GObject 02: A classed type

Now I am going to create a classed type.

Q: What is a class?
A: I don't know for sure.  In object-oriented programming, a class represents a "kind" of data.  It specifies what data there is (the members/fields) in each object (instance) and what can be done (the methods) to the object.

Q: What is a "classed" type in GObject?
A: It is a type that has a "class struct".

Each "classed type" has a unique instance of "class structure".  Note that IN GOBJECT, THERE IS A DIFFERENCE BETWEEN A "Class" AND AN "Instance".  A "Class" IS SOMETHING THAT IS SHARED BY ALL OBJECTS OF A CLASS AND AN "Instance" IS SOMETHING THAT IS SPECIFIC TO EACH OBJECT OF THE CLASS.  You can imagine that whatever in a "Class" are like the static members/fields in C++/Java and whatever in an "Instance" are like non-static members/fields.

The code is here:

/* A fundamental type.  It has a class structure, but is not instantiable. */

#include <stdio.h>
#include <glib-object.h>

typedef struct {
    GTypeClass something_as_boilerplate;
    int foo;
    char* bar;
    void (*baz)();
} myclass_t;

void baz_function() { printf("All your type are belong to us!\n"); }

void my_class_init_func(myclass_t* klass, gpointer data) {
    printf("my_class_init_func called!\n");
    klass->foo = 42;
    klass->bar = "Hello world!";
    klass->baz = baz_function;
}

int main() {
    g_type_init();

    GTypeInfo my_type_info = {
        sizeof(myclass_t),  //guint16                class_size;

        NULL,               //GBaseInitFunc          base_init;
        NULL,               //GBaseFinalizeFunc      base_finalize;

        /* classed types, instantiated types */
        (GClassInitFunc)my_class_init_func, //GClassInitFunc         class_init;
        NULL,               //GClassFinalizeFunc     class_finalize;
        NULL,               //gconstpointer          class_data;

        /* instantiated types */
        0,                  //guint16                instance_size;
        0,                  //guint16                n_preallocs;
        NULL,               //GInstanceInitFunc      instance_init;

        /* value handling */
        NULL,               //const GTypeValueTable *value_table;
    };

    GTypeFundamentalInfo my_fundamental_info = {
        G_TYPE_FLAG_CLASSED
    };

    GType my_type_id = g_type_register_fundamental(
            g_type_fundamental_next(),
            "MyClassedFundamentalType",
            &my_type_info,
            &my_fundamental_info,
            0
            );

    printf("%d\n",my_type_id);
    printf("%s\n",g_type_name(my_type_id));

    myclass_t *klass = (myclass_t*)g_type_class_ref(my_type_id);
    
    printf("foo: %d\n", klass->foo);
    printf("bar: %s\n", klass->bar);
    klass->baz();

    g_type_class_unref(klass);

    return 0;
}

Let's look at the my_type_info struct in the main function.  I filled the class_size field to be sizeof(myclass_t) so that the type-system can help me "malloc" my myclass_t struct.  I also filled the class_init function so that it will call my my_class_init_func to initialize it.

But I didn't fill the instance_size or instance_init so it is not instantiatable.  That is, this type has no instances.

Q: How can it be it useful to have such a type that has no instances?
A: Well... I admit it is not that useful.  It may work as something like a "singleton" (a globally unique object).  Interfaces (like they are in Java/C#) in GObject are also non-instantiatable types.

Then the my_fundamental_info struct following the my_type_info variable in the main function.  I marked this class as "classed".


Let's look at my myclass_t structure. 

Q: Why don't you follow the GObject naming convention and name it as MyClass?
A: It is intentional.  I am emphasizing that it is not the naming-convention, but the function calls (like g_type_register_fundamental) and its parameters (like GTypeInfo, GTypeFundamentalInfo and so on) that make GObject work.  Yeah, you should follow the convention in real-world programming.

typedef struct {
    GTypeClass something_as_boilerplate;
    int foo;  // Not a good idea to store data here.
    char* bar;
    void (*baz)();
} myclass_t;

The first member is a GTypeClass field.  It is required that in GObject type system, all class-structs should begin with a GTypeClass field.

Q: What's inside the GTypeClass?
A: The documentation does not tell me.  The source code is:

typedef struct _GTypeClass              GTypeClass;
struct _GTypeClass
{
  /*< private >*/
  GType g_type;
};

So every class struct starts with a GType field (GType is typedef'ed to be gsize which is then typedef'ed to be unsigned int).  This can be queryed at runtime to find the type-id (GType) of a type.

Then there are an int field "foo", a char pointer "bar" and a function pointer "baz" in myclass_t struct.  foo and bar are static members of my class.  baz, which you may think to be a static member, can also be a virtual method of a class (this will be discussed later).

Let's move on to the my_class_init_func which I filled into the class_init field of the "GTypeInfo my_type_info" struct.

void my_class_init_func(myclass_t* klass, gpointer data) {
    printf("my_class_init_func called!\n");
    klass->foo = 42;
    klass->bar = "Hello world!";
    klass->baz = baz_function;
}

It takes two parameters.  One of them is a myclass_t pointer pointing to the struct to be initialized.  I name it "klass" because "class" is a keyword in C++ and would not work with a C++ compiler.  The other is the class_data field provided in the GTypeInfo struct (where I set it to NULL).

I initialized the myclass_t struct here.  I filled the foo, bar and baz field.


Now the main function.  Here I register my type.  And then:

    myclass_t *klass = (myclass_t*)g_type_class_ref(my_type_id);
    
    printf("foo: %d\n", klass->foo);
    printf("bar: %s\n", klass->bar);
    klass->baz();

    g_type_class_unref(klass);

I call the g_type_class_ref function so that my class struct is initialized.  Then the members of klass are filled.  Finally I call the g_type_class_unref(klass) to unref this class (this may be not necessary here).


Note that it is not a good idea to store static data member into the class struct.  It is difficult to get a pointer to the class struct without having an instance of this class.  You may wish to access the class member in the main function or in a static function.  It is better to use global variables and use naming conventions to show that they belong to a class.