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digitalmars.D.learn - Converting function pointers to delegates

reply "Ryan Voots" <simcop2387 simcop2387.info> writes:
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.
Apr 14 2014
next sibling parent reply "Adam D. Ruppe" <destructionator gmail.com> writes:
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
Apr 14 2014
parent reply "Andrej Mitrovic" <andrej.mitrovich gmail.com> writes:
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.
Apr 14 2014
next sibling parent Artur Skawina <art.08.09 gmail.com> writes:
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
Apr 15 2014
prev sibling next sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
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.
Apr 15 2014
prev sibling next sibling parent reply Artur Skawina <art.08.09 gmail.com> writes:
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
Apr 15 2014
parent "Adam D. Ruppe" <destructionator gmail.com> writes:
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
:)
Apr 15 2014
prev sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
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; }
Apr 15 2014
prev sibling parent "Andrej Mitrovic" <andrej.mitrovich gmail.com> writes:
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.
Apr 14 2014