C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Rethrowing from destructors          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Niklas Matthies Guest Posted: Thu Dec 15, 2005 10:44 am    Post subject: Rethrowing from destructors What is the behavior of the following program? struct X { ~X() { throw; } }; int main() { try { X x; throw 0; } catch (...) { } } Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? -- Niklas Matthies [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Thu Dec 15, 2005 2:21 pm    Post subject: Re: Rethrowing from destructors Niklas Matthies wrote: Quote: What is the behavior of the following program? struct X { ~X() { throw; } }; int main() { try { X x; throw 0; } catch (...) { } } The behavior is that the terminate() function will be called (15.1/. This is because at the time the ~X destructor is called, no exception is being *handled*. Re-throwing of an exception is basically allowed only *after* the catch clause is entered, which is not the case here. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Earl Purple Guest Posted: Thu Dec 15, 2005 2:22 pm    Post subject: Re: Rethrowing from destructors Niklas Matthies wrote: Quote: What is the behavior of the following program? struct X { ~X() { throw; } }; int main() { try { X x; throw 0; } catch (...) { } } Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? -- Niklas Matthies Refer to this: http://www.parashift.com/c++-faq-lite/exceptions.html#faq-17.3 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu Dec 15, 2005 2:23 pm    Post subject: Re: Rethrowing from destructors Niklas Matthies wrote: Quote: What is the behavior of the following program? struct X { ~X() { throw; } }; int main() { try { X x; throw 0; } catch (...) { } } Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? Interesting question. On reading the standard, I'd say that std::terminate() will be called, immediately. The standard says (§15.1/: "If no expcetion is presently being handled, executing a throw-expression with no operand calls terminate()." I don't see an exact definition of "handled" off hand, but given the use of "handler", and "handling an exception" in the title of §15.3, I'd suppose that "being handled" means "executing code in a catch block". Both g++ (4.0.2) and Sun CC (5.5) agree with this interpretation. I do hope that the question is one of idle curiosity, and that you aren't trying to use something like this in actual code:-). -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stefan Näwe Guest Posted: Thu Dec 15, 2005 2:24 pm    Post subject: Re: Rethrowing from destructors Niklas Matthies wrote: Quote: What is the behavior of the following program? struct X { ~X() { throw; } }; You're not catching anything here, so (from the holy standard): 15.1 Throwing an exception (p. 293) 8 If no exception is presently being handled, executing a throw expression with no operand calls terminate() (15.5.1). Quote: int main() { try { X x; throw 0; } catch (...) { } } Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? /S -- Stefan Naewe naewe.s_AT_atlas_DOT_de [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Niklas Matthies Guest Posted: Thu Dec 15, 2005 11:20 pm    Post subject: Re: Rethrowing from destructors On 2005-12-15 14:23, kanze wrote: Quote: Niklas Matthies wrote: What is the behavior of the following program? : Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? Interesting question. On reading the standard, I'd say that std::terminate() will be called, immediately. The standard says (§15.1/: "If no expcetion is presently being handled, executing a throw-expression with no operand calls terminate()." I don't see an exact definition of "handled" off hand, but given the use of "handler", and "handling an exception" in the title of §15.3, I'd suppose that "being handled" means "executing code in a catch block". Yeah, what my question comes down to is that I wasn't sure about the exact meaning of "handled", and older (well, ancient) versions of g++ acted like c). : Quote: I do hope that the question is one of idle curiosity, and that you aren't trying to use something like this in actual code:-). The former; the question crossed my mind when I was thinking about the behavioral differences between destructors and Andrei's on_scope_xxx blocks as defined by his code transformations. -- Niklas Matthies [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Fri Dec 16, 2005 11:55 am    Post subject: Re: Rethrowing from destructors Niklas Matthies wrote: Quote: On 2005-12-15 14:23, kanze wrote: Niklas Matthies wrote: What is the behavior of the following program? Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? Interesting question. On reading the standard, I'd say that std::terminate() will be called, immediately. The standard says (§15.1/: "If no expcetion is presently being handled, executing a throw-expression with no operand calls terminate()." I don't see an exact definition of "handled" off hand, but given the use of "handler", and "handling an exception" in the title of §15.3, I'd suppose that "being handled" means "executing code in a catch block". Yeah, what my question comes down to is that I wasn't sure about the exact meaning of "handled", and older (well, ancient) versions of g++ acted like c). The problem is that the standard doesn't actually define "handled", as far as I can see. I'm just guessing that "being handled" means "executing code in a handler", and it is fairly clear that a "handler" corresponds exactly to what we would generally call a catch block. There are other hints: §15.1/6 mentions that "the value of uncaught_exception() will AGAIN be true," which sort of suggests that the statement is only legal when uncaught_exception() would have been false. And ¹5.1/7 says that "The exception thrown is the one most recently caught and not finished," which seems pretty clear, even if it is not expressed as a constraint on user code. There is an interesting problem, however: take your code, and set it in a catch block of another exception. What does happen when you execute "throw ;" during stack walkback of an exception which occurs when you are handling another exception? The exception "most recently caught and not finished" is the outer exception, so presumably, the "throw" is legal. My guess is that in this case, c) would be the correct behavior, but it's really only a guess. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bob Hairgrove Guest Posted: Fri Dec 16, 2005 1:41 pm    Post subject: Re: Rethrowing from destructors On 15 Dec 2005 05:44:02 -0500, Niklas Matthies <usenet-nospam (AT) nmhq (DOT) net> wrote: Quote: What is the behavior of the following program? struct X { ~X() { throw; } }; int main() { try { X x; throw 0; } catch (...) { } } Does the throw statement in the destructor a) rethrow the outer exception, or does it b) call std::terminate(), or does it c) rethrow the outer exception which then causes std::terminate() to be called upon leaving the destructor? Rethrowing an exception should only be legal within the catch block itself. Clearly, it isn't, so terminate should be called (IMHO -- it is not clear to me exactly in what scope we are when X is destroyed). When the first throw statement is executed, the destructor of X will be called before the code in the catch block is reached because X goes out of scope immediately. The catch block will only catch exceptions which were thrown within the context of the previous try block. The question is, are we still "inside the try block" when X is destroyed? I wouldn't count on it. Two possibilities exist: either we are still inside the scope of the try block, or we are somewhere "in between" the try and catch scopes. If we are inside the try block, then the second exception is thrown while the first is as yet unhandled, thus causing terminate to be called. If we are "in between", then there is no catch block corresponding to that scope, also causing terminate to be called. -- Bob Hairgrove [email]NoSpamPlease (AT) Home (DOT) com[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Niklas Matthies Guest Posted: Fri Dec 16, 2005 1:42 pm    Post subject: Re: Rethrowing from destructors On 2005-12-16 11:55, kanze wrote: : Quote: There is an interesting problem, however: take your code, and set it in a catch block of another exception. What does happen when you execute "throw ;" during stack walkback of an exception which occurs when you are handling another exception? The exception "most recently caught and not finished" is the outer exception, so presumably, the "throw" is legal. My guess is that in this case, c) would be the correct behavior, but it's really only a guess. Indeed interesting. Then what about: struct X { ~X() { try { throw; } catch (...) { } } }; int main() { try { throw 1; } catch (...) { { X x; } // (*) } } Is the exception still being "handled" at (*)? Also note that if such re-throws are indeed legal, it provides a way to inspect _all_ exceptions currently being handled (not just the latest one) from within the innermost handler by using nested destructors. My guess is that this is actually a defect in the standard and that the intent was that re-throws are illegal whenever the topmost exception on the exception stack is not currently being "handled". -- Niklas Matthies [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Niklas Matthies Guest Posted: Fri Dec 16, 2005 1:44 pm    Post subject: Re: Rethrowing from destructors On 2005-12-16 11:55, kanze wrote: : Quote: The problem is that the standard doesn't actually define "handled", as far as I can see. I'm just guessing that "being handled" means "executing code in a handler", and it is fairly clear that a "handler" corresponds exactly to what we would generally call a catch block. Actually 15.3p8 comes close to a definition: "An exception is considered handled upon entry to a handler". Quote: There is an interesting problem, however: take your code, and set it in a catch block of another exception. What does happen when you execute "throw ;" during stack walkback of an exception which occurs when you are handling another exception? The exception "most recently caught and not finished" is the outer exception, so presumably, the "throw" is legal. My guess is that in this case, c) would be the correct behavior, but it's really only a guess. Ignore the first reply I just made to this (hasn't been moderated yet as I'm writing this); destructors are immaterial to what I described. The only interesting thing about destructors here is that apparently they can pull handled exceptions from under unhandled ones, so that the exception stack actually isn't a proper stack any more. My conclusion that this is probably a defect still stands, though. -- Niklas Matthies [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.