export classes from DLLs

[pragma]

In article <e3njr2$lr0$1 at digitaldaemon.com>, John C says...
>
>Anyone ever got classes to export from DLLs correctly? I've searched the 
>forum and it seems each effort has only got so far.
>
>Using a factory method in the DLL works, but that's not what I'm trying 
>to accomplish. If I try to create an instance in a program using the 
>DLL, the compiler complains about an undefined symbol.
>
>mydll.d implementation module
># // DllMain/gc_init etc omitted for brevity
>#
># export class Dog {
># 	export static Dog create() {
># 		return new Dog;
># 	}
># 	char[] bark() {
># 		return "Woof";
># 	}
># }
>
>mydll.di import module
># class Dog {
># 	static Dog create();
># 	char[] bark();
># }
>
>mydll.def
># LIBRARY "mydll.dll"
># EXETYPE NT
># EXPORTS
>
>Compile DLL and create import library
> > dmd mydll mydll.def
> > implib /system mydll.lib mydll.dll
>
>program.d
># import mydll;
># void main() {
># 	// Dog dog = Dog.create(); // This works
>#	Dog dog = new Dog; // This doesn't
># 	dog.bark();
># }
>
>Compile program
> > dmd program mydll.lib
>
>This produces the following error:
> > program.obj
> > Error 42: Symbol undefined __Class_7program3Dog
>
>Adding __Class_7program3Dog to the EXPORTS list kills the error, but 
>then the program throws an access violation.
>
>When I compile classes into a static library (no DLL), this problem 
>doesn't surface.

The problem is that DLL's cannot export type information in the way you'd expect
it to.  In essence, each DLL compiled with D has its own type tree, completely
distinct from the type tree present in your .exe file.  Trace the cause of those
access violations and you'll probably find they all terminate at failing
implicit and explicit casts due to this.  

The problem goes away when you use static linking since the linker will unify
all the type information, yielding totally different behavior in your code.

Anyway, there are *plenty* of other gotchas with DLL's.  I strongly reccomend
you read the following before you get too deep into your design:
http://www.prowiki.org/wiki4d/wiki.cgi?BestPractices/DLL

And before you let that get you down, rest assured, I'm working hard to push DDL
toward release: http://www.dsource.org/projects/ddl.



- EricAnderton at yahoo