Re: [Vala] [Genie] Object members

[Abderrahim Kitouni <a kitouni gmail com>]

Hi,

2009/10/28 Patrick Castle <lemeia bigpond com>:
> Just stuffing about at the moment getting my head about creating classes in
> Genie. From what I gather, every variable you declare outside of a method is a
> member/property of that class.
>
> What I'm finding a bit confusing is there isn't just one way to declare a
> property it seems - but both ways don't work consistently.
>
> For example, I have class "Test" and I want it to have a private property "a"
> not accessible to external callers. It seems like these are the options:
>
> class Test : Object
>    _a : int
>
> or
>
> class Test : Object
>    prop private a : int
>
> or
>
> class Test : Object
>    prop _a : int
the first one actually defines a field, and the two others are properties
> but not
>
> class Test : Object
>    private a : int
>
> My first question - is there any reason why that last one shouldn't compile?
I don't think so (actually I don't know the syntax for private fields,
but this seems fine)
> Also - why actually have so many ways to declare the same thing? Is it
> deliberate or just an accident of the syntax checking?
the field/property thing aside, there is only that a member starting
with an underscrore is implicitly private.
> Also - does Genie have the concept of a class variable, or are they all just
> instance variables? My initial temptation was to use static in the declaration
>
> class Test : Object
>    prop static a : int
>
> This actually starts spitting out all sorts of errors when compiling the C
> source, so perhaps it gets further than it should. Is it an illegal declaration?
You can try removing prop (thus creating a field), but I don't think
this should create errors.
> And if so, should it be caught before it gets as far as the C source being
> created and compiled?
Sure. The rule of thumb here is that a C compiler error is a bug
either in valac or in the vapi (in case you're using a library).
> On the Genie homepage it says:
> All methods, properties and events can also take modifiers to define further
> options.
>
> I assumed that would mean that all modifiers were legal. So I thought I should
> try the other modifiers.
>
> abstract:
>
> class Test : Object
>    prop abstract a : int
>
> This one compiles OK, but if you create an instance of the class and assign a
> value to the property you get a Segmentation Fault when you run it.
this souldn't compile afaik (class Test should be abstract for this to work).
> I looked at the definition of abstract and it seems to say that it should not be
> defined in the class in which it's declared but only in the subclass. So I
> created a subclass and defined it there instead and I still get a clean compile
> but a Segmentation Fault when running.
I think it sould be : (I'm not sure about genie syntax for abstract classes)

class abstract Test : Object
    prop abstract a : int

class Test2 : Test
    _a : int
    prop override a: int
        get
            return _a
        set
            _a = value

> I'm guessing that this is only meant for methods rather than properties - but if
> so, it would probably be a good idea to make a distinction on the web page
> between method modifiers and property modifiers. Seems like the remaining
> modifiers look relevant only to methods as well.
abstract, virtual and override should work for properties as well, the
only thing I see as method only is async. (and readonly is property
only)
> Also - I think rather than getting a Segmentation Fault, improper use of these
> modifiers should probably throw exceptions during the compile.
absolutely.
> So to summarise, declaration of properties implies public access unless keyword
> private is used (or underscore). Your options are: _var, prop private var, prop
> _var.
your options are prop _var or prop private var, but generally you
wouldn't use a private property (unless you need some processing) but
rather private field. (or a readonly property if you don't want the
property to be modified)
> There is no such thing as a class variable in Genie.
there should be both class variables and static variables (I'm not
sure about the difference, but I think that class variables are
specific to a class, so althought they are inherited, they don't
necessarily have the same value in a subclass)

HTH,
Abderrahim