Re: [Vala] Invalid cast Warnings

[Michael Gratton <mike vee net>]

On Mon, Feb 6, 2017 at 4:57 AM, Al Thomas via vala-list 
<vala-list gnome org> wrote:
> I've never used sub-types much. Always found casting to an interface 
> for polymorhism much easier to understand. Sub-type tutorials always 
> end up using animals or car parts :-)
>
> Maybe something like this would make it clearer:
>
> void main () {
>   var a = new Salmon ();
>   Mackerel b = (Mackerel)a; // Invalid cast
>   Fish c = (Fish)a; // Valid cast
> }

In that case, yeah, but there definitely are cases as Daniel suggests 
where it is needed, including his example of integrating with 3rd party 
code.

Eg: If some method defines that parameter with a specific type, but it 
is likely that the actual object will be a subtype, then it's valid to 
check and then cast:

interface Printer {
 void do_print(PrintJob job);
}

class UnixPrinter : Printer {
 void do_print(PrintJob job) {
   UnixPrintJob unix = job as UnixPrintJob;
   ...
 }
}
--
⊨ Michael Gratton, Percept Wrangler.
⚙ <http://mjog.vee.net/>