The bulk of interface definition has already been shown in the section called “Non-instantiable classed types: interfaces” but I feel it is needed to show exactly how to create an interface.

As above, the first step is to get the header right:

#ifndef __MAMAN_IBAZ_H__
#define __MAMAN_IBAZ_H__

#include <glib-object.h>

#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_iface;

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

GType maman_ibaz_get_type (void);

void maman_ibaz_do_action (MamanIbaz *self);

#endif /* __MAMAN_IBAZ_H__ */

This code is the same as the code for a normal GType which derives from a GObject except for a few details:

The implementation of the MamanIbaz type itself is trivial:

static void
maman_ibaz_base_init (gpointer g_class)
{
  static gboolean is_initialized = FALSE;

  if (!is_initialized)
    {
      /* add properties and signals to the interface here */

      is_initialized = TRUE;
    }
}

GType
maman_ibaz_get_type (void)
{
  static GType iface_type = 0;
  if (iface_type == 0)
    {
      const GTypeInfo info = {
        sizeof (MamanIbazInterface),
        maman_ibaz_base_init,   /* base_init */
        NULL,   /* base_finalize */
      };

      iface_type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz",
                                           &info, 0);
    }

  return iface_type;
}

void
maman_ibaz_do_action (MamanIbaz *self)
{
  g_return_if_fail (MAMAN_IS_IBAZ (self));

  MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
}