multiple dispatch using run & compile time reflection

[Craig Black]

This is very cool!  Is it necessary to place all methods inside a single 
class?  It is sometimes necessary to allow multimethods to be extended in 
multiple classes.  I figure there's probably a way to do this with an 
approach similar to this.

-Craig

"Christian Kamm" <kamm.incasoftware at shift-at-left-and-remove-this.de> wrote 
in message news:f84ufe$27e3$1 at digitalmars.com...
>I was experimenting with the new __traits and used it to create a
> RTTI-dispatched visitor here:
> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D.announce&artnum=9348
>
> I've extended the code to allow multiple dispatch and be a bit more 
> generic.
> The code is attached, an example is inline:
>
> ----
> import multipledispatch, std.stdio;
>
> class A {}
> class B : A {}
>
> class Foo
> {
>  void bar(A a1, A a2)
>  { writefln("AA"); }
>
>  void bar(A a, B b)
>  { writefln("AB"); }
>
>  void bar(B a, B b)
>  { writefln("BB"); }
>
>  // generates void dispatch(Object,Object)
>  // that uses RTTI to call one of the overloads
>  // defined above
>  mixin MultipleDispatch!("bar", _0, _1);
> }
>
> void main()
> {
>  A a = new A;
>  A b = new B;
>
>  auto foo = new Foo;
>
>  foo.dispatch(a, a); // writes AA
>  foo.dispatch(a, b); // writes AB
>  foo.dispatch(b, b); // writes BB
>
>  foo.dispatch(b, a); // throws error
> }
> ----
>
> I have also included a MultipleDispatchFallthrough template that'll call 
> the
> best match instead of throwing an error if there's no exact match. 
> However,
> the implementation is pretty inefficient. (feel free to optimize)
>
> Here are some rough benchmarks, relative to a virtual function call:
>
> virtual function call: 1
> MultipleDispatch with one DispatchArg: 2.5
> MultipleDispatch with three DispatchArgs: 2.9
> MultipleDispatch with one DispatchArg and Fallthrough: 10
>
> As you can see, a call through MultipleDispatch will take roughly three
> times as long as a virtual function call. Since a call to dispatch will
> contain two virtual function calls, that's almost optimal.
>
> Regards,
> Christian