multiple dispatch using run & compile time reflection

[Pragma]

Christian Kamm wrote:
> 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
> 

Neat.

I gather that since you're using near-trival function bodies for testing, these timings represent the relative call 
dispatch overhead?  If so, then that's great research - thanks.

-- 
- EricAnderton at yahoo