C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Catching class exception          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Dave Guest Posted: Thu Jan 27, 2005 4:34 pm    Post subject: Catching class exception try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? Is it guaranteed to work, guaranteed to not work or implementation defined? Thanks, Dave Back to top Victor Bazarov Guest Posted: Thu Jan 27, 2005 4:45 pm    Post subject: Re: Catching class exception Dave wrote: Quote: try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? What do you mean by "the right thing"? Quote: Is it guaranteed to work, guaranteed to not work or implementation defined? The 'what' member function returns some valid pointer. Displaying it is what it's there for. What specifically it will return is defined by the implementation. If you catch the base class like this, it will return something "suitable for conversion and display", but not necessarily meaningful. The code will definitely not call the derived class' "what()" member. V Back to top Dave Guest Posted: Thu Jan 27, 2005 4:56 pm    Post subject: Re: Catching class exception "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: Dave wrote: try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? What do you mean by "the right thing"? By "the right thing", I'm asking if it will return a pointer to the string that was used to construct the *derived* object as opposed to a pointer to some other string. Is it guaranteed to work, guaranteed to not work or implementation defined given that the base what() is being called? Quote: Is it guaranteed to work, guaranteed to not work or implementation defined? The 'what' member function returns some valid pointer. Displaying it is what it's there for. What specifically it will return is defined by the implementation. If you catch the base class like this, it will return something "suitable for conversion and display", but not necessarily meaningful. The code will definitely not call the derived class' "what()" member. V Back to top Victor Bazarov Guest Posted: Thu Jan 27, 2005 5:22 pm    Post subject: Re: Catching class exception Dave wrote: Quote: "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message news:2V8Kd.39238$NC6.13387 (AT) newsread1 (DOT) mlpsca01.us.to.verio.net... Dave wrote: try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? What do you mean by "the right thing"? By "the right thing", I'm asking if it will return a pointer to the string that was used to construct the *derived* object What derived object? What string passed to construct it? std::exception has *no idea* about them. Quote: as opposed to a pointer to some other string. Of course it's a pointer to some other string. 'std::exception' has no constructor that takes a C string or a C++ string. Quote: Is it guaranteed to work, guaranteed to not work or implementation defined given that the base what() is being called? What guaranteed to work? I write the class: class Base { public: Base(); virtual ~Base(); virtual int getnumber() const { return 42; } }; How in the world should *I* know that *you* derived from it and made *your* class to be constructed with some other number, which *you* intended to *your* overridden 'getnumber' to return? Just think a bit before replying. V Back to top Dave Guest Posted: Thu Jan 27, 2005 6:21 pm    Post subject: Re: Catching class exception "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: Dave wrote: "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message news:2V8Kd.39238$NC6.13387 (AT) newsread1 (DOT) mlpsca01.us.to.verio.net... Dave wrote: try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? What do you mean by "the right thing"? By "the right thing", I'm asking if it will return a pointer to the string that was used to construct the *derived* object What derived object? What string passed to construct it? std::exception has *no idea* about them. as opposed to a pointer to some other string. Of course it's a pointer to some other string. 'std::exception' has no constructor that takes a C string or a C++ string. Is it guaranteed to work, guaranteed to not work or implementation defined given that the base what() is being called? What guaranteed to work? I write the class: class Base { public: Base(); virtual ~Base(); virtual int getnumber() const { return 42; } }; How in the world should *I* know that *you* derived from it and made *your* class to be constructed with some other number, which *you* intended to *your* overridden 'getnumber' to return? Just think a bit before replying. V OK, I guess I have to spell this out for you... The definition of class std::exception contains the following declaration: virtual const char* what() const throw(); Now take class std::logic_error, which derives publicly from class exception, as an example. class logic_error *must* be constructed with a std::string (for later use by what()), class exception *may not* be constructed with a std::string. It may only be default or copy constructed. The question comes down to where the string that what() uses lives. It can live either in class exception or it can live in the classes derived from exception. Where the string lives determines what happens when I catch a class exception by *value* and therefore end up calling the *base* what(). If the std::string lives in class exception (i.e. class exception holds the string for the derived classes and they inherit it), the right thing will print out upon using what(). If each derived class is responsible for holding its own "what" string, the right thing will not print out upon using what(). In my mind, the behavior is implementation defined, bu tI was hoping to get confirmation of that. Just think a bit before replying. Back to top Victor Bazarov Guest Posted: Thu Jan 27, 2005 6:29 pm    Post subject: Re: Catching class exception Dave wrote: Quote: [...] OK, I guess I have to spell this out for you... :-) Quote: The definition of class std::exception contains the following declaration: virtual const char* what() const throw(); Now take class std::logic_error, which derives publicly from class exception, as an example. class logic_error *must* be constructed with a std::string (for later use by what()), "for later use" by WHOSE "what()"? It's a virtual function. Quote: class exception *may not* be constructed with a std::string. It may only be default or copy constructed. The question comes down to where the string that what() uses lives. It can live either in class exception or it can live in the classes derived from exception. Where the string lives determines what happens when I catch a class exception by *value* and therefore end up calling the *base* what(). If the std::string lives in class exception (i.e. class exception holds the string for the derived classes and they inherit it), the right thing will print out upon using what(). If each derived class is responsible for holding its own "what" string, the right thing will not print out upon using what(). Yes, you get it now! Quote: In my mind, the behavior is implementation defined, bu tI was hoping to get confirmation of that. The Standard says NOTHING about all this, as I already told you. Quote: Just think a bit before replying. Get your own copy of the Standard and stop wasting our time. V Back to top Dave Guest Posted: Thu Jan 27, 2005 6:38 pm    Post subject: Re: Catching class exception "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: Dave wrote: [...] OK, I guess I have to spell this out for you... :-) The definition of class std::exception contains the following declaration: virtual const char* what() const throw(); Now take class std::logic_error, which derives publicly from class exception, as an example. class logic_error *must* be constructed with a std::string (for later use by what()), "for later use" by WHOSE "what()"? It's a virtual function. class exception *may not* be constructed with a std::string. It may only be default or copy constructed. The question comes down to where the string that what() uses lives. It can live either in class exception or it can live in the classes derived from exception. Where the string lives determines what happens when I catch a class exception by *value* and therefore end up calling the *base* what(). If the std::string lives in class exception (i.e. class exception holds the string for the derived classes and they inherit it), the right thing will print out upon using what(). If each derived class is responsible for holding its own "what" string, the right thing will not print out upon using what(). Yes, you get it now! I got it all along. You did not understand what I was asking. Quote: In my mind, the behavior is implementation defined, bu tI was hoping to get confirmation of that. The Standard says NOTHING about all this, as I already told you. Just think a bit before replying. Get your own copy of the Standard and stop wasting our time. You are absolutely free to not respond to any question if you so choose. Nobody has an obligation to provide an answer in this or any NG. Nobody can waste your time but you. Quote: V Back to top Efrat Regev Guest Posted: Thu Jan 27, 2005 9:49 pm    Post subject: Re: Catching class exception I'm not sure if this is what you're asking, but Alexandrescu and Sutter explain in "C++ Coding Standards" a rationale for throwing by value and catching by reference. HTH "Dave" <better_cs_now (AT) yahoo (DOT) com> wrote Quote: try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? Is it guaranteed to work, guaranteed to not work or implementation defined? Thanks, Dave Back to top Andrey Tarasevich Guest Posted: Thu Jan 27, 2005 10:04 pm    Post subject: Re: Catching class exception Dave wrote: Quote: try { ... } catch (exception e) { cout << e.what() << endl; } In the code above, e is caught by value rather than polymorphically (assume that what was thrown is derived from exception). What does the Standard say about whether or not the call to e.what() will do the right thing? Is it guaranteed to work, guaranteed to not work or implementation defined? This code will, of course, slice the exception object, i.e 'std::exception::what()' will be called, not the derived class' 'what()'. The string, pointer to which is returned by 'std::exception::what()', is implementation defined, which means that in general case it will not "do the right thing". In order to make it "do the right thing" the exception has to be caught by reference. -- Best regards, Andrey Tarasevich Back to top Old Wolf Guest Posted: Thu Jan 27, 2005 10:27 pm    Post subject: Re: Catching class exception Victor Bazarov wrote: Quote: Dave wrote: "Victor Bazarov" wrote: Dave wrote: try { ... } catch (exception e) { cout << e.what() << endl; } What does the Standard say about whether or not the call to e.what() will do the right thing? What do you mean by "the right thing"? He obviously means, if the ... above contained 'throw runtime_error("foo");', will "foo" be written to cout? Quote: as opposed to a pointer to some other string. Of course it's a pointer to some other string. 'std::exception' has no constructor that takes a C string or a C++ string. Non-sequitur. std::exception could contain a string internally, even though it has no constructor that takes one. Quote: Is it guaranteed to work, guaranteed to not work or implementation defined given that the base what() is being called? What guaranteed to work? The above code example ! Quote: I write the class: class Base { public: Base(); virtual ~Base(); virtual int getnumber() const { return 42; } }; How in the world should *I* know that *you* derived from it and made *your* class to be constructed with some other number, which *you* intended to *your* overridden 'getnumber' to return? The question is, is that a valid implementation for std::exception? It would be easy to implement std::exception in a way that does 'work'. Quote: Just think a bit before replying. You can't solve that one by thinking, without reading the Standard. And if he should follow your suggestion and not bother this newsgroup with questions, perhaps we should shut down the newsgroup entirely? Back to top Victor Bazarov Guest Posted: Thu Jan 27, 2005 10:57 pm    Post subject: Re: Catching class exception Old Wolf wrote: Quote: [...] You can't solve that one by thinking, without reading the Standard. And if he should follow your suggestion and not bother this newsgroup with questions, perhaps we should shut down the newsgroup entirely? Look, the Standard is available, and we're here not to re-type or copy/paste it into our answers. That's one. Second, I am fairly certain that even in the absence of the Standard, the OP has some sort of documentation describing the behaviour of the 'std::exception' class, which undoubtedly says something about this. That's two. Third, there is other documentation readily available on the Web, which probably describes this just as well. If that all means we need to shut down the newsgroup, then we need to shut it down. *I* don't think so. But the OP's question was not about something that cannot be figured out without the newsgroup. And I still think that questions, answers to which can be found elsewhere, should only be asked here after a bit of thinking. That is what I suggested. And get off my case on this. Now, Quote: The question is, is that a valid implementation for std::exception? Or course it is. The Standard says nothing about what 'what' should return except that it's an NTBS suitable for conversion and display. Quote: It would be easy to implement std::exception in a way that does 'work'. That is totally irrelevant. Back to top BigBrian Guest Posted: Fri Jan 28, 2005 4:38 am    Post subject: Re: Catching class exception Quote: Old Wolf wrote: [...] You can't solve that one by thinking, without reading the Standard. And if he should follow your suggestion and not bother this newsgroup with questions, perhaps we should shut down the newsgroup entirely? Look, the Standard is available, and we're here not to re-type or copy/paste it into our answers. That's one. Second, I am fairly certain that even in the absence of the Standard, the OP has some sort of documentation describing the behaviour of the 'std::exception' class, which undoubtedly says something about this. That's two. Third, there is other documentation readily available on the Web, which probably describes this just as well. If that all means we need to shut down the newsgroup, then we need to shut it down. *I* don't think so. But the OP's question was not about something that cannot be figured out without the newsgroup. And I still think that questions, answers to which can be found elsewhere, should only be asked here after a bit of thinking. That is what I suggested. And get off my case on this. That may all be true, but you don't have to be a jerk! The Standard may not be understandable, or the OP may not have access to the Standard, or any number of other reasons may exist why they're posting the question to this newsgroup. Quote: Now, The question is, is that a valid implementation for std::exception? Or course it is. The Standard says nothing about what 'what' should return except that it's an NTBS suitable for conversion and display. It would be easy to implement std::exception in a way that does 'work'. That is totally irrelevant. I could be completely wrong, but I believe the answer the OP is looking for is that he needs to catch by reference so that the call to what() behaves polymorphically instead of slicing the thrown object down to std::exception. 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