I'm trying to register member function of one of my class as expat handler. Of course expat functions are taking pointer to a function and are declared in extern(C) scope. I understand that member functions are in fact normal functions invisibly taking "this" argument and so is this possible to do such thing in D? If answer is "yes": how and "WOW I love this language"; if answer is "no": what would be the best workaround.

For clarifying this is what I'm trying to do:

extern(C){

 void XML_SetElementHandler(XML_Parser p, XML_StartElementHandler start,
   XML_EndElementHandler end);

 typedef void function(void *userData, XML_Char *name, XML_Char **atts)
   XML_StartElementHandler;

 typedef void function(void *userData, XML_Char *name)
   XML_EndElementHandler;
}

class Foo{

 this(){
 _parser = XML_ParserCreate(null);
 XML_SetElementHandler(_parser,
  cast(XML_StartElementHandler)&this.startHandle,
  cast(XML_EndElementHandler)&this.endHandle);
 }

extern(C){
 void startHandle(void *userData,XML_Char *name,XML_Char **atts){
  printf("started: %sn",name);
 }
 void endHandle(void *userData,XML_Char *name){
  printf("ended: %sn",name);
 }

}

I get:
e2ir: cannot cast from void delegate(void*userData,char*name,char**atts) to
void(C *)(void*userData,char*name,char**atts)
e2ir: cannot cast from void delegate(void*userData,char*name) to void(C *
(void*userData,char*name)

Regards,
-- 
Dawid Ciężarkiewicz | arael
jid: arael@fov.pl

I think you can do that by making the class members static ...

- Kris

In article <cteolm$12va$1@digitaldaemon.com>, Dawid =?ISO-8859-2?Q?Ci=EA=BFarkiewicz?= says...
>
>I'm trying to register member function of one of my class as expat handler. Of course expat functions are taking pointer to a function and are declared in extern(C) scope. I understand that member functions are in fact normal functions invisibly taking "this" argument and so is this possible to do such thing in D? If answer is "yes": how and "WOW I love this language"; if answer is "no": what would be the best workaround.
>
>For clarifying this is what I'm trying to do:
>
>extern(C){
>
> void XML_SetElementHandler(XML_Parser p, XML_StartElementHandler start,
>   XML_EndElementHandler end);
>
> typedef void function(void *userData, XML_Char *name, XML_Char **atts)
>   XML_StartElementHandler;
>
> typedef void function(void *userData, XML_Char *name)
>   XML_EndElementHandler;
>}
>
>class Foo{
>
> this(){
> _parser = XML_ParserCreate(null);
> XML_SetElementHandler(_parser,
>  cast(XML_StartElementHandler)&this.startHandle,
>  cast(XML_EndElementHandler)&this.endHandle);
> }
>
>extern(C){
> void startHandle(void *userData,XML_Char *name,XML_Char **atts){
>  printf("started: %sn",name);
> }
> void endHandle(void *userData,XML_Char *name){
>  printf("ended: %sn",name);
> }
>
>}
>
>I get:
>e2ir: cannot cast from void delegate(void*userData,char*name,char**atts) to
>void(C *)(void*userData,char*name,char**atts)
>e2ir: cannot cast from void delegate(void*userData,char*name) to void(C *
>(void*userData,char*name)
>
>Regards,
>-- 
>Dawid Ciê¿arkiewicz | arael
>jid: arael@fov.pl

These are the culprit lines from your post:

>  cast(XML_StartElementHandler)&this.startHandle,
>  cast(XML_EndElementHandler)&this.endHandle);

I understand what you're trying to do, as I thought the same thing myself: since D passes 'this' as the first argument to a method, why not pass along my class's methods to Expat since they happen to match the same spec.

This would work amazingly well in C++ but D doesn't like it (obviously). There's a lot more going on in those lines than meets the eye:

1) D converts "&this.startHandle" into a *delegate*, which contains a reference
to 'this' as well as the address of "startHandle".
2) D does not provide a means for casting a delegate into a function, so the
cast to XML_StartElementHandler fails miserably.

A more complete explaination of the underpinnings of delegates is here:

http://www.prowiki.org/wiki4d/wiki.cgi?DocComments/Function

Now what you're asking for is a hack to crack the function pointer out of a delegate.  I don't advocate this in the slightest, since it's probing around a grey area that may or may not be changed in the D ABI.

> import std.stdio;
> 
> struct DelegateStruct {
> 	void *ptr;
> 	void *func;
> };
> 
> class Test{
> 	extern(C) int myMethod(int a){
> 		writefln("A is %d",a);
> 		return a+1;
> 	}
> }
> 
> extern(C) typedef int delegate(int) TestDelegate;
> extern(C) typedef int function(void*,int) TestFunctionHandle;
> 
> int testFunction(void* thisPtr,TestFunctionHandle handle,int a){
> 	return handle(thisPtr,a);
> }
> 
> void main(){
> 	Test t = new Test();
> 	writefln("calling directly");
> 	writefln("result is: %d",t.myMethod(42));
> 	TestDelegate del = &t.myMethod;
> 	DelegateStruct ds = *cast(DelegateStruct*)cast(void**)&del;
> 	TestFunctionHandle handle = cast(TestFunctionHandle)ds.func;
> 	writefln("calling indirectly");
> 	writefln("result is: %d",testFunction(cast(void*)t,handle,42));
> }

I'm sure there's probably a way to use a template and/or mixin to make this much more clean.

The secret here is the liberal use of "extern(C)".  The default D linkage likes to throw the last argument around as EAX, which happens to be "this" for object.method calls (at least for DMD, I don't know what GDC is up to).


- EricAnderton at yahoo

Kris wrote:

> I think you can do that by making the class members static ...

Well, right but this way I lose information stored in "this" and I don't know on which object I work. I was trying to write multithreaded server application that communicates with xml. Each connection has to be separately parsed. I need tons of parser - each knowing almost nothing about other.
-- 
Dawid Ciężarkiewicz | arael
jid: arael@fov.pl

pragma wrote:

> I'm sure there's probably a way to use a template and/or mixin to make this much more clean.
> 
> The secret here is the liberal use of "extern(C)".  The default D linkage likes to throw the last argument around as EAX, which happens to be "this" for object.method calls (at least for DMD, I don't know what GDC is up to).

This is nice hack, but I was thinking about something more portable, stable etc. As I understand whole hack is set on fact that EAX isn't changed and stores "this" pointer, right? Can I really depend on this in multithreaded environment using external C callback? I mean - I will register such function as handler, then many things will happen (EAX will definitely change) and then expat will call my handler and I don't think it will set EAX to something usable. If I understand this will end with random "this" and segfault.

I'm not sure if I understood you so sorry if I'm jabbering.
-- 
Dawid Ciężarkiewicz | arael
jid: arael@fov.pl

Dawid Ciężarkiewicz wrote:

> I'm trying to register member function of one of my class as expat handler. Of course expat functions are taking pointer to a function and are declared in extern(C) scope. I understand that member functions are in fact normal functions invisibly taking "this" argument and so is this possible to do such thing in D? If answer is "yes": how and "WOW I love this language"; if answer is "no": what would be the best workaround.

I've found my personal solution. Because every class instance that need his own parser is new thread I register static member function as handler that use "getThis()" to run good instance of member function.

Like this:

static void StartHandler(void *userData,XML_Char *name,XML_Char **atts){
  (cast(ConnectionHandler)getThis()).startHandle(userData,name,atts);
}


-- 
Dawid Ciężarkiewicz | arael
jid: arael@fov.pl

In article <ctfqta$287m$1@digitaldaemon.com>, Dawid =?ISO-8859-2?Q?Ci=EA=BFarkiewicz?= says...
>
>pragma wrote:
>
>> I'm sure there's probably a way to use a template and/or mixin to make this much more clean.
>> 
>> The secret here is the liberal use of "extern(C)".  The default D linkage likes to throw the last argument around as EAX, which happens to be "this" for object.method calls (at least for DMD, I don't know what GDC is up to).
>
>This is nice hack, but I was thinking about something more portable, stable etc. As I understand whole hack is set on fact that EAX isn't changed and stores "this" pointer, right?

As long as you use "extern(C)" in all the places illustrated, D will compile
those methods/functions as 'stdcall' which is the C standard (no EAX mojo).  If
you try removing "extern(C)", so the compiler uses the D linkage instead, it
segfaults all over the place.

A hack is a hack. I don't advocate using it unless it gives you a sound advantage that can't be met otherwise.  If it passes muster with testing on all your target platforms, then techically its sound code.  I for one never code with a hack unless it achives a requirement, like better performance or a reduced memory footprint; and even then its a last resort.  Plus I document the heck out of it (flagging it with "//HACK:") to warn other developers of what, why and how.


- EricAnderton at yahoo

Hi, Dawid,

Why not to use something simple as below?
(It's just a scetch, never compiled it)

class MyHandler
{
 void startHandler(XML_Char *name,XML_Char **atts)
 {
  printf("started: %sn",name);
 }
 void endHandler(XML_Char *name)
 {
  printf("ended: %sn",name);
 }

 extern(C) static void _startHandler(void *userData,XML_Char *name,XML_Char
**atts)
 {
  (cast(MyHandler)(userData)).startHandler(name,atts);
 }
 extern(C) static void _endHandler(void *userData,XML_Char *name)
 {
  (cast(MyHandler)(userData)).endHandler(name);
 }
 void setup(XML_Parser p)
 {
     XML_SetUserData(p, cast(void*) this);
     XML_SetElementHandler(p,_startHandle,_endHandle);
 }
}

You can wrap it into template if you want to use it in multiple places.

Dawid Ciężarkiewicz wrote:
> I'm trying to register member function of one of my class as expat handler.
> Of course expat functions are taking pointer to a function and are declared
> in extern(C) scope. I understand that member functions are in fact normal
> functions invisibly taking "this" argument and so is this possible to do
> such thing in D? If answer is "yes": how and "WOW I love this language"; if
> answer is "no": what would be the best workaround.
> 
> For clarifying this is what I'm trying to do:
> 
> extern(C){
> 
>  void XML_SetElementHandler(XML_Parser p, XML_StartElementHandler start,
>    XML_EndElementHandler end);
> 
>  typedef void function(void *userData, XML_Char *name, XML_Char **atts)
>    XML_StartElementHandler;
> 
>  typedef void function(void *userData, XML_Char *name)
>    XML_EndElementHandler;
> }
> 
> class Foo{
> 
>  this(){
>  _parser = XML_ParserCreate(null);
>  XML_SetElementHandler(_parser,
>   cast(XML_StartElementHandler)&this.startHandle,
>   cast(XML_EndElementHandler)&this.endHandle);
>  }
> 
> extern(C){
>  void startHandle(void *userData,XML_Char *name,XML_Char **atts){
>   printf("started: %sn",name);
>  }
>  void endHandle(void *userData,XML_Char *name){
>   printf("ended: %sn",name);
>  }
> 
> }

You need to create an extern(C) wrapper for the class function, and store the 'this' pointer in a struct, which you use as the userdata. Here's the bare bones of an example, with some nice translation of C strings to D strings thrown in, to boot:

// override this class for your own uses...
class ExpatParser {
  void startHandler(XML_char[] name,XML_Char[][] attrs) { return; }
  void endHandler  (XML_char[] name) { return; }

  this() {
    _parser = XML_ParserCreate(null);

    ExpatParserUserData *ptr = new ExpatParserUserData[1];
    ptr.ref = this;
    XML_SetUserData(_parser, cast(void*)ptr);

    XML_SetElementHandler(_parser, &startHandler_c, &endHandler_c);
  };
};

struct ExpatParserUserData {
  ExpatParser ref;
};

extern(C) void startHandler_c(void *userdata_void,
                              XML_Char  *name_c,
                              XML_Char **attrs_c) {
  ExpatParserUserData *userdata =
          cast(ExpatParserUserData*)userdata_void;
  XML_Char[]   name = name_c[0..strlen(name_c)];
  XML_Char[][] attrs;
    for( ; *attrs_c != null; attrs_c++)
      attrs ~= (*attrs_c)[0..strlen(*attrs_c)];

  userdata.ref.startHandler(name,attrs);
}

you can write your own endHandler_c() the same way...

Andrew Fedoniouk wrote:

> Hi, Dawid,
> 
> Why not to use something simple as below?

Brilinat! I didn't know that user that is working like this. Thank you. This is very good answer to my problem.
-- 
Dawid Ciężarkiewicz | arael
jid: arael@fov.pl