Once the interface is defined, implementing it is rather trivial.

The first step is to define a normal GObject class, like:

#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_IS_BAZ(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
#define MAMAN_BAZ_CLASS(klass)     (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAZ, MamanbazClass))
#define MAMAN_IS_BAZ_CLASS(klass)  (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAZ))
#define MAMAN_BAZ_GET_CLASS(obj)   (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAZ, MamanbazClass))


typedef struct _MamanBaz        MamanBaz;
typedef struct _MamanBazClass   MamanBazClass;

struct _MamanBaz
{
  GObject parent_instance;

  int instance_member;
};

struct _MamanBazClass
{
  GObjectClass parent_class;
};

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 MamanBaz by defining its GType. Instead of using G_DEFINE_TYPE we use G_DEFINE_TYPE_WITH_CODE and the G_IMPLEMENT_INTERFACE macros.

static void maman_ibaz_interface_init (MamanIbazInterface *iface);

G_DEFINE_TYPE_WITH_CODE (MamanBar, maman_bar, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
                                                maman_ibaz_interface_init));

This definition is very much like all the similar functions we looked at previously. The only interface-specific code present here is the call to G_IMPLEMENT_INTERFACE.

maman_baz_interface_init, the interface initialization function: inside it every virtual method of the interface must be assigned to its implementation:

static void
maman_baz_do_action (MamanBaz *self)
{
  g_print ("Baz implementation of IBaz interface Action: 0x%x.\n",
           self->instance_member);
}

static void
maman_ibaz_interface_init (MamanIbazInterface *iface)
{
  iface->do_action = baz_do_action;
}

static void
maman_baz_init (MamanBaz *self)
{
  MamanBaz *self = MAMAN_BAZ (instance);
  self->instance_member = 0xdeadbeaf;
}