Re: glib interfaces

[James Henstridge <james daa com au>]

Miroslaw Dobrzanski-Neumann wrote:

> Hi all
> I have just two questions
According to Tim, interfaces in glib are closer to gcc signatures (see 
the info pages) than java style interfaces.  

> 1. Why it is not possible to derive from interfaces? My intention was to
>   extend an interface by deriving a new one from the other.
Deriving interfaces doesn't really fit into glib's interface model.  You 
can instead define a new interface that provides the aditional 
functionality.  You can add the "parent" interface as a prerequisite to 
the new interface in order to ensure that types that implement your new 
interface also implement the parent one.

> 2. What is purpose of g_type_interface_add_prerequisite()? How it behaves?
>   What constraints does it place on given interface?
add_prerequisite() adds a prerequisite for implementing a type.  It says 
that if a type wishes to implement an interface I, then it must be a T 
(for some type T).

For instance, you might require that all types implementing your 
interface be subclasses of GObject (it is possible for instantiable 
types other than GObjects to implement interfaces, although you will 
probably never make use of this).  Or as above, to simulate derivation 
of interfaces, you might add a prerequisite that all types implementing 
the "derived" interface must also implement the "parent" interface.

Adding a prerequisite on GObject is quite useful for an interface, as it 
lets you use that type id for arguments to a signal (since 
implementations of an interface need not be GObjects, the signal code 
wouldn't know how to manage the argument otherwise).

Hope this helps.

James.