C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  why arent C++ exceptions declared in function declaration?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message mentat79@gmail.com Guest Posted: Fri Dec 23, 2005 3:14 am    Post subject: why arent C++ exceptions declared in function declaration? Hello, In Java, if a function body has a throw(SomeException) statement, then, this must be explicitly declared in the function declaration via a throws SomeException clause. (yes yes as long as SomeExceptions is a descendant of Throwable or something like that) In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? Thanks! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Zara Guest Posted: Fri Dec 23, 2005 1:00 pm    Post subject: Re: why arent C++ exceptions declared in function declarati On 22 Dec 2005 22:14:19 -0500, "mentat79 (AT) gmail (DOT) com" <mentat79 (AT) gmail (DOT) com> wrote: Quote: Hello, In Java, if a function body has a throw(SomeException) statement, then, this must be explicitly declared in the function declaration via a throws SomeException clause. (yes yes as long as SomeExceptions is a descendant of Throwable or something like that) In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? First: Java is a really bad example, for two reasons: a) it is OT here and b) it has two categories of exceptions: rutime errors and exceptions, and you must declare only the latter. In C++, if you declare an exception list, you must stick to it. If you ever throw a different exception, you will have an unexpected exception situation. Really Bad Thing. And you are not responsible for the behaviour of any function you call. Compilers must create code for testing the execptions on every exception listed function, even when exception list is throw(), so thta code grows. Not listing exevptions has the advantage of letting any exception go fast and easily to its destination (catch or exit, depending on the structure of the program), putting no overhead on the functions that do not need exception processing. If exception lists were compulsory, any programmer should be forced to keep track of the exceptions thrown by functions called, thus enforcing recompilation with every change from the libraries used. BTW, that isone of the reasons why Java does not force runtime error lists. IMHO, Java is neither consistent nor safe in this respect, and C++ is, at least, safe and clearly easy to use. Best regards, Zara [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Luke Meyers Guest Posted: Fri Dec 23, 2005 1:06 pm    Post subject: Re: why arent C++ exceptions declared in function declaratio It is not purely stylistic. The semantics of a C++ exception specification ("throws declaration" [sic -- the 's' on the end is a Java-ism]) are quite different from what you would expect based on the Java idiom. Whereas a Java throws clause declares "these exceptions may be thrown and must be checked (enforced by the compiler); no other exceptions (other than RuntimeExceptions) will be thrown," a C++ exception specification declares "I promise that if I do throw an exception, it will be one of these types -- or, if one occurs that's not in this list, it will be thrown as a std::unexpected." The compiler does not have to do any checking for you like in Java, so putting in an exception specification doesn't force people to catch all your declared exceptions. They're not generally used (by informed programmers) because they easily lead to confusion, and don't provide much in the way of useful semantic guarantees. All they really do is provide documentation, and that's what comments are for. See Stroustrup 14.6 for more on these, and also see the relevant section in Sutter's style book (101 C++ Coding Standards) or whatever it's called. L [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top P.J. Plauger Guest Posted: Fri Dec 23, 2005 4:19 pm    Post subject: Re: why arent C++ exceptions declared in function declarati <mentat79 (AT) gmail (DOT) com> wrote Quote: In Java, if a function body has a throw(SomeException) statement, then, this must be explicitly declared in the function declaration via a throws SomeException clause. (yes yes as long as SomeExceptions is a descendant of Throwable or something like that) In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? C++ exception specifications are a paper tiger -- they were standardized before they were implemented. Unsurprisingly, they have proved not to be as useful as originally hoped. P.J. Plauger Dinkumware, Ltd. http://www.dinkumware.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Fri Dec 23, 2005 4:22 pm    Post subject: Re: why arent C++ exceptions declared in function declarati In article <1135274202.983557.310930 (AT) g47g2000cwa (DOT) googlegroups.com>, "mentat79 (AT) gmail (DOT) com" <mentat79 (AT) gmail (DOT) com> writes Quote: Hello, In Java, if a function body has a throw(SomeException) statement, then, this must be explicitly declared in the function declaration via a throws SomeException clause. (yes yes as long as SomeExceptions is a descendant of Throwable or something like that) In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? Because by the time that exceptions were introduced to the development of C++ the language was already 10 years old with millions of lines of existing code. The only way to make such a rule work would be by having the default (no exception specification) be 'nothrow' rather than the current 'can throw anything'. Actually experience suggests that exception specifications are flawed beyond salvation (and they are nowhere near as valuable in Java as the designers originally expected) in C++ where the existence of templates and generic programming cause major obstacles to exception specifications. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrew Koenig Guest Posted: Fri Dec 23, 2005 6:42 pm    Post subject: Re: why arent C++ exceptions declared in function declarati <mentat79 (AT) gmail (DOT) com> wrote Quote: In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? One fundamental reason is to permit, but not require, implementations to throw exceptions in the case of conditions such as overflow. If function declarations had to name all possible exceptions, then it would be necessary to define what exceptions could possibly occur in the case of overflow and related circumstances, and also to mention those exceptions in every function that does arithmetic. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top puzzlecracker Guest Posted: Mon Dec 26, 2005 2:13 am    Post subject: Re: why arent C++ exceptions declared in function declaratio Francis Glassborow wrote: Quote: In article <1135274202.983557.310930 (AT) g47g2000cwa (DOT) googlegroups.com>, "mentat79 (AT) gmail (DOT) com" <mentat79 (AT) gmail (DOT) com> writes Hello, In Java, if a function body has a throw(SomeException) statement, then, this must be explicitly declared in the function declaration via a throws SomeException clause. (yes yes as long as SomeExceptions is a descendant of Throwable or something like that) In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? Because by the time that exceptions were introduced to the development of C++ the language was already 10 years old with millions of lines of existing code. The only way to make such a rule work would be by having the default (no exception specification) be 'nothrow' rather than the current 'can throw anything'. Actually experience suggests that exception specifications are flawed beyond salvation (and they are nowhere near as valuable in Java as the designers originally expected) in C++ where the existence of templates and generic programming cause major obstacles to exception specifications. Quote: in C++ where the existence of templates and generic programming cause major obstacles to exception specifications. That would make java 5.0 suffer the same problem as generic code in C++ does. But, it didn't! It would be valid to assert that exception is C++ have the same share of usefulness as in Java given non-generic code. I agree with aforementioned statement - that programmers never really learnt how to use exception nor designed system based on latter before their implementation, thus avoiding them altogether, whereas in Java, exception specifications existed since the inception. [ 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: Wed Dec 28, 2005 12:37 pm    Post subject: Re: why arent C++ exceptions declared in function declaratio puzzlecracker wrote: Quote: Francis Glassborow wrote: In article [email]1135274202.983557.310930 (AT) g47g2000cwa (DOT) googlegroups.com[/email]>, "mentat79 (AT) gmail (DOT) com" <mentat79 (AT) gmail (DOT) com> writes In Java, if a function body has a throw(SomeException) statement, then, this must be explicitly declared in the function declaration via a throws SomeException clause. [...] Quote: in C++ where the existence of templates and generic programming cause major obstacles to exception specifications. That would make java 5.0 suffer the same problem as generic code in C++ does. But, it didn't! That's because, unlike what the original poster claims, Java exceptions aren't checked. Java only checks exceptions which don't derive from either RuntimeException or Error. In other words, errors which should be reported by a return value rather than an exception anyway. (Java is often more or less forced to use exceptions here because it doesn't have out parameters. Sort of a case of trying to correct one mistake with another.) In practice, I'm not convinced that there is a real conflict between exception specifications and generic programming, when exception specifications are used in a reasonable manner. Basically, the only really useful information in an exception specification is whether the function throws or not. Or rather, whether the function guarantees or not that it will not throw as part of its contractual interface. Quote: It would be valid to assert that exception is C++ have the same share of usefulness as in Java given non-generic code. I agree with aforementioned statement - that programmers never really learnt how to use exception nor designed system based on latter before their implementation, thus avoiding them altogether, whereas in Java, exception specifications existed since the inception. But they don't work any better than those in C++. (In fact, they actually are less useful, since there is no way in Java to declare that a function guarantees that it will not throw anything. In sum, the only useful declaration is not available.) -- 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 wij@seed.net.tw Guest Posted: Wed Dec 28, 2005 5:13 pm    Post subject: Re: why arent C++ exceptions declared in function declaratio kanze wrote: Quote: In practice, I'm not convinced that there is a real conflict between exception specifications and generic programming, when exception specifications are used in a reasonable manner. Basically, the only really useful information in an exception specification is whether the function throws or not. Or rather, whether the function guarantees or not that it will not throw as part of its contractual interface. Throw specification is not part of the function signature and can't be in typedef. Quote: It would be valid to assert that exception is C++ have the same share of usefulness as in Java given non-generic code. I agree with aforementioned statement - that programmers never really learnt how to use exception nor designed system based on latter before their implementation, thus avoiding them altogether, whereas in Java, exception specifications existed since the inception. But they don't work any better than those in C++. (In fact, they actually are less useful, since there is no way in Java to declare that a function guarantees that it will not throw anything. In sum, the only useful declaration is not available.) I don't use Java, but as mentioned by Andrew Koening, there are many cases like ++i or a+b with arithmetic types. throw specification is not easy to happen. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kk_oop@yahoo.com Guest Posted: Sat Dec 31, 2005 1:25 pm    Post subject: Re: why arent C++ exceptions declared in function declaratio [email]mentat79 (AT) gmail (DOT) com[/email] wrote: <snip> Quote: In C++ the throws declaration is optional. I would like to know why this is so. Is it just a stylistic issue or are there deeper reasons related to the C++ programming model? Here's an important consideration. If function f( ) has a "throws" declaration and f( ) throws an exception not in that declaration, unexpected(), defined in the C++ standard library, is invoked. The default behavior of unexpected() is to call terminate(). This is really dangerous if f( ) is using a third party library that may throw exceptions now or in future releases that the owner/maintainer of f( ) is not aware of. If one of those exceptions gets thrown in f( ), and f( ) lets it escape (i.e., does not catch it), termination will occur. If you are writing any safety critical code that must not abort, this makes f( ) very fragile, since it must be aware of every possible exception that can bubble up from any of its function calls now and forever. If you do not use an exception specification, such an exception would just quietly bubble up, hopefully to some generic catch at some higher level function. Note that if that generic catch were not there, the program would still abort, but it's far easier to have a generic catch in one place, than fragile exception specifications all over the place. - Ken [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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