## How to define and implement interfaces

## How to define 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:

    #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 `_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:

- `maman_ibaz_get_type` registers the type in the type system.

- `maman_ibaz_base_init` is expected to register the interface's signals if there are any (we will see a bit (later how to use them). Make sure to use a static local boolean variable to make sure not to run the initialization code twice (as described in the section called “Interface Initialization”, `base_init` is run once for each interface implementation instantiation)

- `maman_ibaz_do_action` dereferences the class structure to access its associated class function and calls it.

    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);
    }