C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  try / catch syntax Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Gregg Guest Posted: Thu Mar 04, 2004 10:51 pm    Post subject: try / catch syntax What was the rationale behind requiring the bodies of try and catch to be blocks instead of statements (possibly compound)? It seems inconsistent with other similar constructs such as if / else. Why not allow try <stmt1> catch (std::exception&) <stmt2> where <stmt2> and <stmt2> could be either simple or compound (with braces) exactly as if / else? Thanks --- [ 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 Ray Lischner Guest Posted: Fri Mar 05, 2004 4:05 am    Post subject: Re: try / catch syntax On Thursday 04 March 2004 05:51 pm, Gregg wrote: Quote: What was the rationale behind requiring the bodies of try and catch to be blocks instead of statements (possibly compound)? You need the braces to tell which catch goes with which try. For example: try try try_this(); catch (exception1&) handle1(); catch (exception2&) handle2(); catch (exception3&) handle3(); -- Ray Lischner, author of C++ in a Nutshell http://www.tempest-sw.com/cpp --- [ 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 Nicola Musatti Guest Posted: Fri Mar 05, 2004 3:04 pm    Post subject: Re: try / catch syntax [email]rl.news (AT) tempest-sw (DOT) com[/email] (Ray Lischner) wrote in message news:<1785.4047f27d.858ca (AT) prospero (DOT) island.local>... Quote: On Thursday 04 March 2004 05:51 pm, Gregg wrote: What was the rationale behind requiring the bodies of try and catch to be blocks instead of statements (possibly compound)? You need the braces to tell which catch goes with which try. For example: try try try_this(); catch (exception1&) handle1(); catch (exception2&) handle2(); catch (exception3&) handle3(); I'm not convinced this is the only reason, because it would have been just as reasonable to stipulate that catch'es go with the closest try, as it happens with else's and if's. If this were the case, the example above would result in a "try without catch" error. Cheers, Nicola Musatti --- [ 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 Joe Greer Guest Posted: Fri Mar 05, 2004 3:04 pm    Post subject: Re: try / catch syntax [email]rl.news (AT) tempest-sw (DOT) com[/email] (Ray Lischner) wrote in message news:<1785.4047f27d.858ca (AT) prospero (DOT) island.local>... Quote: On Thursday 04 March 2004 05:51 pm, Gregg wrote: What was the rationale behind requiring the bodies of try and catch to be blocks instead of statements (possibly compound)? You need the braces to tell which catch goes with which try. For example: try try try_this(); catch (exception1&) handle1(); catch (exception2&) handle2(); catch (exception3&) handle3(); I personally don't find this a compelling argument. You run into a similar situation with if statements: if (this) if (that) else .... and one just knows that if you are going to nest, then you must use braces. The same rules could apply here. I am not really advocating a change, but there really should be a better rationale than that. joe --- [ 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 Ray Lischner Guest Posted: Fri Mar 05, 2004 3:35 pm    Post subject: Re: try / catch syntax On Friday 05 March 2004 10:04 am, Nicola Musatti wrote: Quote: I'm not convinced this is the only reason, because it would have been just as reasonable to stipulate that catch'es go with the closest try, as it happens with else's and if's. If this were the case, the example above would result in a "try without catch" error. The difference is that try can have mutliple catch parts. An if statement can have only one else part. -- Ray Lischner, author of C++ in a Nutshell http://www.tempest-sw.com/cpp --- [ 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 P.J. Plauger Guest Posted: Fri Mar 05, 2004 7:55 pm    Post subject: Re: try / catch syntax "Ray Lischner" <rl.news (AT) tempest-sw (DOT) com> wrote Quote: On Friday 05 March 2004 10:04 am, Nicola Musatti wrote: I'm not convinced this is the only reason, because it would have been just as reasonable to stipulate that catch'es go with the closest try, as it happens with else's and if's. If this were the case, the example above would result in a "try without catch" error. The difference is that try can have mutliple catch parts. An if statement can have only one else part. True, but there *still* could be a simple disambiguation rule for catch clauses. Each could bind to the immediately preceding try clause. But the whole discussion is academic because that's now the grammar that was chosen, for whatever reason. P.J. Plauger Dinkumware, Ltd. http://www.dinkumware.com --- [ 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 Tony Burrows Guest Posted: Sat Mar 06, 2004 4:38 am    Post subject: Re: try / catch syntax Slightly differently Why no finally which gets executed whether try or catch happens. Ie, code that always runs before a function ends? As a Java programmer I find it useful at times. Tony On Fri, 05 Mar 2004 15:35:01 +0000, Ray Lischner wrote: Quote: On Friday 05 March 2004 10:04 am, Nicola Musatti wrote: I'm not convinced this is the only reason, because it would have been just as reasonable to stipulate that catch'es go with the closest try, as it happens with else's and if's. If this were the case, the example above would result in a "try without catch" error. The difference is that try can have mutliple catch parts. An if statement can have only one else part. --- [ 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 Gregg Guest Posted: Sat Mar 06, 2004 4:38 am    Post subject: Re: try / catch syntax [email]rl.news (AT) tempest-sw (DOT) com[/email] (Ray Lischner) wrote in message news:<1785.4047f27d.858ca (AT) prospero (DOT) island.local>... Quote: You need the braces to tell which catch goes with which try. I hadn't considered the case of nested handlers with multipe catch clauses, and it seems obvious now that that was the reason for requiring the braces. However, it still strikes me as an ugly linguistic inconsistency. It seems that it would have been better to apply rules similar to those for disambiguating nested if/else statements, extended to handle mulitple catch clauses. Gregg --- [ 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 Clem Dickey Guest Posted: Sat Mar 06, 2004 4:46 pm    Post subject: Re: try / catch syntax [email]gregglastname (AT) hotmail (DOT) com[/email] (Gregg) wrote in message news:<20d0a2b2.0403041413.13e57e69 (AT) posting (DOT) google.com>... Quote: What was the rationale behind requiring the bodies of try and catch to be blocks instead of statements Perhaps blocks are required to avoid a gratuitous difference with function definitions ;-) --- [ 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 Ken Hagan Guest Posted: Sun Mar 07, 2004 6:06 am    Post subject: Re: try / catch syntax It was an arbitrary decision. Bjarne considered allowing the braces to be absent but decided it would be safer to insist on them. (I think this is mentioned in "The Design and Evolution of C++", along with the point that, strictly speaking, the "try" is redundant too.) As for why the standard kept the rule rather than making it consistent with other parts of the language, I can only guess, but I would guess that the suggestion never came up. "try" and "catch" are suffiently rare in real programs that it probably doesn't matter. "Gregg" <gregglastname (AT) hotmail (DOT) com> wrote... Quote: What was the rationale behind requiring the bodies of try and catch to be blocks instead of statements (possibly compound)? It seems inconsistent with other similar constructs such as if / else. --- [ 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 Edward Diener Guest Posted: Sun Mar 07, 2004 7:10 am    Post subject: Re: try / catch syntax Tony Burrows wrote: Quote: Slightly differently Why no finally which gets executed whether try or catch happens. Ie, code that always runs before a function ends? As a Java programmer I find it useful at times. Java needs it because it has no deterministic destruction of objects. Java programmers have to make sure they manually release resources whereas C++ programmers don't have to worry about this at all. That is why a 'finally' has much less importance in C++ than it does in Java ( or C# ). So C++ never adopted such a measure, and I don't believe it needs it. A number of C++ compiler implementations have added it as an extension to C++. Quote: Tony On Fri, 05 Mar 2004 15:35:01 +0000, Ray Lischner wrote: On Friday 05 March 2004 10:04 am, Nicola Musatti wrote: I'm not convinced this is the only reason, because it would have been just as reasonable to stipulate that catch'es go with the closest try, as it happens with else's and if's. If this were the case, the example above would result in a "try without catch" error. The difference is that try can have mutliple catch parts. An if statement can have only one else part. --- [ 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 Andrew Koenig Guest Posted: Sun Mar 07, 2004 7:10 am    Post subject: Re: try / catch syntax ""P.J. Plauger"" <pjp (AT) dinkumware (DOT) com> wrote Quote: True, but there *still* could be a simple disambiguation rule for catch clauses. Each could bind to the immediately preceding try clause. Unfortunately, that rule does the wrong thing for the following case: try f(); try g(); catch(...) x(); catch(...) y(); Because each try must have at least one catch, there is only one sensible way to parse this example: The first catch must go with the second try, and the second catch must go with the first try. If each catch binds to the immediately preceding try, then the first try winds up without a corresponding catch. So I think a disambiguation rule would have to be more complicated to be useful. --- [ 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 P.J. Plauger Guest Posted: Sun Mar 07, 2004 5:20 pm    Post subject: Re: try / catch syntax ""Andrew Koenig"" <ark (AT) acm (DOT) org> wrote Quote: ""P.J. Plauger"" <pjp (AT) dinkumware (DOT) com> wrote in message news:RO22c.65045$C65.38591 (AT) nwrddc01 (DOT) gnilink.net... True, but there *still* could be a simple disambiguation rule for catch clauses. Each could bind to the immediately preceding try clause. Unfortunately, that rule does the wrong thing for the following case: try f(); try g(); catch(...) x(); catch(...) y(); Because each try must have at least one catch, there is only one sensible way to parse this example: The first catch must go with the second try, and the second catch must go with the first try. If each catch binds to the immediately preceding try, then the first try winds up without a corresponding catch. So I think a disambiguation rule would have to be more complicated to be useful. Agreed, but there are other simple rules in C/C++ that don't always give the "useful" result, such as x+++++y. Yet we've kept them for simplicity, in some dimension. This quibble doesn't alter the fact that it was probably wise to require compound statements after try and catch. P.J. Plauger Dinkumware, Ltd. http://www.dinkumware.com --- [ 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 Joshua Lehrer Guest Posted: Sun Mar 07, 2004 5:20 pm    Post subject: Re: try / catch syntax [email]tony (AT) tonyburrows (DOT) com[/email] (Tony Burrows) wrote in message news:<pan.2004.03.05.17.10.05.427664 (AT) tonyburrows (DOT) com>... Quote: Slightly differently Why no finally which gets executed whether try or catch happens. Ie, code that always runs before a function ends? As a Java programmer I find it useful at times. Tony Because in C++, stack based objects get destroyed deterministically. Whatever you would put into a finally block, just put into the destructor of a stack based object. Now, you might say that writing stack based objects just to have specialized destructors is a pain, and you'd be correct. Enter ScopeGuard, C++'s finally! http://www.cuj.com/documents/s=8000/cujcexp1812alexandr/alexandr.htm http://tinyurl.com/26vzp ScopeGuard lets you create stack-based object's, on the fly, who's destructor does whatever you want: FILE *pFile = FOPEN(...); ON_BLOCK_EXIT(fclose,pFile); -joshua lehrer factset research systems NYSE:FDS --- [ 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 Craig Henderson Guest Posted: Sun Mar 07, 2004 6:03 pm    Post subject: Re: try / catch syntax Quote: Why not allow try stmt1 catch (std::exception&) stmt2 where <stmt2> and <stmt2> could be either simple or compound (with braces) exactly as if / else? There can be many catch() statements associated with each try: try { } catch (std::exception const &) { } catch (...) { } and trys can be nested try { try { } catch (std::exception const &) { // handle known exception and absorb it } catch (...) { // report unknown exception and rethrow ... throw; } } catch (...) { // handle the unknown exception at an outer scope } Now, if the curlies were removed, this becomes try // (1) try // (2) <stmt1> catch (std::exception const &) <stmt2> catch (...) // (3) <stmt3> catch (...) // (4) -- ambigous <stmt4> The catch(...) at (4) has lost context, and will look as though it belongs to try (2), but has already been defined at (3). This is different to the if..else example you cited, as each 'if' can have zero or one else clauses, but try can have one or more catch-s. -- Craig --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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