Re: [Vala] Confused with constructors

[Sam Danielson <samdanielson gmail com>]

Thanks for the reply. I found the thread you started in Nov. 2008 to
be of some help too.

http://mail.gnome.org/archives/vala-list/2008-November/msg00113.html


On Wed, Feb 25, 2009 at 2:30 PM, Feng Yu <rainwoodman gmail com> wrote:
> On Sun, Feb 22, 2009 at 2:02 PM, Sam Danielson <samdanielson gmail com> wrote:
> > The following code causes c compilation to fail.
> >
> > public class Baz : Object {
> >
> >        public string baz_string { get; construct; }
> >
> >        public Baz (string arg) {
> >                if (arg.length > 2) this.baz_string = arg;
> >                else this.baz_string = "done";
> >        }
> > }
> >
> > COMPILE:
> > construct.c: In function 'baz_construct':
> > construct.c:34: error: '__params_it' undeclared (first use in this function)
> >
> >
> > It appears that the constructoresque Baz () may be limited to setting
> > properties only in the form (this.baz_string = arg). Next question...
> >
> > public class Bar : Object {
> >        public string bar_string { get; construct; default = "not set";}
> >        public int bar_int = 42;
> >
> >        public Bar () {
> >                message ("In Bar ()");
> >                bar_int = 12;
> >                bar_string = "hello";
> >        }
> >        construct {
> >                message ("In Bar's constructor with bar_string = %s and bar_int =
> > %d.", this.bar_string, this.bar_int);
> >        }
> > }
> > public void main () {
> >        var obj = new Bar ();
> > }
> >
> > OUTPUT:
> > ** Message: construct.vala:32: In Bar's constructor with bar_string =
> > hello and bar_int = 42.
> > ** Message: construct.vala:27: In Bar ()
> >
> >
> > Additional lines in Bar () are executed AFTER the property is set to
> > "hello" and after the construct {} block is called, even though
> > this.bar_string = "hello" is the last line in Bar (). How does vala
> > choose which code in bar to execute first? Should vala be made to
> > abort with an error if it sees something besides setting a property
> > in the constructoresque property setting method?
> It used to abort with an error in 0.4.x or earlier versions.
>
> The old behavior was to filter the property assignments, if the
> construct properties were passed by g_object_new and the ordinary
> properties were assigned after g_object_new.
>
> The new behavior is to compile the entire CreationMethod -- especially
> after the improvement on GTypeInstance based classes. Apparently the
> old behavior(which should be fully preserved for GObject based
> classes) was only partially preserved.
>
> That explains why you were getting the bazzar errors: the property
> assignments were inside an if block, and valac was confused -- the new
> behavior and old behavior don't work well together.
>
> > If Bar () sets member variables instead of properties the members will
> > be initialized after the construct {} block is called. bar_int retains
> > the value 42 until after the construct block is called.
> >
> > Perhaps my confusion stems from a general lack of understanding
> > concerning properties and GObject. When would I use a property
> > vs. just a regular instance variable and what is a managed property? I
> > have used the C api to Gtk and dug into the glib/gobject source but I
> > never really grasped the reason properties exist.
>
> I guess one of the rules of the thumb is to always use getter/setters.
> GTK is already sealing all the public member variables anyways.
>
> The inability to
> > compile certain things in the constructoresque property setter plus
> > its control flow complexity has me scratching my head.
> Try some different approach. Why is a control flow needed? is it
> possible to eliminate the control flow from the CreationMethod?
>
> The Vala
> > tutorial and faq are correct in their treatment of these issues but
> > they are also
> > very minimal.
> >
> > Another odd thing:
> >
> > public class Foo : Object {
> >        public int foo_int = 42;
> >        public Foo foo_foo { get; construct; }
> >
> >        public Foo () {
> >                foo_foo = this;
> >        }
> >        construct {
> >                message ("%d", foo_foo.foo_int);
> >        }
> > }
> > public void main () {
> >        var obj = new Foo ();
> > }
> >
> > The above code segfaults. foo_foo is a property so it should be set in
> > Foo before construct {} is called but apparently it isn't because the
> > fault occurs in foo_constructor(). Is this a bug? Can properties have
> > an Object type?
>
> This behavior should be related to Bug 572079.
>
> > Also, is it (or will it be) possible to overload the constructoresque
> > property setter on type and arity? I currently get an "already
> > defined" error.
>
> Never tried to do so.
> > Finally, what is the constructoresque property setter actually called,
> > or is "Constructoresque property setter" a good name?
> No idea. Isn't it just a regular setter? 'construct' stands for a
> special Flag when calling g_object_install_property.
>
> > -Sam Danielson
> > _______________________________________________
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