Use class template as a type

[Namespace via Digitalmars-d-learn]

> We have a handy dandy syntax for this:
>
> if (MyClassInt subclass = cast(MyClassInt)value) {
> 	writeln(subclass.value);
> }
>
> If it doesn't cast to said type (it will be null) that branch 
> won't execute.

Just out of interest: it looks like a dynamic_cast in C++ which 
is considered as slow operation. Is that D cast also a dynamic 
cast and also slow? I've never used it, so I'm a bit curious.