Re: [Vala] Using 'as' for casting?

[Yu Feng <rainwoodman gmail com>]

On Fri, 2009-08-07 at 17:38 +0200, Jiří Zárevúcky wrote:
> On 08/07/2009 05:19 PM, Daniel Svensson wrote:
> > When converting my application to use Gtk.Builder I ended up with a
> > lot of code like this:
> >
> > var lbl = builder.get_object("foo") as Gtk.Label;
> >
> > And this caused me to try the following:
> >
> > var time = Time.gm (unxtime as time_t);
> >
> > as it was more readable than:
> >
> > var time = Time.gm ((time_t) unxtime);
> >
> > This however is not valid syntax, but maybe it should be? On IRC I got
> > the answer that if an 'as' would fail it would return null, and that's
> > why it shouldn't work.
> >
> > I thought it was best to drop a mail to the list and see what people think.
> >
> >    
>
> That's right, but it's not just the fact it can return null. You can't 
> use 'as' for simple types because there is no way to do run-time type 
> check for them, so no way to find out whether cast is valid (until the 
> segfault, of course). So there is no way 'as' could work for them.
So the difference between 'as' and '(Type)' is that
(1) 'as' is run-time, aka, dynamic
(2) '(Type)' is compile-type, aka, static

For structs doesn't carry sufficient type information for a run-time
casting, '(type)' casting is the only reasonable way for it.


Yu
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