Casts

[Denis Koroskin]

On Wed, 22 Oct 2008 22:39:10 +0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> With reddit I have found a page that shows casts in C#:
> http://rusek.org/stefan/default.aspx/2008/10/22/the-3-cast-operators-in-c/73/
>
> The default cast of C# throws an exception if the dynamic cast can't be  
> done. Do you see this as safer than the current D1 cast?
>
> The "as" of C# is similar to the current dynamic cast of D1.
>
> The "is" operatotor of C# can be implemented with a cast plus test:
> if (cast(Foo)o !is null) { ...
> That sometimes I write:
> if (IsInstance!(Foo)(o)) { ...
>
> This syntax of C#
> something = o as Foo ?? new Foo(0);
>
> Becomes in D:
> something = cast(Foo)o;
> if (something is null) something = new Foo(0);
>
> Because this shorter code requires two casts, it's slower:
> something = cast(Foo)o is null ? new Foo(0) : cast(Foo)o;
>
> I think in D may be useful to split the current cast() into two  
> different syntaxes, one similar the current dynamc cast, and the other  
> that performs a non-aliasing re-interepretation of the given bitpattern  
> (the kind of conversion unions are often used for in C). I find those  
> two cases quite different, but I confuse them in the current D1, so I  
> think they may deserve two different syntaxes.
>
> (This may look like an increase of the complexity of D, but when two (or  
> more) usages are conflated into a single syntax (or one common usage  
> doesn't have a hansy syntax) then the complexity of the language seems  
> actually higher).
>
> --------------
>
> Sometimes you need to test a given object to many classes:
>
> if (cast(Foo1)obj !is null) {
>     ...
> } else if (cast(Foo2)obj !is null) {
>     ...
> } else if (cast(Foo3)obj !is null) {
>     ...
> } else if (cast(Foo4)obj !is null) {
>     ...
> } else {
>     ...
> }
>
> If such idioms are used often enough (in my code I don't use them often)  
> then something shorter and/or faster may be adopted.
>
> Bye,
> bearophile

You can use the following short-cut to avoid double casting:

if (auto foo = cast(Foo)obj) {
     ...
} else if (auto bar = cast(Bar)bar) {
     ...
} else {
     ...
}