## Non-instantiable classed types: interfaces

GType's interfaces are very similar to Java's interfaces. They allow to describe a common API that several classes will adhere to. Imagine the play, pause and stop buttons on hi-fi equipment - those can be seen as a playback interface. Once you know what they do, you can control your CD player, MP3 player or anything that uses these symbols. To declare an interface you have to register a non-instantiable classed type which derives from GTypeInterface. The following piece of code declares such an interface.

    #define MAMAN_TYPE_IBAZ                (maman_ibaz_get_type ())
    #define MAMAN_IBAZ(obj)                (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
    #define MAMAN_IS_IBAZ(obj)             (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
    #define MAMAN_IBAZ_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))

    typedef struct _MamanIbaz MamanIbaz; /* dummy object */
    typedef struct _MamanIbazInterface MamanIbazInterface;

    struct _MamanIbazInterface {
      GTypeInterface parent;

      void (*do_action) (MamanIbaz *self);
    };

    GType maman_ibaz_get_type (void);

    void maman_ibaz_do_action (MamanIbaz *self);

The interface function, `maman_ibaz_do_action` is implemented in a pretty simple way:

    void maman_ibaz_do_action (MamanIbaz *self)
    {
      MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
    }

`maman_ibaz_get_type` registers a type named MamanIBaz which inherits from G_TYPE_INTERFACE. All interfaces must be children of G_TYPE_INTERFACE in the inheritance tree.

An interface is defined by only one structure which must contain as first member a GTypeInterface structure. The interface structure is expected to contain the function pointers of the interface methods. It is good style to define helper functions for each of the interface methods which simply call the interface' method directly: `maman_ibaz_do_action` is one of these.

Once an interface type is registered, you must register implementations for these interfaces. The function named `maman_baz_get_type` registers a new GType named MamanBaz which inherits from GObject and which implements the interface MamanIBaz.

    static void maman_baz_do_action (MamanIbaz *self)
    {
      g_print ("Baz implementation of IBaz interface Action.\n");
    }

    static void
    baz_interface_init (gpointer         g_iface,
                        gpointer         iface_data)
    {
      MamanIbazInterface *iface = (MamanIbazInterface *)g_iface;
      iface->do_action = maman_baz_do_action;
    }

    GType
    maman_baz_get_type (void)
    {
      static GType type = 0;
      if (type == 0) {
        const GTypeInfo info = {
          sizeof (MamanBazInterface),
          NULL,   /* base_init */
          NULL,   /* base_finalize */
          NULL,   /* class_init */
          NULL,   /* class_finalize */
          NULL,   /* class_data */
          sizeof (MamanBaz),
          0,      /* n_preallocs */
          NULL    /* instance_init */
        };
        const GInterfaceInfo ibaz_info = {
          (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
          NULL,               /* interface_finalize */
          NULL          /* interface_data */
        };
        type = g_type_register_static (G_TYPE_OBJECT,
                                       "MamanBazType",
                                       &info, 0);
        g_type_add_interface_static (type,
                                     MAMAN_TYPE_IBAZ,
                                     &ibaz_info);
      }
      return type;
    }

`g_type_add_interface_static` records in the type system that a given type implements also FooInterface (`foo_interface_get_type` returns the type of FooInterface). The GInterfaceInfo structure holds information about the implementation of the interface:

    struct _GInterfaceInfo
    {
      GInterfaceInitFunc     interface_init;
      GInterfaceFinalizeFunc interface_finalize;
      gpointer               interface_data;
    };

When having no special requirements you also can use the `G_DEFINE_INTERFACE` macro:

    G_DEFINE_INTERFACE (MamanBaz, maman_baz, G_TYPE_OBJECT)

### Interface Initialization

When an instantiable classed type which registered an interface implementation is created for the first time, its class structure is initialized following the process described in the section called “Instantiable classed types: objects”. After that, the interface implementations associated with the type are initialized.

First a memory buffer is allocated to hold the interface structure. The parent's interface structure is then copied over to the new interface structure (the parent interface is already initialized at that point). If there is no parent interface, the interface structure is initialized with zeros. The g_type and the g_instance_type fields are then initialized: g_type is set to the type of the most-derived interface and g_instance_type is set to the type of the most derived type which implements this interface.

Finally, the interface' most-derived `base_init` function and then the implementation's `interface_init` function are invoked. It is important to understand that if there are multiple implementations of an interface the `base_init` and `interface_init` functions will be invoked once for each implementation initialized.

It is thus common for base_init functions to hold a local static boolean variable which makes sure that the interface type is initialized only once even if there are multiple implementations of the interface:

    static void
    maman_ibaz_base_init (gpointer g_iface)
    {
      static gboolean initialized = FALSE;

      if (!initialized) {
        /* create interface signals here. */
        initialized = TRUE;
      }
    }

If you have found the stuff about interface hairy, you are right: it is hairy but there is not much I can do about it. What I can do is summarize what you need to know about interfaces:

The above process can be summarized as follows:

Table 2. Interface Initialization

Invocation time | Function Invoked | Function's parameters | Remark
--- | --- | --- | ---
First call to `g_type_create_instance` for type implementing interface | interface' base_init function | On interface' vtable | Register interface' signals here (use a local static boolean variable as described above to make sure not to register them twice.).
interface' interface_init function | On interface' vtable | Initialize interface' implementation. That is, initialize the interface method pointers in the interface structure to the function's implementation.

It is highly unlikely (i.e. I do not know of anyone who actually used it) you will ever need other more fancy things such as the ones described in the following section (the section called “Interface Destruction”).

### Interface Destruction

When the last instance of an instantiable type which registered an interface implementation is destroyed, the interface's implementations associated to the type are destroyed.

To destroy an interface implementation, GType first calls the implementation's `interface_finalize` function and then the interface's most-derived `base_finalize` function.

Again, it is important to understand, as in the section called “Interface Initialization”, that both `interface_finalize` and `base_finalize` are invoked exactly once for the destruction of each implementation of an interface. Thus, if you were to use one of these functions, you would need to use a static integer variable which would hold the number of instances of implementations of an interface such that the interface's class is destroyed only once (when the integer variable reaches zero).

The above process can be summarized as follows:

Table 3. Interface Finalization

Invocation time | Function Invoked | Function's parameters
--- | --- | ---
Last call to `g_type_free_instance` for type implementing interface | interface' interface_finalize function | On interface' vtable
interface' base_finalize function | On interface' vtable