digitalmars.D - Out parameters and the strong exception guarantee
- Michel Fortin (31/31) Jun 08 2010 The strong exception guarantee guaranties that if an exception is
- bearophile (6/33) Jun 08 2010 In a recent post here:
- Justin Johansson (9/46) Jun 09 2010 I re-read that recent post of yours in the context of this new post and
- Nick Sabalausky (5/11) Jun 09 2010 Existential typing, is that the thing where there's a type that indicate...
- FeepingCreature (2/4) Jun 13 2010 I think that guarantee is too constraining for too little payoff.
The strong exception guarantee guaranties that if an exception is thrown, the function will have no side effect. Of course, not all function can support this (a file I/O error in the middle of writing will have side effects), but often it can and it's generally good practice to offer the guaranty whenever possible. <http://en.wikipedia.org/wiki/Exception_guarantees> 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. -- Michel Fortin michel.fortin michelf.com http://michelf.com/
Jun 08 2010
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
Jun 08 2010
bearophile wrote:Michel Fortin: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 JustinBut 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
Jun 09 2010
"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 :)
Jun 09 2010
On 08.06.2010 14:28, Michel Fortin wrote:The strong exception guarantee guaranties that if an exception is thrown, the function will have no side effect.I think that guarantee is too constraining for too little payoff.
Jun 13 2010