I'm porting some code from D1 to D2 and this is the final error i'm getting from DMD and GDC.  I have no idea what is going on with it though.

/**
 * Convert any function pointer to a delegate.
 * _ From: http://www.digitalmars.com/d/archives/digitalmars/D/easily_convert_any_method_function_to_a_delegate_55827.html */
R delegate(P) toDelegate(R, P...)(R function(P) fp)
{	struct S
	{	R Go(P p) // P is the function args.
		{	return (cast(R function(P))(cast(void*)this))(p);
		}
	}
	return &(cast(S*)(cast(void*)fp)).Go;
}

DMD tells me:
src/yage/core/misc.d(164): Error: e2ir: cannot cast this of type S to type void*


The code itself you can see at: https://bitbucket.org/simcop2387/yage/src/0542cea5eefabf4c90b822cf26ec1062ed0fcb64/src/yage/core/misc.d?at=default#cl-161

and where it gets called: https://bitbucket.org/simcop2387/yage/src/0542cea5eefabf4c90b822cf26ec1062ed0fcb64/src/yage/core/repeater.d?at=default#cl-47

To be perfectly honest I'm not sure what it's doing to figure out how to fix it.

On Monday, 14 April 2014 at 17:45:52 UTC, Ryan Voots wrote:
> src/yage/core/misc.d(164): Error: e2ir: cannot cast this of type S to type void*


Try taking the address of this before casting it. So more like

cast(void*)&this


IIRC in D1 this was a pointer, whereas in D2 this is a reference. You can't cast a reference to pointer directly (at least not without overloading the cast operator) but you can take its address to fetch a pointer out of it and then work with that.

BTW see also: http://dlang.org/phobos/std_functional.html#toDelegate

On Monday, 14 April 2014 at 17:45:52 UTC, Ryan Voots wrote:
> /**
>  * Convert any function pointer to a delegate.
>  * _ From: http://www.digitalmars.com/d/archives/digitalmars

You can replaced it with std.functional.toDelegate.

As for its use-case, if some API or function supports only delegates but you want to pass in a function, you would wrap the function with toDelegate and then pass the delegate to the API/function.

On Monday, 14 April 2014 at 17:48:31 UTC, Adam D. Ruppe wrote:
> On Monday, 14 April 2014 at 17:45:52 UTC, Ryan Voots wrote:
>> src/yage/core/misc.d(164): Error: e2ir: cannot cast this of type S to type void*
>
>
> Try taking the address of this before casting it. So more like
>
> cast(void*)&this
>
>
> IIRC in D1 this was a pointer, whereas in D2 this is a reference. You can't cast a reference to pointer directly.

That's not true.

And I think the issue in his diagnostic is that he used it with a struct. You can't cast 'this' of a struct to a pointer (you'd have to use &this), but you can cast a class reference to a pointer.

On 04/14/14 19:51, Andrej Mitrovic wrote:
> On Monday, 14 April 2014 at 17:48:31 UTC, Adam D. Ruppe wrote:
>> On Monday, 14 April 2014 at 17:45:52 UTC, Ryan Voots wrote:
>>> src/yage/core/misc.d(164): Error: e2ir: cannot cast this of type S to type void*
>>
>>
>> Try taking the address of this before casting it. So more like
>>
>> cast(void*)&this
>>
>>
>> IIRC in D1 this was a pointer, whereas in D2 this is a reference. You can't cast a reference to pointer directly.
> 
> That's not true.
> 
> And I think the issue in his diagnostic is that he used it with a struct. You can't cast 'this' of a struct to a pointer (you'd have to use &this), but you can cast a class reference to a pointer.

It *is* true. Classes are /reference types/, not references to classes.
[There's no such thing as a "class payload" in D. Also, you can have a
 D-ref to a D-class.]

D doesn't have "true" references, just a storage-class based hack, that only supports a small subset of ref functionality. But these pseudo-refs still act as the objects they point to.

artur

On 4/15/14, Artur Skawina <art.08.09@gmail.com> wrote:
> It *is* true. Classes are /reference types/, not references to classes.

I meant the part where he said you can't cast a reference to a pointer. You can.

On 04/15/14 13:30, Andrej Mitrovic wrote:
> On 4/15/14, Artur Skawina <art.08.09@gmail.com> wrote:
>> It *is* true. Classes are /reference types/, not references to classes.
> 
> I meant the part where he said you can't cast a reference to a pointer. You can.

He obviously meant that you can't get a pointer to the object, that the reference points to, just by casting and w/o address-of.

   int* f(ref int r) { return cast(int*)r; }

   void main() {
      int a = 42;

      import std.stdio;
      writeln(&a);
      writeln(f(a));
   }

Yes, this will compile, and, yes, you can "cast a reference to a pointer", but this does not mean that you will get a pointer to 'a'.

The D situation wrt to refs and classes is bad enough; saying that you "can cast a reference to a pointer" will only confuse people.

artur

On Tuesday, 15 April 2014 at 12:05:09 UTC, Artur Skawina wrote:
> He obviously meant that you can't get a pointer to the object, that the> reference points to, just by casting and w/o address-of.

Yea, you can cast a class reference to void* (which does include
this inside a class), but not a ref storage class thingy which
includes this inside a struct. Since the topic at hand was a
struct I got a bit sloppy in my wording.

So I'd say we're all partially right, just in different contexts
:)

On 4/15/14, Artur Skawina <art.08.09@gmail.com> wrote:
> He obviously meant that you can't get a pointer to the object, that the reference points to, just by casting and w/o address-of.

Ah right. I don't do this often so I forgot. The proper code would be:

int* f(ref int r) { return &r; }