C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  template argument deduction for reference to const array          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Guest Posted: Fri Apr 14, 2006 8:06 am    Post subject: template argument deduction for reference to const array I'm getting inconsistent results using templates with argument type of 'reference to array of const'. Example: template<int len> void doit( char const (&buffer)[len] ) {} int main() { char ch[10]; doit( ch ); } Should this code compile? MSVC.Net 2003 says 'yes' GCC 3.4.4/4.0.2 say 'yes' GCC 3.2.3 says 'no' Comeau 4.3.3 beta says 'no' I can't find a specific reference to this in the core issue list. It might be related to core issue 450 (Binding a reference to const to a cv-qualified array rvalue) Can anyone shed any light? Regards, Roger Orr -- MVP in C++ at www.brainbench.com Roger. --- [ 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 ] Back to top Tomás Guest Posted: Fri Apr 14, 2006 4:06 pm    Post subject: Re: template argument deduction for reference to const array Quote: template<int len void doit( char const (&buffer)[len] ) {} int main() { char ch[10]; doit( ch ); } I would haveve thought this would compile just fine, but I got an error when I tried to compile it there using g++. -Tomás --- [ 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 ] Back to top Martin Vejnár Guest Posted: Fri Apr 14, 2006 6:06 pm    Post subject: Re: template argument deduction for reference to const array rogero (AT) howzatt (DOT) demon.co.uk wrote: Quote: I'm getting inconsistent results using templates with argument type of 'reference to array of const'. Example: template<int len void doit( char const (&buffer)[len] ) {} int main() { char ch[10]; doit( ch ); } Should this code compile? Is it related only to templates? Should the following compile? void doit(char const (&)[10]) {} int main() { char ch[10]; doit( ch ); } AFAICS, "char []" -> "const char []" is not a standard conversion and should not compile. It, however, compiles fine with MSVC8 and Comeau, which leads me to believe, that I've overlooked something. Can someone point me to where the Standard says that the above is a standard conversion (or that it is legal to bind "char []" to "const char (&)[]")? -- Martin --- [ 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 ] Back to top Guest Posted: Fri Apr 14, 2006 8:06 pm    Post subject: Re: template argument deduction for reference to const array The code compiles successfully if a non-template function is used, and also if the template argument is specified: doit<10>( ch ); The problem seems to be to do with template argument deduction. Regards, Roger Orr. -- MVP in C++ at www.brainbench.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.comeaucomputing.com/csc/faq.html ] Back to top Graeme Prentice Guest Posted: Sat Apr 15, 2006 3:06 am    Post subject: Re: template argument deduction for reference to const array On Fri, 14 Apr 2006 02:35:39 CST, rogero (AT) howzatt (DOT) demon.co.uk wrote: Quote: I'm getting inconsistent results using templates with argument type of 'reference to array of const'. Example: template<int len void doit( char const (&buffer)[len] ) {} int main() { char ch[10]; doit( ch ); } Should this code compile? MSVC.Net 2003 says 'yes' GCC 3.4.4/4.0.2 say 'yes' GCC 3.2.3 says 'no' Comeau 4.3.3 beta says 'no' I can't find a specific reference to this in the core issue list. It might be related to core issue 450 (Binding a reference to const to a cv-qualified array rvalue) Can anyone shed any light? This is a bug in Comeau 4.3.3 (EDG) and fixed in the latest Comeau release. Daveed Vandevoorde described it here http://makeashorterlink.com/?U1C8216FC which I'm reproducing below for convenience <<<<<<<<<<, quote Daveede >>>>>>>>>> OK. Just work your way through 14.8.2.1. Para 1: Our A is "int[10]" and our P is "T const(&)[S]". Para 2: P is a reference type, so skip the 3 bullets. P is not a cv-qualified type (references never are), so skip the first sentence after the bullets. P is a reference type, so we use P' = "T const [S]" for deduction purposes. Para 3: We try to find T, S such that P' becomes identical to A. That's not possible, so we looks at the three bullets. The 2nd and 3rd don't apply, but the first implies that T=int, S=10 is OK because "int const[10]" is more cv-qualified that "int[10]" (see 3.9/5). So deduction succeeds. <<<<<<<<<<<<< end quote >>>>>>>>>>>>. However the wording in 14.8.2.1 is being tinkered with in active issue 522 http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#522 where the word transformed is added (the deduced A can be more CV qualified than the transformed A), but in this case, A and transformed A are the same. Note also issue 214 partial ordering, where references are first removed, then top level CV qualifiers are removed. http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#214 The standard doesn't define what a "top level" CV qualifier is. Presumably with int const arr[10]; the const is a top level CV qualifier. In 5.2.8/5, const&D is implied as having a top level CV qualifier, which suggests to me that 14.8.2.1/2 would benefit from pointing out that references are never "CV qualified types" as Daveede does above. Graeme --- [ 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 ] Back to top Martin Vejnár Guest Posted: Sun Apr 16, 2006 5:06 pm    Post subject: Re: template argument deduction for reference to const array Martin Vejnár wrote: Quote: [...] Should the following compile? void doit(char const (&)[10]) {} int main() { char ch[10]; doit( ch ); } AFAICS, "char []" -> "const char []" is not a standard conversion and should not compile. It, however, compiles fine with MSVC8 and Comeau, which leads me to believe, that I've overlooked something. Can someone point me to where the Standard says that the above is a standard conversion (or that it is legal to bind "char []" to "const char (&)[]")? Ok, I see it now. [8.3.4/1, note at the end of the paragraph] ..an “array of N cv-qualifier-seq T” has cv-qualified type... That makes the 'const' in "const char []" a top-level cv-qualifier. According to 8.5.3, the "char []" can be bound directly to "const char (&)[]". -- Martin --- [ 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 ] Back to top Bronek Kozicki Guest Posted: Sun Apr 16, 2006 5:06 pm    Post subject: Re: template argument deduction for reference to const array rogero (AT) howzatt (DOT) demon.co.uk wrote: Quote: Comeau 4.3.3 beta says 'no' it compiles successfully on Como 4.3.4.1 (beta), which is based on fixed version of EDG frontend B. --- [ 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 ] Back to top Roger Orr Guest Posted: Sun Apr 16, 2006 7:06 pm    Post subject: Re: template argument deduction for reference to const array "Graeme Prentice" <gp.kiwi (AT) gmail (DOT) com> wrote in message news:pph042pknqqulr25djf25ocl7ltnbujg31 (AT) 4ax (DOT) com... Quote: On Fri, 14 Apr 2006 02:35:39 CST, rogero (AT) howzatt (DOT) demon.co.uk wrote: [snip] This is a bug in Comeau 4.3.3 (EDG) and fixed in the latest Comeau release. Daveed Vandevoorde described it here http://makeashorterlink.com/?U1C8216FC which I'm reproducing below for convenience Thanks, I missed that thread. Quote: However the wording in 14.8.2.1 is being tinkered with in active issue 522 http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#522 where the word transformed is added (the deduced A can be more CV qualified than the transformed A), but in this case, A and transformed A are the same. 14.8.2.1 is pretty complex ...but I believe the existing wording covers this case already. But I alway worry when I disagree with Comeau because it's usually me at fault. Regards, Roger Orr -- MVP in C++ at www.brainbench.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.comeaucomputing.com/csc/faq.html ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language, library and standards All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.