OOP Using GObject (4) – An Inheritable Class

Now, we will begin to implement some real OO mechanism using GObject library. In this article, we will make our fundamental type Inheritable.

Here’s comes our Base type:

NOTE: PLEASE READ ALL COMMENT CAREFULLY.

// base.h
#ifndef BASE_H_
#define BASE_H_

#include <glib-object.h>

/* Base object struct */
typedef struct _Base {
    GTypeInstance parent;
    /* instance variable, should be hidden */
    gint base_instance_i;
} Base;

/* Base class struct */
typedef struct _BaseClass {
    GTypeClass parent;
    /* instance method, used as a virtual method */
    void (*base_instance_dump)(struct _Base *instance);
} BaseClass;

/* static method of Base class */
void base_class_set_i(gint i);

/* non-virtual public method for Base object */
void base_instance_set_i(Base *instance, gint i);

/* virtual public method for Base object, both version are supported */
void base_instance_dump(Base *instance);

/* type method */
GType base_get_type();


#endif /* BASE_H_ */

// base.h

#ifndef BASE_H_

#define BASE_H_

#include <glib-object.h>

/* Base object struct */

typedef struct _Base {

GTypeInstance parent;

/* instance variable, should be hidden */

gint base_instance_i;

} Base;

/* Base class struct */

typedef struct _BaseClass {

GTypeClass parent;

/* instance method, used as a virtual method */

void (*base_instance_dump)(struct _Base *instance);

} BaseClass;

/* static method of Base class */

void base_class_set_i(gint i);

/* non-virtual public method for Base object */

void base_instance_set_i(Base *instance, gint i);

/* virtual public method for Base object, both version are supported */

void base_instance_dump(Base *instance);

/* type method */

GType base_get_type();

#endif /* BASE_H_ */

// base.c
#include "base.h"

/* static field of Base class */
gint base_class_i;

/* static method of Base class */
void base_class_set_i(gint i) {
    base_class_i = i;
    g_print("Invoking base_class_set_i(): base_class_i=%d\n", base_class_i);
}

void base_instance_set_i(Base *instance, gint i) {
    instance->base_instance_i = i;
    g_print("Invoking base_instance_set_i(): base_instance_i=%d\n", instance->base_instance_i);
}

void base_instance_dump(Base *instance) {
    g_print("Invoking base_instance_dump(): base_instance_i=%d\n", instance->base_instance_i);
}

static void base_class_init(BaseClass *klass, gpointer data) {
    base_class_i = 100;
    g_print("Calling base_class_init(): base_class_i=%d\n", base_class_i);
    klass->base_instance_dump = base_instance_dump;
}

static void base_instance_init(Base *instance, gpointer data) {
    instance->base_instance_i = 200;
    g_print("Calling base_instance_init(): base_instance_i=%d\n", instance->base_instance_i);
}

GType base_get_type() {
    static GType base_type = 0;
    if (base_type == 0) {
        static const GTypeInfo base_type_info = {
            sizeof(BaseClass),  /* class_size */
            NULL,               /* base_init */
            NULL,               /* base_finalize */
            (GClassInitFunc)base_class_init, /* class_init */
            NULL,               /* class_finalize */
            NULL,               /* class_data */
            sizeof(Base),       /* instance_size */
            0,                  /* n_preallocs */
            (GInstanceInitFunc)base_instance_init, /* instance_init */
            NULL                /* value_table */
        };
        GTypeFundamentalInfo foo_type_fundamental_info = {
            G_TYPE_FLAG_CLASSED           /* Indicates a classed type */
            | G_TYPE_FLAG_INSTANTIATABLE  /* Indicates an instantiable type */
            | G_TYPE_FLAG_DERIVABLE       /* Indicates a flat derivable type */
            | G_TYPE_FLAG_DEEP_DERIVABLE  /* Indicates a deep derivable type */
        };
        base_type = g_type_register_fundamental(g_type_fundamental_next(),
            "BaseFundamentalType", &base_type_info, &foo_type_fundamental_info, 0);
    }
    return base_type;
}

// base.c

#include "base.h"

/* static field of Base class */

gint base_class_i;

/* static method of Base class */

void base_class_set_i(gint i) {

base_class_i = i;

g_print("Invoking base_class_set_i(): base_class_i=%d\n", base_class_i);

}

void base_instance_set_i(Base *instance, gint i) {

instance->base_instance_i = i;

g_print("Invoking base_instance_set_i(): base_instance_i=%d\n", instance->base_instance_i);

}

void base_instance_dump(Base *instance) {

g_print("Invoking base_instance_dump(): base_instance_i=%d\n", instance->base_instance_i);

}

static void base_class_init(BaseClass *klass, gpointer data) {

base_class_i = 100;

g_print("Calling base_class_init(): base_class_i=%d\n", base_class_i);

klass->base_instance_dump = base_instance_dump;

}

static void base_instance_init(Base *instance, gpointer data) {

instance->base_instance_i = 200;

g_print("Calling base_instance_init(): base_instance_i=%d\n", instance->base_instance_i);

}

GType base_get_type() {

static GType base_type = 0;

if (base_type == 0) {

static const GTypeInfo base_type_info = {

sizeof(BaseClass), /* class_size */

NULL, /* base_init */

NULL, /* base_finalize */

(GClassInitFunc)base_class_init, /* class_init */

NULL, /* class_finalize */

NULL, /* class_data */

sizeof(Base), /* instance_size */

0, /* n_preallocs */

(GInstanceInitFunc)base_instance_init, /* instance_init */

NULL /* value_table */

};

GTypeFundamentalInfo foo_type_fundamental_info = {

G_TYPE_FLAG_CLASSED /* Indicates a classed type */

| G_TYPE_FLAG_INSTANTIATABLE /* Indicates an instantiable type */

| G_TYPE_FLAG_DERIVABLE /* Indicates a flat derivable type */

| G_TYPE_FLAG_DEEP_DERIVABLE /* Indicates a deep derivable type */

};

base_type = g_type_register_fundamental(g_type_fundamental_next(),

"BaseFundamentalType", &base_type_info, &foo_type_fundamental_info, 0);

}

return base_type;

}

In base_instance_init(), we assigned the base_instance_dump() callback. Thus, we can invoke this function by both global function or instance function of BaseClass class. Additional flags G_TYPE_FLAG_DERIVABLE and G_TYPE_FLAG_DEEP_DERIVABLE are also passed to the GTypeFundamentalInfo struct to enable inheritance.

It’s time to define our Derived type:

// derived.h
#ifndef DERIVED_H_
#define DERIVED_H_

#include "base.h"
#include <glib-object.h>

/* Derived object struct */
typedef struct _Derived {
    /* The GTypeClass structure is still the first member of the class structure */
    Base parent;
    /* should be hidden */
    gint derived_instance_i;
} Derived;

/* Derived class struct */
typedef struct _DerivedClass {
    /* The TypeInstance structure is still the first member of the instance structure */
    BaseClass parent;
} DerivedClass;

/* non-virtual public method for Derived object */
void derived_instance_set_i(Derived *instance, gint i);

/* (Overwrite) virtual public method for Derived object, both version are supported */
void derived_instance_dump(Base *instance);

/* type method */
GType derived_get_type();


#endif /* DERIVED_H_ */

// derived.h

#ifndef DERIVED_H_

#define DERIVED_H_

#include "base.h"

#include <glib-object.h>

/* Derived object struct */

typedef struct _Derived {

/* The GTypeClass structure is still the first member of the class structure */

Base parent;

/* should be hidden */

gint derived_instance_i;

} Derived;

/* Derived class struct */

typedef struct _DerivedClass {

/* The TypeInstance structure is still the first member of the instance structure */

BaseClass parent;

} DerivedClass;

/* non-virtual public method for Derived object */

void derived_instance_set_i(Derived *instance, gint i);

/* (Overwrite) virtual public method for Derived object, both version are supported */

void derived_instance_dump(Base *instance);

/* type method */

GType derived_get_type();

#endif /* DERIVED_H_ */

// derived.c
#include "derived.h"

void derived_instance_set_i(Derived *instance, gint i) {
    instance->derived_instance_i = i;
    g_print("Invoking derived_instance_set_i(): derived_instance_i=%d\n", instance->derived_instance_i);
}

void derived_instance_dump(Base *instance) {
    Derived *derived = G_TYPE_CHECK_INSTANCE_CAST(instance, derived_get_type(), Derived);
    g_print("Invoking derived_instance_dump(): base_instance_i=%d, derived_instance_i=%d\n", instance->base_instance_i, derived->derived_instance_i);
}

static void derived_class_init(DerivedClass *klass, gpointer data) {
    g_print("Calling derived_class_init()\n");
    base_class_set_i(300);
    /* override */
    BaseClass *base_klass = G_TYPE_CHECK_CLASS_CAST(klass, base_get_type(), BaseClass);
    base_klass->base_instance_dump = derived_instance_dump;
}

static void derived_instance_init(Derived *instance, gpointer data) {
    instance->derived_instance_i = 400;
    g_print("Calling derived_instance_init(): derived_instance_i=%d\n", instance->derived_instance_i);
}

GType derived_get_type() {
    static GType derived_type = 0;
    if(derived_type == 0) {
        static const GTypeInfo derived_type_info = {
            sizeof(DerivedClass), /* class_size */
            NULL,               /* base_init */
            NULL,               /* base_finalize */
            (GClassInitFunc)derived_class_init, /* class_init */
            NULL,               /* class_finalize */
            NULL,               /* class_data */
            sizeof(Derived),    /* instance_size */
            0,                  /* n_preallocs */
            (GInstanceInitFunc)derived_instance_init, /* instance_init */
            NULL                /* value_table */
        };
        derived_type = g_type_register_static(
            base_get_type(), "DerivedStaticClass", &derived_type_info, 0);
    }
    return derived_type;
}

// derived.c

#include "derived.h"

void derived_instance_set_i(Derived *instance, gint i) {

instance->derived_instance_i = i;

g_print("Invoking derived_instance_set_i(): derived_instance_i=%d\n", instance->derived_instance_i);

}

void derived_instance_dump(Base *instance) {

Derived *derived = G_TYPE_CHECK_INSTANCE_CAST(instance, derived_get_type(), Derived);

g_print("Invoking derived_instance_dump(): base_instance_i=%d, derived_instance_i=%d\n", instance->base_instance_i, derived->derived_instance_i);

}

static void derived_class_init(DerivedClass *klass, gpointer data) {

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

base_class_set_i(300);

/* override */

BaseClass *base_klass = G_TYPE_CHECK_CLASS_CAST(klass, base_get_type(), BaseClass);

base_klass->base_instance_dump = derived_instance_dump;

}

static void derived_instance_init(Derived *instance, gpointer data) {

instance->derived_instance_i = 400;

g_print("Calling derived_instance_init(): derived_instance_i=%d\n", instance->derived_instance_i);

}

GType derived_get_type() {

static GType derived_type = 0;

if(derived_type == 0) {

static const GTypeInfo derived_type_info = {

sizeof(DerivedClass), /* class_size */

NULL, /* base_init */

NULL, /* base_finalize */

(GClassInitFunc)derived_class_init, /* class_init */

NULL, /* class_finalize */

NULL, /* class_data */

sizeof(Derived), /* instance_size */

0, /* n_preallocs */

(GInstanceInitFunc)derived_instance_init, /* instance_init */

NULL /* value_table */

};

derived_type = g_type_register_static(

base_get_type(), "DerivedStaticClass", &derived_type_info, 0);

}

return derived_type;

}

Our Derived type inherits Base by replacing GTypeClass and GTypeInstance with the corresponding struct of the Base type. According to the memory layout of structs, GTypeClass and GTypeInstance are still the first member of corresponding struct. In derived_get_type(), we register Derived type using g_type_register_static() since it’s not a fundamental at all. And the first parameter is the type id of Base type.

Let’s have some time to look up how to implement polymorphism. In derived_class_init(), we re-assign the base_instance_dump() callback to from the Base‘s implementation to Derived‘s implementation.

Test code:

int main() {
    g_type_init();
    my_dump_type(base_get_type());
    my_dump_type(derived_get_type());

    /*
     * Official document:
     * Use of g_type_create_instance() is reserved for implementators of
     * fundamental types only. E.g. instances of the GObject hierarchy should
     * be created via g_object_new() and never directly through
     * g_type_create_instance() which doesn't handle things like singleton
     * objects or object construction.
     */
    Base *base = (Base *)g_type_create_instance(base_get_type());
    base_class_set_i(101);
    base_instance_set_i(base, 201);
    Derived *derived = (Derived *)g_type_create_instance(derived_get_type());
    derived_instance_set_i(derived, 401);

    /* Test polymorphism */
    Base *instances[2] = { base, (Base *)derived };
    int i;
    for (i = 0; i < 2; i++) {
        Base *inst = instances[i];
        BaseClass *klass = G_TYPE_INSTANCE_GET_CLASS(inst, base_get_type(), BaseClass);
        klass->base_instance_dump(inst);
    }

    return 0;
}

int main() {

g_type_init();

my_dump_type(base_get_type());

my_dump_type(derived_get_type());

* Official document:

* Use of g_type_create_instance() is reserved for implementators of

* fundamental types only. E.g. instances of the GObject hierarchy should

* be created via g_object_new() and never directly through

* g_type_create_instance() which doesn't handle things like singleton

* objects or object construction.

Base *base = (Base *)g_type_create_instance(base_get_type());

base_class_set_i(101);

base_instance_set_i(base, 201);

Derived *derived = (Derived *)g_type_create_instance(derived_get_type());

derived_instance_set_i(derived, 401);

/* Test polymorphism */

Base *instances[2] = { base, (Base *)derived };

int i;

for (i = 0; i < 2; i++) {

Base *inst = instances[i];

BaseClass *klass = G_TYPE_INSTANCE_GET_CLASS(inst, base_get_type(), BaseClass);

klass->base_instance_dump(inst);

}

return 0;

}

All source code is available in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestGObject-{date}.zip/TestGObject4 folder.

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