From 3999515605481792952 X-Google-Thread: f78e5,adf0c498ff865076,start X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news4.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local02.nntp.dca.giganews.com!nntp.speakeasy.net!news.speakeasy.net.POSTED!not-for-mail NNTP-Posting-Date: Fri, 21 Jul 2006 02:10:01 -0500 Return-Path: X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Original-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@mailman.ucar.edu From: "Oleg" Newsgroups: comp.std.c++ Subject: Exception handling. Extension request Organization: http://groups.google.com Message-ID: <1153464622.485595.254900@i42g2000cwa.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" X-Complaints-To: groups-abuse@google.com User-Agent: G2/0.2 X-HTTP-UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; ru; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4,gzip(gfe),gzip(gfe) X-HTTP-Via: 1.1 academ.org:7777 (squid/2.5.STABLE9) Complaints-To: groups-abuse@google.com Injection-Info: i42g2000cwa.googlegroups.com; posting-host=81.222.113.153; posting-account=TxfnwQ0AAABWD8cPztY8NZeutFlPwa7C X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Approved: Fergus Henderson , moderator of comp.std.c++ X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Date: Fri, 21 Jul 2006 02:09:46 CST Lines: 187 NNTP-Posting-Host: 65.182.171.162 X-Trace: sv3-DgYkhMDwnKeC9DOe8q1unU43QpHG4ylQQ9r+Mz5XZ89UrFMtZw7hPF1w2HSKeGvZoNCnWjcDGcMyvJK!PDy5fj05gCEPC2UXkJo9Lun6j+ca2TMni7S/7Ry0J2AvRtXiT6F2bIZEPZB6clJWcX8LCAYEw0EE!89yGL7+wS/bhNlylNijnZU2A2DX8ResKyKhXs1wPHZw= X-Complaints-To: abuse@speakeasy.net X-DMCA-Complaints-To: abuse@speakeasy.net X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.32 Xref: g2news2.google.com comp.std.c++:2928 Hello! In current C++ language exception handling is defined with well structured exception hierarchies in mind. In this case exception handling is simple: catch(base_exception& e) and be happy. Problem arises when one have to handle many unrelated exceptions that does not correspond to single hierarchy or even in a not so rare case where one needs to provide different responses for different exceptions in the same hierarchy. The first is common on subsystem boundaries like C++ - C or C++ - GUI event handlers. The second is common in system-level errors handling as it is in boost::filesystem library. There are workarounds exists that are based on "throw in catch(...)" idiom: try { //... } catch(...) { handle_exceptions(); } void handle_exceptions() { assert(std::uncaught_exception()); try { throw; } catch (exception_type1& e) { //handle it } catch (exception_type2& e) { //handle it } catch (exception_type3& e) { //handle it } // etc. } But I believe that the problem mentioned can be elegantly solved on a language level. All we need is just allowing the following: class A {}; class B { public : B(A const& a); }; void foo() try { throw A; } catch (B b) { //caught } The idea behind it is simple: if A was thrown and B can be constructed from it then the catch(B b) handler is called. With this extension one can do the following: typedef boost::variant< int, std::string > my_exception; class my_exception_visitor : public boost::static_visitor<> { public: void operator()(int & i) const { //... } void operator()(std::string & str) const { //... } }; void foo() try { if(...) throw 1; else throw std::string("Hello world!"); } catch (my_exception e) { boost::apply_visitor( my_exception_visitor(), e ); } In order to use polymorphic exceptions one can do: #include #include class base { public : virtual void print() const { std::cout << "base\n"; } }; class child : public base { public : void print() const { std::cout << "child\n"; } }; class my_visitor : public boost::static_visitor<> { public: void operator()(base& b) const { std::cout << "visit base\n"; b.print(); } // it is not called void operator()(child& c) const { std::cout << "visit child\n"; c.print(); } void operator()(int i) const { std::cout << "int\n"; } }; #include int main() { child c; typedef boost::variant, int> var_t; // boost::reference_wrapper should be patched to allow the next line to compile // the following constructor should be added to the boost::reference_wrapper class: // template // reference_wrapper(reference_wrapper const& rw): t_(rw.get_pointer()) {} var_t v = boost::ref(c); boost::apply_visitor( my_visitor(), v ); // prints: // visit base // child base b; v = boost::ref(b); boost::apply_visitor( my_visitor(), v ); // prints: // visit base // base std::cin.get(); return 0; } For boost::filesystem library it means that its clients can use uniform "exception handling" even without exceptions at all. Versions of functions with error_code could be replaced with templates that can accept any visitor instead of error_code. It would be a high-level, type-safe, reusable and maintainable way to handle system-level errors in this particular case and in common case where catch(base_exception&) is not enough. This post is a call for comments/opinions/improvements of the basic idea presented. Best, Oleg Abrosimov. --- [ 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.comeaucomputing.com/csc/faq.html ]