OOP Using GObject (9) – A Dynamic Type

Recall there are 3 types in GObject type system: Fundamental, static and dynamic. A fundamental type is a top-most type which has no parent type. Most of them are pre-defined. Static types never load/unload its class type (say, their class struct) at runtime, since they are static. On the contrary, dynamic types can be dynamically loaded/unloaded at runtime. They are normally used within a module.

We can call g_type_register_dynamic() to register a dynamic type. When used in a module of GObject library (may be a GTypeModule type), We can also call g_type_module_register_type() to create your dynamic types. g_type_register_dynamic() is invoked for you in that function. Let’s go through the code:

NOTE: PLEASE READ ALL COMMENT CAREFULLY.

// bartype.h
#ifndef BAR_TYPE_H_
#define BAR_TYPE_H_

#include <glib-object.h>

/* Bar type class struct */
typedef struct _BarTypeClass {
    GObjectClass parent;
} BarTypeClass;

/* Far type object struct */
typedef struct _BarType {
    GObject parent;
} BarType;

/* type function */
GType bar_type_get_type();

/* register type */
void bar_type_register_type(GTypeModule *type_module);


#endif /* BAR_TYPE_H_ */

// bartype.h

#ifndef BAR_TYPE_H_

#define BAR_TYPE_H_

#include <glib-object.h>

/* Bar type class struct */

typedef struct _BarTypeClass {

GObjectClass parent;

} BarTypeClass;

/* Far type object struct */

typedef struct _BarType {

GObject parent;

} BarType;

/* type function */

GType bar_type_get_type();

/* register type */

void bar_type_register_type(GTypeModule *type_module);

#endif /* BAR_TYPE_H_ */

// bartype.c
#include "bartype.h"

static gpointer parent_klass = NULL;
static GType bar_type_type_id = 0;

static void bar_type_instance_init(BarType *self) {
    g_print("Calling bar_type_instance_init()\n");
}

static void bar_type_instance_finalize(GObject *object) {
    /* do some finalize, maybe release some dynamically allocated memory */
    g_print("Calling bar_type_instance_finalize()\n");
    /* chain to parent's finalize */
    G_OBJECT_CLASS(parent_klass)->finalize(object);
}

static void bar_type_class_init(BarTypeClass *klass) {
    g_print("Calling bar_type_class_init()\n");
    parent_klass = g_type_class_peek_parent(klass);
    G_OBJECT_CLASS(klass)->finalize = bar_type_instance_finalize;
}

static void bar_type_class_finalize(BarTypeClass *klass) {
    g_print("Calling bar_type_class_finalize()\n");
}

GType bar_type_get_type() {
    return bar_type_type_id;
}

void bar_type_register_type(GTypeModule *type_module) {
    const GTypeInfo type_info = {
        sizeof(BarTypeClass), /* class_size */
        NULL,                   /* base_init */
        NULL,                   /* base_finalize */
        (GClassInitFunc)bar_type_class_init, /* class_init */
        (GClassFinalizeFunc)bar_type_class_finalize, /* class_finalize */
        NULL,                   /* class_data */
        sizeof(BarType),        /* instance_size */
        0,                      /* n_preallocs */
        (GInstanceInitFunc)bar_type_instance_init, /* instance_init */
        NULL                    /* value_table */
    };
    bar_type_type_id = g_type_module_register_type(
        type_module, G_TYPE_OBJECT, "BarTypeDynamicClass", &type_info, 0);
}

// bartype.c

#include "bartype.h"

static gpointer parent_klass = NULL;

static GType bar_type_type_id = 0;

static void bar_type_instance_init(BarType *self) {

g_print("Calling bar_type_instance_init()\n");

}

static void bar_type_instance_finalize(GObject *object) {

/* do some finalize, maybe release some dynamically allocated memory */

g_print("Calling bar_type_instance_finalize()\n");

/* chain to parent's finalize */

G_OBJECT_CLASS(parent_klass)->finalize(object);

}

static void bar_type_class_init(BarTypeClass *klass) {

g_print("Calling bar_type_class_init()\n");

parent_klass = g_type_class_peek_parent(klass);

G_OBJECT_CLASS(klass)->finalize = bar_type_instance_finalize;

}

static void bar_type_class_finalize(BarTypeClass *klass) {

g_print("Calling bar_type_class_finalize()\n");

}

GType bar_type_get_type() {

return bar_type_type_id;

}

void bar_type_register_type(GTypeModule *type_module) {

const GTypeInfo type_info = {

sizeof(BarTypeClass), /* class_size */

NULL, /* base_init */

NULL, /* base_finalize */

(GClassInitFunc)bar_type_class_init, /* class_init */

(GClassFinalizeFunc)bar_type_class_finalize, /* class_finalize */

NULL, /* class_data */

sizeof(BarType), /* instance_size */

0, /* n_preallocs */

(GInstanceInitFunc)bar_type_instance_init, /* instance_init */

NULL /* value_table */

};

bar_type_type_id = g_type_module_register_type(

type_module, G_TYPE_OBJECT, "BarTypeDynamicClass", &type_info, 0);

}

The implementation structure may be a little different with the stuff when creating a static type. An additional parameter GTypeModule is passed in. It represents the module your dynamic type belongs to. So, when the module is unloaded, all dynamic types in it are unaccessible.

Also note the bar_type_class_finalize() function. We use it to override the finalize() virtual function in GObjectClass. Now you can do un-initialiation in this function. It is like the destructor in a C++ class.

Let’s move on to the module type. This type inherits GTypeModule:

// fakemodule.h
#ifndef FAKE_MODULE_H_
#define FAKE_MODULE_H_

#include <glib-object.h>

/* module object struct */
typedef struct _FakeModule {
    GTypeModule parent;
} FakeModule;

/* module class struct */
typedef struct _FakeModuleClass {
    GTypeModuleClass parent;
} FakeModuleClass;

/* type method */
GType fake_module_get_type();


#endif /* FAKE_MODULE_H_ */

// fakemodule.h

#ifndef FAKE_MODULE_H_

#define FAKE_MODULE_H_

#include <glib-object.h>

/* module object struct */

typedef struct _FakeModule {

GTypeModule parent;

} FakeModule;

/* module class struct */

typedef struct _FakeModuleClass {

GTypeModuleClass parent;

} FakeModuleClass;

/* type method */

GType fake_module_get_type();

#endif /* FAKE_MODULE_H_ */

// fakemodule.c
#include "fakemodule.h"

/*
 * If you implement a real shared library module, you
 * can init module variables, assign virtual function here.
 */
gboolean fake_module_load(GTypeModule *module) {
    g_print("Invoking fake_module_load()\n");
    /* successfully loaded */
    return TRUE;
}

/*
 * If you implement a real shared library module, you
 * can uninit module variables, and make all cleanups here.
 */
void fake_module_unload(GTypeModule *module) {
    /* noop */
    g_print("Invoking fake_module_unload()\n");
}

static void fake_module_class_init(FakeModuleClass *klass, gpointer data) {
    g_print("Calling fake_module_class_init()\n");
    GTypeModuleClass *module_class = G_TYPE_MODULE_CLASS(klass);
    module_class->load = fake_module_load;
    module_class->unload = fake_module_unload;
}

static void fake_module_instance_init(FakeModule *instance, gpointer data) {
    g_print("Calling fake_module_instance_init()\n");
}

GType fake_module_get_type() {
    static GType type_id = 0;
    if (type_id == 0) {
        static const GTypeInfo type_info = {
            sizeof(FakeModuleClass), /* class_size */
            NULL,                   /* base_init */
            NULL,                   /* base_finalize */
            (GClassInitFunc)fake_module_class_init, /* class_init */
            NULL,                   /* class_finalize */
            NULL,                   /* class_data */
            sizeof(FakeModule),     /* instance_size */
            0,                      /* n_preallocs */
            (GInstanceInitFunc)fake_module_instance_init, /* instance_init */
            NULL                    /* value_table */
        };
        type_id = g_type_register_static(
            G_TYPE_TYPE_MODULE, "FakeModuleStaticClass", &type_info, 0);
    }
    return type_id;
}

// fakemodule.c

#include "fakemodule.h"

* If you implement a real shared library module, you

* can init module variables, assign virtual function here.

gboolean fake_module_load(GTypeModule *module) {

g_print("Invoking fake_module_load()\n");

/* successfully loaded */

return TRUE;

}

* If you implement a real shared library module, you

* can uninit module variables, and make all cleanups here.

void fake_module_unload(GTypeModule *module) {

/* noop */

g_print("Invoking fake_module_unload()\n");

}

static void fake_module_class_init(FakeModuleClass *klass, gpointer data) {

g_print("Calling fake_module_class_init()\n");

GTypeModuleClass *module_class = G_TYPE_MODULE_CLASS(klass);

module_class->load = fake_module_load;

module_class->unload = fake_module_unload;

}

static void fake_module_instance_init(FakeModule *instance, gpointer data) {

g_print("Calling fake_module_instance_init()\n");

}

GType fake_module_get_type() {

static GType type_id = 0;

if (type_id == 0) {

static const GTypeInfo type_info = {

sizeof(FakeModuleClass), /* class_size */

NULL, /* base_init */

NULL, /* base_finalize */

(GClassInitFunc)fake_module_class_init, /* class_init */

NULL, /* class_finalize */

NULL, /* class_data */

sizeof(FakeModule), /* instance_size */

0, /* n_preallocs */

(GInstanceInitFunc)fake_module_instance_init, /* instance_init */

NULL /* value_table */

};

type_id = g_type_register_static(

G_TYPE_TYPE_MODULE, "FakeModuleStaticClass", &type_info, 0);

}

return type_id;

}

GTypeModule is an abstract type. We should implements its load() and unload() virtual function.

Our test code:

// main.c
#include "footype.h"
#include "bartype.h"
#include "fakemodule.h"
#include <glib-object.h>

/*
 * Module entry point. If you implement a real shared library module,
 * you can use dlopen()/dlsym() or g_module_open()/g_module_symbol() to
 * load this module dynamically.
 */
void module_init(GTypeModule *type_module) {
    foo_type_register_type(type_module);
    bar_type_register_type(type_module);
}

int main() {
    g_type_init();

    FakeModule *module = (FakeModule *)g_object_new(fake_module_get_type(), NULL);
    module_init((GTypeModule *)module);

    /*
     * Add a reference to foo type class here. Otherwise, the fake module
     * will be unloaded right after the free() of foo type object and cause error.
     */
    FooTypeClass *foo_type_class = (FooTypeClass *)g_type_class_ref(foo_type_get_type());

    FooType *foo_type = (FooType *)g_object_new(foo_type_get_type(), NULL);
    BarType *bar_type = (BarType *)g_object_new(bar_type_get_type(), NULL);

    /* Test for override finalize() */
    g_object_unref(foo_type);
    g_object_unref(bar_type);

    g_type_class_unref(foo_type_class);

    return 0;
}

// main.c

#include "footype.h"

#include "bartype.h"

#include "fakemodule.h"

#include <glib-object.h>

* Module entry point. If you implement a real shared library module,

* you can use dlopen()/dlsym() or g_module_open()/g_module_symbol() to

* load this module dynamically.

void module_init(GTypeModule *type_module) {

foo_type_register_type(type_module);

bar_type_register_type(type_module);

}

int main() {

g_type_init();

FakeModule *module = (FakeModule *)g_object_new(fake_module_get_type(), NULL);

module_init((GTypeModule *)module);

* Add a reference to foo type class here. Otherwise, the fake module

* will be unloaded right after the free() of foo type object and cause error.

FooTypeClass *foo_type_class = (FooTypeClass *)g_type_class_ref(foo_type_get_type());

FooType *foo_type = (FooType *)g_object_new(foo_type_get_type(), NULL);

BarType *bar_type = (BarType *)g_object_new(bar_type_get_type(), NULL);

/* Test for override finalize() */

g_object_unref(foo_type);

g_object_unref(bar_type);

g_type_class_unref(foo_type_class);

return 0;

}

Another dynamic type BarType is defined in addition to FooType to demo the usage. The output maybe:

Calling fake_module_class_init()
Calling fake_module_instance_init()
Invoking fake_module_load()
Calling foo_type_class_init()
Calling foo_type_instance_init()
Calling bar_type_class_init()
Calling bar_type_instance_init()
Calling foo_type_instance_finalize()
Calling bar_type_instance_finalize()
Calling bar_type_class_finalize()
Calling foo_type_class_finalize()
Invoking fake_module_unload()

Calling fake_module_class_init()

Calling fake_module_instance_init()

Invoking fake_module_load()

Calling foo_type_class_init()

Calling foo_type_instance_init()

Calling bar_type_class_init()

Calling bar_type_instance_init()

Calling foo_type_instance_finalize()

Calling bar_type_instance_finalize()

Calling bar_type_class_finalize()

Calling foo_type_class_finalize()

Invoking fake_module_unload()

See the init/finalize process?

At the end of my note, Let me summarize to compare GObject library with C++ implementation:

1. Member Variables:

GObject	C++
in class struct	class meta info
in object struct	class instance member
global variable	class static member

2. Function Callbacks:

GObject	C++
base_init	init class dynamic meta info
base_finalize	finalize dynamic class meta info, only dynamic types use it
class_init	init class static meta info
class_finalize	finalize class static meta info, only dynamic types use it
instance_init	init instace, like C++ constructor
override finalize in GObjectClass	finalize instance, like C++ destructor

All source code is available in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestGObject-{date}.zip/TestGObject7 folder.

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