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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. 
    
Classes can implement multiple interfaces by using multiple
    calls to G_IMPLEMENT_INTERFACE inside the call
    to G_DEFINE_TYPE_WITH_CODE.
      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;
}