Dispatching on a variant

[Justin Johansson]

language_fan Wrote:

> Sat, 26 Sep 2009 09:32:55 -0400, Justin Johansson thusly wrote:
> 
> > I've had a good poke around the forums and couldn't find anything on
> > this so ...
> > 
> > What's the recommended method for dispatching code off the runtime type
> > of a variant variable (Phobos D2 std.variant)?
> > 
> > Does one use a bunch of
> > 
> > if ( var.peek!(type1)) { ... }
> > else if ( var.peek!(type2)  { ... }
> > 
> > for all N possible types, or is there a better & faster way with a
> > switch or jump table of sorts?
> 
> If the type count gets large, how fast it is depends on the backend 
> optimizations of the compiler. In the worst case it is a O(n) time linear 
> search. A jump table or almost any other way of dispatching would be 
> faster. If the variant had an integral tag field, it could be used in a 
> switch; that way the compiler could easily optimize it further with the 
> currently available constructs.
> 
> This problem is solved in higher level languages by providing pattern 
> matching constructs. The compiler is free to optimize the code the way it 
> likes:
> 
>   case var of
>     type1 => ...
>     type2 => ...
>     ...
> 
> But since no C-like language has ever implemented pattern matching, it 
> might be too radical to add it to D.

Thanks both for replies.

I've got about 2 dozen types in the variant so the O(n) really hurts.
The variant thing seemed like a really cool idea at the time but now ...
Without something like suggested above or a computed goto on typeid or Andrei's visitator,
it almost pushes me to backout from using variants and having to redesign around some common base class or interface and using virtual function dispatch. :-(