C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Derived catch clauses preceding base catch clauses          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Scott Meyers Guest Posted: Mon Nov 17, 2003 10:34 pm    Post subject: Derived catch clauses preceding base catch clauses Recently I was reminded of something in C++ that I just don't understand. If a base class catch clause precedes a derived class catch clause, the second clause can never be executed: struct B { ... }; struct D: B { ... }; try { ... } catch(B) { ... }; catch(D) { ... }; // can never be executed As I recall, this was disallowed in the ARM, but the rule was changed for the final standard. Can somebody please explain to me why it's not an error for a base catch clause to precede a derived catch clause? If it's an issue of accessibility (e.g., it might make sense if D privately inherits from B), then why not prohibit it when D can be implicitly converted to B? I'm sure there's a good reason for the current rules. I'd just like to know what it is. Thanks, Scott --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Sergey P. Derevyago Guest Posted: Wed Nov 19, 2003 12:26 am    Post subject: Re: Derived catch clauses preceding base catch clauses Scott Meyers wrote: Quote: Recently I was reminded of something in C++ that I just don't understand. If a base class catch clause precedes a derived class catch clause, the second clause can never be executed: struct B { ... }; struct D: B { ... }; try { ... } catch(B) { ... }; catch(D) { ... }; // can never be executed As I recall, this was disallowed in the ARM, but the rule was changed for the final standard. Can somebody please explain to me why it's not an error for a base catch clause to precede a derived catch clause? If it's an issue of accessibility (e.g., it might make sense if D privately inherits from B), then why not prohibit it when D can be implicitly converted to B? I'm sure there's a good reason for the current rules. I'd just like to know what it is. IMHO 308 "Catching exceptions with ambiguous base classes" NAD issue gives some points. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_closed.html#308 -- With all respect, Sergey. http://cpp3.virtualave.net/ mailto : ders at skeptik.net --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Richard Smith Guest Posted: Wed Nov 19, 2003 12:27 am    Post subject: Re: Derived catch clauses preceding base catch clauses Scott Meyers wrote: Quote: Recently I was reminded of something in C++ that I just don't understand. If a base class catch clause precedes a derived class catch clause, the second clause can never be executed: struct B { ... }; struct D: B { ... }; try { ... } catch(B) { ... }; catch(D) { ... }; // can never be executed [BTW: These two semicolons are illegal.] It *is* possible for the catch block for D to be entered. If B is an ambiguous public base class of the exception, but D is not, then the second catch block will be entered. For example, Imagine the code within the try block throws an object of type E, defined below. struct E : D, B {}; -- Richard Smith --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top James Kuyper Guest Posted: Wed Nov 19, 2003 4:25 pm    Post subject: Re: Derived catch clauses preceding base catch clauses [email]richard (AT) ex-parrot (DOT) com[/email] (Richard Smith) wrote in message news:<Pine.LNX.4.55.0311181738000.27380 (AT) sphinx (DOT) mythic-beasts.com>... Quote: Scott Meyers wrote: .... struct B { ... }; struct D: B { ... }; try { ... } catch(B) { ... }; catch(D) { ... }; // can never be executed [BTW: These two semicolons are illegal.] Not quite - they both are quite legal, though pointless, since they each terminate empty statements that follow the preceeding catch block (which is probably NOT what the author intended them to represent). The first ';' renders the following 'catch' illegal, since there's no corresponding 'try' for it; it's no long part of the same try-block as the first catch. The second semicolon terminates a perfectly legal empty statement. But both semicolons are themselves legal. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Richard Smith Guest Posted: Thu Nov 20, 2003 3:19 am    Post subject: Re: Derived catch clauses preceding base catch clauses James Kuyper wrote: Quote: Richard Smith wrote: Scott Meyers wrote: .... try { ... } catch(B) { ... }; catch(D) { ... }; // can never be executed [BTW: These two semicolons are illegal.] Not quite - they both are quite legal, though pointless, since they each terminate empty statements that follow the preceeding catch block (which is probably NOT what the author intended them to represent). The first ';' renders the following 'catch' illegal, since there's no corresponding 'try' for it; it's no long part of the same try-block as the first catch. The second semicolon terminates a perfectly legal empty statement. But both semicolons are themselves legal. It's a bit pointless arguing whether the semicolons themselves are legal or not if the it makes the code as a whole ill-formed. The Standard does not have any notion of whether a code fragment, such as an semicolon in isolation, is well-formed: it only has the notion of whether a whole translation unit is well-formed. Unless the preprocessor is used to change and/or hide the code, the first semicolon prevents the code _as a whole_ from being legal in any context. Given that, I think it's reasonable to say the first semicolon is illegal. The second semicolon is perfectly OK -- my mistake. Sorry. -- Richard Smith --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Adam H. Peterson Guest Posted: Thu Nov 20, 2003 10:54 pm    Post subject: Re: Derived catch clauses preceding base catch clauses Quote: struct B { ... }; struct D: B { ... }; try { ... } catch(B) { ... }; catch(D) { ... }; // can never be executed [BTW: These two semicolons are illegal.] Actually, I believe only the first one is . (The second one is useless, but not illegal.) Adam H. Peterson --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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