• bearophile <bearophileHUGS lycos.com> Jun 08 2010
• Justin Johansson <no spam.com> Jun 09 2010
• "Nick Sabalausky" <a a.a> Jun 09 2010

bearophile <bearophileHUGS lycos.com> writes:

Michel Fortin:

 But if one of your function has an 'out' parameter, it's impossible to 
 implement the strong guarantee, as illustrated by this trivial example:
 
 	void testOut(out int a) {
 		throw new Exception("hello!");
 	}
 
 	void main() {
 		int a = 2;
 		try
 			testOut(a);
 		finally
 			writeln(a);
 	}
 
 Prints:
 
 	0
 	object.Exception: hello!
 
 This happens because the out parameter gets reset to its default value 
 as soon as you enter the function, so you can't throw an exception 
 before it has been changed.
 
 So should 'out' be reformed to behave more like a return value? I'm not 
 sure. But I think this is something to keep in mind when using out 
 parameters.



In a recent post here:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=110908
and in some successive answers I have tried to explain that out arguments are a
hack, they aren't as logically clean as multiple return values. I didn't think
about exceptions too, your example adds one more case to what I was saying
there. Thank you.

Bye,
bearophile

Justin Johansson <no spam.com> writes:

bearophile wrote:
 Michel Fortin:
 
 But if one of your function has an 'out' parameter, it's impossible to 
 implement the strong guarantee, as illustrated by this trivial example:

 	void testOut(out int a) {
 		throw new Exception("hello!");
 	}

 	void main() {
 		int a = 2;
 		try
 			testOut(a);
 		finally
 			writeln(a);
 	}

 Prints:

 	0
 	object.Exception: hello!

 This happens because the out parameter gets reset to its default value 
 as soon as you enter the function, so you can't throw an exception 
 before it has been changed.

 So should 'out' be reformed to behave more like a return value? I'm not 
 sure. But I think this is something to keep in mind when using out 
 parameters.


 
 In a recent post here:
 http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=110908
 and in some successive answers I have tried to explain that out arguments are
a hack, they aren't as logically clean as multiple return values. I didn't
think about exceptions too, your example adds one more case to what I was
saying there. Thank you.
 
 Bye,
 bearophile


I re-read that recent post of yours in the context of this new post and 
I must say that I agree with you.  The ability to return mutiple return 
values,  as a tuple of sorts, is a good idea.  Short of this a stronger 
type system with existential typing (cardinalities) would suffice.  Of 
course that might bring us into a discussion about nullable types which 
is which is which is which is which is a recursive topic on this ng.

Regards
Justin

"Nick Sabalausky" <a a.a> writes:

"Justin Johansson" <no spam.com> wrote in message 
news:huo7rk$9ae$1 digitalmars.com...
 I re-read that recent post of yours in the context of this new post and I 
 must say that I agree with you.  The ability to return mutiple return 
 values,  as a tuple of sorts, is a good idea.  Short of this a stronger 
 type system with existential typing (cardinalities) would suffice.


Existential typing, is that the thing where there's a type that indicates 
"this doesn't return"?

 Of course that might bring us into a discussion about nullable types which 
 is which is which is which is which is a recursive topic on this ng.


He he he :)