How to define and implement interfaces

Defining interfaces
Implementing interfaces
Interface definition prerequisites
Interface properties
Overriding interface methods

Defining interfaces

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. This interface defines two methods:

#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);
  void (*do_something) (MamanIbaz *self);
};

GType maman_ibaz_get_type (void);

void maman_ibaz_do_action    (MamanIbaz *self);
void maman_ibaz_do_something (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 _GET_CLASS macro is called _GET_INTERFACE and not implemented with G_TYPE_INSTANCE_GET_CLASS but with G_TYPE_INSTANCE_GET_INTERFACE.
The instance type, MamanIbaz is not fully defined: it is used merely as an abstract type which represents an instance of whatever object which implements the interface.
The parent of the MamanIbazInterface is not GObjectClass but GTypeInterface.

The implementation of the MamanIbaz type itself is trivial:

G_DEFINE_INTERFACE creates a maman_ibaz_get_type function which registers the type in the type system. The third argument is used to define a prerequisite interface (which we'll talk about more later). Just pass 0 for this argument when an interface has no prerequisite.
maman_ibaz_default_init is expected to register the interface's signals if there are any (we will see a bit later how to use them).
The interface methods maman_ibaz_do_action and maman_ibaz_do_something dereference the interface structure to access its associated interface function and call it.

G_DEFINE_INTERFACE (MamanIbaz, maman_ibaz, 0);

static void
maman_ibaz_default_init (gpointer g_class)
{
    /* add properties and signals to the interface here */
}

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

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

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

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