Once the interface is defined, implementing it is rather trivial. Source code showing how to do this for the IBaz interface defined in the previous section is located in sample/interface/maman-baz.{h|c}.

The first step is to define a normal GType. Here, we have decided to use a GType which derives from GObject. Its name is MamanBaz:

#ifndef MAMAN_BAZ_H
#define MAMAN_BAZ_H

#include <glib-object.h>

#define MAMAN_TYPE_BAZ             (maman_baz_get_type ())
#define MAMAN_BAZ(obj)             (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
#define MAMAN_BAZ_CLASS(vtable)    (G_TYPE_CHECK_CLASS_CAST ((vtable), MAMAN_TYPE_BAZ, MamanbazClass))
#define MAMAN_BAZ_GET_CLASS(inst)  (G_TYPE_INSTANCE_GET_CLASS ((inst), MAMAN_TYPE_BAZ, MamanbazClass))

typedef struct _MamanBaz MamanBaz;
typedef struct _MamanBazClass MamanBazClass;

struct _MamanBaz {
  GObject parent;
  int instance_member;

struct _MamanBazClass {
  GObjectClass parent;

GType maman_baz_get_type (void);

#endif //MAMAN_BAZ_H

There is clearly nothing specifically weird or scary about this header: it does not define any weird API or derives from a weird type.

The second step is to implement maman_baz_get_type:

maman_baz_get_type (void)
  static GType type = 0;
  if (type == 0) {
    static 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 */
      baz_instance_init    /* instance_init */
    static const GInterfaceInfo ibaz_info = {
      (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
      NULL,               /* interface_finalize */
      NULL                /* interface_data */
    type = g_type_register_static (G_TYPE_OBJECT,
                                   &info, 0);
    g_type_add_interface_static (type,
  return type;

This function is very much like all the similar functions we looked at previously. The only interface-specific code present here is the call to g_type_add_interface_static which is used to inform the type system that this just-registered GType also implements the interface MAMAN_TYPE_IBAZ.

baz_interface_init, the interface initialization function, is also pretty simple:

static void baz_do_action (MamanBaz *self)
  g_print ("Baz implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
static void
baz_interface_init (gpointer   g_iface,
                    gpointer   iface_data)
  MamanIbazInteface *iface = (MamanIbazInteface *)g_iface;
  iface->do_action = (void (*) (MamanIbaz *self))baz_do_action;
static void
baz_instance_init (GTypeInstance   *instance,
                   gpointer         g_class)
  MamanBaz *self = MAMAN_BAZ(instance);
  self->instance_member = 0xdeadbeaf;

baz_interface_init merely initializes the interface methods to the implementations defined by MamanBaz: maman_baz_do_action does nothing very useful but it could :)