export classes from DLLs

[John C]

pragma wrote:
> 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

Thanks for the explanation and the link. It's a shame it won't work - I 
was hoping to compile a fairly sizable class library into a DLL which 
other projects would link to, instead of bringing in all the code. So 
much for that idea!

I've been keeping an eye on DDL for a while. It looks like it'll be 
suitable for the plug-in system I've wanted to work on.

John C.