dynamic classes and duck typing

[Simen kjaeraas]

On Sun, 29 Nov 2009 11:44:56 +0100, biozic <dransic at free.fr> wrote:

> Le 29/11/09 00:36, Walter Bright a écrit :
>> And here it is (called opDispatch, Michel Fortin's suggestion):
>>
>> http://www.dsource.org/projects/dmd/changeset?new=trunk%2Fsrc@268&old=trunk%2Fsrc@267
>>
>
> Seems interesting, but for now the error message when no opDispatch  
> template can be instantiated looks confusing when trying to use a class  
> with an opDispatch implemented, and making e.g. a typo error:
>
> =============================================
> module lib;
> class Test
> {
>      string opDispatch(string name)()
>      {
>          static if (name == "foo")
>              return "foo";
>      }
> }
> =============================================
> module main;
> import lib;
> import std.stdio;
>
> void main()
> {
>      auto test = new Test;
>      writeln(test.foo); // OK
>      writeln(test.fooo); // Error
> }
> =============================================
>
> Error is: """
> lib.d(5): Error: function lib.Test.opDispatch!("fooo").opDispatch  
> expected to return a value of type string
> lib.d(9): Error: template instance lib.Test.opDispatch!("fooo") error  
> instantiating
> """
>
> nicolas

That is because your opDispatch is instantiated no matter what the name  
is, but only does something sensible if it's foo. Try this:

string opDispatch( string name )( ) {
   static if ( name == "foo" ) {
     return "foo";
   } else {
     static assert( false, "Invalid member name." );
   }
}

-- 
Simen