue.

A good, detailed tutorial is available here, which I've been working through. After getting the gist of something, I get itchy for some sample code to play around with. So I Googled and found this how-to. An older version of the documentation mentioned some sample code that I was never able to find.

After giving up on that, I decided that I should be able to use the tutorial to scrap together my own sample. The tutorial was pretty detailed, after all, and apparently referenced a sample that did (or does) exist somewhere. So that's what I did. I've created a fully functioning sample based more-or-less on the tutorial mentioned above. The code is below, with comments.

I have compiled this on my Ubuntu 8.10 and 9.04 machines using the following command:

gcc `pkg-config --libs gtk+-2.0` `pkg-config --cflags gtk+-2.0` maman-bar.c

maman-bar.h

/*
 * Copyright/Licensing information.
 *
 * Reference:
 *
 * http://library.gnome.org/devel/gobject/unstable/howto-gobject.html
 * http://library.gnome.org/devel/gobject/unstable/chapter-gobject.html
 *
 *
 */


/* inclusion guard */
#ifndef __MAMAN_BAR_H__
#define __MAMAN_BAR_H__

#include <glib-object.h>

/*
 * Potentially, include other headers on which this header depends.
 */

/*
 * Type macros.
 */
#define MAMAN_TYPE_BAR                  (maman_bar_get_type ())
#define MAMAN_BAR(obj)                  (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
#define MAMAN_IS_BAR(obj)               (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
#define MAMAN_BAR_CLASS(klass)          (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
#define MAMAN_IS_BAR_CLASS(klass)       (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
#define MAMAN_BAR_GET_CLASS(obj)        (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))

typedef struct _MamanBar        MamanBar;
typedef struct _MamanBarClass   MamanBarClass;

/* 
 * Private instance fields 
 * Uses the Pimpl method:
 *
 * http://www.gotw.ca/gotw/024.htm
 * http://www.gotw.ca/gotw/028.htm
 *
 */
typedef struct _MamanBarPrivate MamanBarPrivate;


/* object */
struct _MamanBar
{
    GObject parent_instance;

    /* public */ 
    int public_int;


    /*< private >*/    
    MamanBarPrivate *priv;
};

/* class */
struct _MamanBarClass
{
    GObjectClass parent_class;

    /* class members */
  
    /* Virtual public method */
    void (*do_action_virt) (MamanBar *self, gchar *msg);

};


/*
 * Non-virtual public method
 */
void maman_bar_do_action (MamanBar *self, gchar *msg /*, other params */);

/* Virtual method call declaration */
void maman_bar_do_action_virt (MamanBar *self, gchar *msg /*, other params */);
/* Virtual method default 'super' class method */
void maman_bar_do_action_virt_default (MamanBar *self, gchar *msg);


#endif /* __MAMAN_BAR_H__ */

maman-bar.c

#include "maman-bar.h"

/*
    http://library.gnome.org/devel/gobject/2.21/gobject-Type-Information.html#G-DEFINE-TYPE--CAPS

    A convenience macro for type implementations, which declares a class 
    initialization function, an instance initialization function (see GTypeInfo
    for information about these) and a static variable named t_n_parent_class 
    pointing to the parent class. Furthermore, it defines a *_get_type() 
    function. See G_DEFINE_TYPE_EXTENDED() for an example.
*/
G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);


/* Define the private structure in the .c file */
#define MAMAN_BAR_GET_PRIVATE(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), MAMAN_TYPE_BAR, MamanBarPrivate))

struct _MamanBarPrivate
{
  int hsize;
  gchar *msg;
};


/* Init functions */
static void
maman_bar_class_init (MamanBarClass *klass)
{
    g_type_class_add_private (klass, sizeof (MamanBarPrivate));

    /* Setup the default handler for virtual method */
    klass->do_action_virt = maman_bar_do_action_virt_default;
}


static void
maman_bar_init (MamanBar *self)
{
    
    g_print("maman_bar_init() - init object\n");
    

    /* Initialize all public and private members to reasonable default values. */
    
    /* Initialize public fields */
    self->public_int = 99;

    g_print("  initializing public_int to %d\n", self->public_int);
 

    /* Initialize private fields */
    MamanBarPrivate *priv;
    self->priv = priv = MAMAN_BAR_GET_PRIVATE(self);
    priv->hsize = 42;

    g_print("  init'd private variable priv->hsize to %d\n", priv->hsize);


    /* If you need specific construction properties to complete initialization,
     * delay initialization completion until the property is set. 
     */

}


/* Object non-virtual method */
void maman_bar_do_action (MamanBar *self, gchar *msg) {
    /* First test that 'self' is of the correct type */
    g_return_if_fail (MAMAN_IS_BAR (self));


    // Assign to private 'msg' 
    self->priv->msg = msg;

    g_print("maman_bar_do_action() - %s\n", self->priv->msg);

}

/* Object virtual method call - performs the override */
void maman_bar_do_action_virt (MamanBar *self, gchar *msg) {
     /* First test that 'self' is of the correct type */
    g_return_if_fail (MAMAN_IS_BAR (self));

    g_print("maman_bar_do_action_virt() -> ");
    MAMAN_BAR_GET_CLASS (self)->do_action_virt(self, msg);  
}

/* Object virtual method default action (can be overridden) */
void maman_bar_do_action_virt_default (MamanBar *self, gchar *msg) {

    g_print("maman_bar_do_action_virt_default() - %s\n", msg );

}

int
main (int argc, char *argv[])
{
    /*
     * Prior to any use of the type system, g_type_init() has to be called 
     * to initialize the type system and assorted other code portions 
     * (such as the various fundamental type implementations or the signal 
     * system).
     */
    g_type_init();

    /* Create our object */
    MamanBar *bar = g_object_new (MAMAN_TYPE_BAR, NULL);

    bar->public_int +=1;
    g_print("incremented bar->public_int:  %d\n", bar->public_int);

    /* Call object method */
    maman_bar_do_action(bar, "helowrld");

    /* Call virtual object method - we could subclass and override... */
    maman_bar_do_action_virt(bar, "HELOWRLD");

    return 0; 
}

And here's what I get when I run a.out:

ok ./a.out
maman_bar_init() - init object
  initializing public_int to 99
  init'd private variable priv->hsize to 42
incremented bar->public_int:  100
maman_bar_do_action() - helowrld
maman_bar_do_action_virt() -> maman_bar_do_action_virt_default() - HELOWRLD

You can download the source files directly from here.