Using delegates for C callbacks.

[Leandro Lucarella]

Hi! I'm doing some code to interface with C and I need to use D delegates
as C callbacks.

I've tested a lot of posibilities to do this, asking on IRC channels, but
nothing seems to work entirely.

This is the closer I got:

 1  import std.stdio;
 2
 3  extern (C) void f(void function(void*) cb, void* arg)
 4  {
 5  	cb(arg);
 6  }
 7
 8  extern (C) static void thunk(alias Fn)(void* arg)
 9  {
10  	Fn(arg);
11  }
12
13  void fcb(void* arg)
14  {
15  	writefln("fcb: ", *cast (int*) arg);
16  }
17
18  class C
19  {
20  	int x = 1;
21  	void dcb(void* arg)
22  	{
24  		writefln("dcb: ", *cast (int*) arg, " = ", x);
25  	}
26  }
27
28  void main()
29  {
30  	int x = 1;
31  	C c = new C;
32  	thunk!(fcb)(&x);
33  	//thunk!(c.dcb)(&x);
34  	f(&thunk!(fcb), &x);
35  	//f(&thunk!(c.dcb), &x);
36  }

The thunk for the plain function adaptation works fine, but not the
delegate. If I uncomment the code at line 33 I get:

thunk.d:10: Error: need 'this' to access member dcb

I have a void* pointer to use, I can "inject" the this pointer, but I
don't know how. Doing something like:

 8  extern (C) static void thunk(alias Fn)(void* arg)
 9  {
10  	Fn.ptr = arg;
11  	Fn();
12  }

Doesn't work, it says:
thunk.d:10: Error: no property 'ptr' for type 'void'
thunk.d:10: Error: constant dcb().ptr is not an lvalue
thunk.d:10: Error: cannot implicitly convert expression (arg) of type void* to int

Which I don't understand (specially the part of "no property 'ptr' for
type 'void'", why dcb is void? I don't think I understand very well the
semantics of an alias template parameter =S


Is there any recomended solution for this? I think it (or should be) a
fairly common problem (at least when making D bindings for C libraries).

-- 
Leandro Lucarella (luca) | Blog colectivo: http://www.mazziblog.com.ar/blog/
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