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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.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | #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:
| 1 2 3 4 | 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.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 | 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 (MamanBazClass), 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:
| 1 2 3 4 5 6 | struct _GInterfaceInfo { GInterfaceInitFunc interface_init; GInterfaceFinalizeFunc interface_finalize; gpointer interface_data; }; | 
If you have no special requirements you can use the G_IMPLEMENT_INTERFACE macro to implement an interface:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | static void maman_baz_do_action (MamanIbaz *self) { g_print ("Baz implementation of Ibaz interface Action.\n"); } static void maman_ibaz_interface_init (MamanIbazInterface *iface) { iface->do_action = maman_baz_do_action; } G_DEFINE_TYPE_WITH_CODE (MamanBaz, maman_baz, G_TYPE_OBJECT, G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ, maman_ibaz_interface_init)); | 
When an instantiable classed type which implements an interface (either directly or by inheriting an implementation from a superclass) 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.
            The interface's base_init function is called,
            and then the interface's default_init is invoked.
            Finally if the type has registered an implementation of the interface,
            the implementation's interface_init
            function is invoked. 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 recommended to use a default_init function to
            initialize an interface. This function is called only once for the interface no
            matter how many implementations there are. The
            default_init function is declared by
            G_DEFINE_INTERFACE
            which can be used to define the interface:
| 1 2 3 4 5 6 7 | G_DEFINE_INTERFACE (MamanIbaz, maman_ibaz, G_TYPE_OBJECT); static void maman_ibaz_default_init (MamanIbazInterface *iface) { /* add properties and signals here, will only called once */ } | 
Or you can do that yourself in a GType function for your interface:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 | GType maman_ibaz_get_type (void) { static volatile gsize type_id = 0; if (g_once_init_enter (&type_id)) { const GTypeInfo info = { sizeof (MamanIbazInterface), NULL, /* base_init */ NULL, /* base_finalize */ maman_ibaz_default_init, /* class_init */ NULL, /* class_finalize */ NULL, /* class_data */ 0, /* instance_size */ 0, /* n_preallocs */ NULL /* instance_init */ }; GType type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz", &info, 0); g_once_init_leave (&type_id, type); } return type_id; } static void maman_ibaz_default_init (MamanIbazInterface *iface) { /* add properties and signals here, will only called once */ } | 
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:
Table 2. Interface Initialization
| Invocation time | Function Invoked | Function's parameters | Remark | 
|---|---|---|---|
| First call to g_type_create_instancefor any type implementing interface | interface's base_initfunction | On interface's vtable | Rarely necessary to use this. Called once per instantiated classed type implementing the interface. | 
| First call to g_type_create_instancefor each type implementing interface | interface's default_initfunction | On interface's vtable | Register interface's signals, properties, etc. here. Will be called once. | 
| First call to g_type_create_instancefor any type implementing interface | implementation's interface_initfunction | On interface's vtable | Initialize interface implementation. Called for each class that that implements the interface. Initialize the interface method pointers in the interface structure to the implementing class'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”).
        
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_instancefor type
                    implementing interface | interface' interface_finalize function | On interface' vtable | 
| interface' base_finalize function | On interface' vtable |