OOP Using GObject (3) – An Instantiatable Class

We will make fundamental type instantiatable and complete our first usage sample in this article. An object class should be defined firstly:

NOTE: PLEASE READ ALL COMMENT CAREFULLY.

/* Foo object struct */
typedef struct _Foo {
    /*
     * Official document:
     * All instance structures must contain as first member a TypeInstance structure.
     */
    GTypeInstance parent;
    /* instance variable */
    int foo_instance_i;
} Foo;

/* Foo object struct */

typedef struct _Foo {

* Official document:

* All instance structures must contain as first member a TypeInstance structure.

GTypeInstance parent;

/* instance variable */

int foo_instance_i;

} Foo;

Also, we re-define the class struct:

/* Foo class struct */
typedef struct _FooClass {
    /*
     * Official document:
     * All class structures must contain as first member a GTypeClass structure.
     */
    GTypeClass parent;
} FooClass;

/* static field of Foo class */
int foo_class_i;

/* static method of Foo class */
void foo_class_set_i(int i) {
    foo_class_i = i;
    printf("Invoking foo_class_set_i(): foo_class_i=%d\n", foo_class_i);
}

/* Foo class struct */

typedef struct _FooClass {

* Official document:

* All class structures must contain as first member a GTypeClass structure.

GTypeClass parent;

} FooClass;

/* static field of Foo class */

int foo_class_i;

/* static method of Foo class */

void foo_class_set_i(int i) {

foo_class_i = i;

printf("Invoking foo_class_set_i(): foo_class_i=%d\n", foo_class_i);

}

GTypeClass should be the first member of a class struct, while TypeInstance the first member of a object struct. You may wonder why there’s two int variable in both struct. The foo_class_i is like a static variable in C++ class, while The foo_instance_i is like an instance variable in C++ class. And remember fields in a class struct? It is used as meta info.

The registry function also need modification:

GType foo_get_type() {
    static GType foo_type = 0;
    if (foo_type == 0) {
        static const GTypeInfo foo_type_info = {
            sizeof(FooClass),   /* class_size */
            NULL,               /* base_init */
            NULL,               /* base_finalize */
            (GClassInitFunc)foo_class_init, /* class_init */
            NULL,               /* class_finalize */
            NULL,               /* class_data */
            sizeof(Foo),        /* instance_size */
            0,                  /* n_preallocs */
            (GInstanceInitFunc)foo_instance_init, /* instance_init */
            NULL                /* value_table */
        };
        /* G_TYPE_FLAG_INSTANTIATABLE: Indicates an instantiable type (implies classed) */
        GTypeFundamentalInfo foo_type_fundamental_info = {
            G_TYPE_FLAG_CLASSED | G_TYPE_FLAG_INSTANTIATABLE
        };
        foo_type = g_type_register_fundamental(g_type_fundamental_next(),
            "FooClassedFundamentalType", &foo_type_info, &foo_type_fundamental_info, 0);
    }
    return foo_type;
}

GType foo_get_type() {

static GType foo_type = 0;

if (foo_type == 0) {

static const GTypeInfo foo_type_info = {

sizeof(FooClass), /* class_size */

NULL, /* base_init */

NULL, /* base_finalize */

(GClassInitFunc)foo_class_init, /* class_init */

NULL, /* class_finalize */

NULL, /* class_data */

sizeof(Foo), /* instance_size */

0, /* n_preallocs */

(GInstanceInitFunc)foo_instance_init, /* instance_init */

NULL /* value_table */

};

/* G_TYPE_FLAG_INSTANTIATABLE: Indicates an instantiable type (implies classed) */

GTypeFundamentalInfo foo_type_fundamental_info = {

G_TYPE_FLAG_CLASSED | G_TYPE_FLAG_INSTANTIATABLE

};

foo_type = g_type_register_fundamental(g_type_fundamental_next(),

"FooClassedFundamentalType", &foo_type_info, &foo_type_fundamental_info, 0);

}

return foo_type;

}

We assigned the instance_init() callback. It is called when a instance of our Foo class is created. You may ask where is the corresponding instance_finalize() callback? Hey, we will discuss it in upcoming articles. The instance_init() callback can be regarded as the constructor of a C++ class. Note, an additional G_TYPE_FLAG_INSTANTIATABLE flag is also added in the GTypeFundamentalInfo struct.

Let’s see how to create an instance:

int main() {
    g_type_init();
    my_dump_type(foo_get_type());

    /* Use g_type_create_instance if implement a fundamental class */
    Foo *foo = (Foo *)g_type_create_instance(foo_get_type());
    foo_class_set_i(101);
    foo_instance_set_i(foo, 201);

    printf("Is instance of int? %s\n",
        G_TYPE_CHECK_INSTANCE_TYPE(foo, G_TYPE_INT) ? "yes" : "no");
    printf("Is instance of FooClassedFundamentalType? %s\n",
        G_TYPE_CHECK_INSTANCE_TYPE(foo, foo_get_type()) ? "yes" : "no");

    return 0;
}

int main() {

g_type_init();

my_dump_type(foo_get_type());

/* Use g_type_create_instance if implement a fundamental class */

Foo *foo = (Foo *)g_type_create_instance(foo_get_type());

foo_class_set_i(101);

foo_instance_set_i(foo, 201);

printf("Is instance of int? %s\n",

G_TYPE_CHECK_INSTANCE_TYPE(foo, G_TYPE_INT) ? "yes" : "no");

printf("Is instance of FooClassedFundamentalType? %s\n",

G_TYPE_CHECK_INSTANCE_TYPE(foo, foo_get_type()) ? "yes" : "no");

return 0;

}

Congratulations! You’ve finished the learning of our fundamental sample.

All source code is available in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestGObject-{date}.zip/TestGObject3 folder.

Leave a Reply Cancel reply