Re: Custom container + Child type with interface

[David Nečas <yeti physics muni cz>]

On Sun, Dec 20, 2009 at 04:43:57PM +0000, Matthew Bucknall wrote:
> Next question - The interface I have defined basically just mandates a
> few properties. The container that requires children to implement this
> interface only ever reads these properties.
> 
> Some implementers of the interface will determine the value of the
> interface properties based on various bits of internal state
> information. Other implementers will be passed the property values via
> their constructors.
> 
> So as far as the container and some implementers are concerned, the
> interface properties only need to be G_PARAM_READABLE. In other cases,
> the properties might need to be (G_PARAM_READWRITE |
> G_PARAM_CONSTRUCT_ONLY).
> 
> I assume that the interface definition has to go with the lowest common
> denominator, which I think is G_PARAM_READWRITE. In that case, what
> should implementers do where the properties are not settable (because
> they are derived from other state information)?

I've never had to override properties (fortunately), so just some OO
theory...

You do not go with `the least common denominator'.  The is-a relation
between subclases and classes or implementations of interfaces means that
you can always say MyFoo is a GpanePanel if it implements the interface.
In other words, you must be always able to substitute the derived object
for the parent.

You can't do this if the parent has writable properties but the child
has not.

So to mandate the is-a relation in case of your properties, the parent
must have the most *restrictive* flags (typically G_PARAM_READABLE).
Subclasses can then permit more operations.

Regards,

Yeti