C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Can "export" be used with explicit specialization?          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Hubert Holin Guest Posted: Fri Oct 21, 2005 2:47 pm    Post subject: Can "export" be used with explicit specialization? Somewhere in the E.U., le 21/10/2005 Bonjour Reading 14.7.3, it is not clear to me if it is possible to export an explicit specialization of a non-exported template function. In the example below, "grabuge.hpp" declares a template and two explicit specialization, one of which is defined (and exported) in the file "grabuge.cpp" (and the second of which is never defined). Is the example legal or not? Since the optional "export" keyword is not listed in the grammar detailing explicit specialization, I would guess the program is perhaps, after all, not legal. In that case, it would appear to be a deficiency of the standard. The closest thing I could find to that topic is clause 8, which does not seem to be entirely relevant. Merci Hubert Holin 8>< ------------ main.cpp ------------ ><8 #include "grabuge.hpp" int main (int argc, char * const argv[]) { ::std::cout << "There are " << argc << " arguments, with values: " << ::std::endl; for (int idx = 0; idx < argc; ++idx) { switch (idx) { case 0: ::std::cout << grabuge<0> << argv[idx] << ::std::endl; break; default: ::std::cout << grabuge<-1> << argv[idx] << ::std::endl; break; } } return 0; } 8>< --------------------------------- ><8 8>< ---------- grabuge.hpp----------- ><8 #include template<int selector> ::std::ostream & grabuge(::std::ostream & s) { return(s); } template<> ::std::ostream & grabuge<0>(::std::ostream & s); template<> ::std::ostream & grabuge<1>(::std::ostream & s); // actually not defined! 8>< --------------------------------- ><8 8>< ---------- grabuge.cpp----------- ><8 #include "grabuge.hpp" export template<> ::std::ostream & grabuge<0>(::std::ostream & s) { return(s << "The 0th argument, by tradition, is: "); } 8>< --------------------------------- ><8 --- [ 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 google@vandevoorde.com Guest Posted: Sat Oct 22, 2005 6:40 pm    Post subject: Re: Can "export" be used with explicit specialization? Hubert Holin wrote: Quote: Somewhere in the E.U., le 21/10/2005 Bonjour Reading 14.7.3, it is not clear to me if it is possible to export an explicit specialization of a non-exported template function. No, the grammar doesn't allow it. Only the grammar rule in 14/1 allows for the keyword "export". Quote: In the example below, "grabuge.hpp" declares a template and two explicit specialization, one of which is defined (and exported) in the file "grabuge.cpp" (and the second of which is never defined). Is the example legal or not? Since the optional "export" keyword is not listed in the grammar detailing explicit specialization, I would guess the program is perhaps, after all, not legal. Right. Quote: In that case, it would appear to be a deficiency of the standard. Can you elaborate? Why is it a deficiency? Daveed --- [ 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 Hubert Holin Guest Posted: Mon Oct 24, 2005 4:25 pm    Post subject: Re: Can "export" be used with explicit specialization? Somewhere in the E.U., le 24/10/2005 Bonjour In article <1129933108.040492.203720 (AT) z14g2000cwz (DOT) googlegroups.com>, [email]google (AT) vandevoorde (DOT) com[/email] wrote: Quote: Hubert Holin wrote: Somewhere in the E.U., le 21/10/2005 Bonjour Reading 14.7.3, it is not clear to me if it is possible to export an explicit specialization of a non-exported template function. [SNIP] Quote: Is the example legal or not? Since the optional "export" keyword is not listed in the grammar detailing explicit specialization, I would guess the program is perhaps, after all, not legal. Right. In that case, it would appear to be a deficiency of the standard. Can you elaborate? Why is it a deficiency? Daveed My belief that this represents a deficiency stems from the assumption that the "export" keyword exists to provide some amount of code modularity (in addition to potential code hiding): the user of the template need only know the methods of the template, not their implementation. I therefore find it odd that one can export general methods (the unspecialized template), but not specific ones (the explicitly specialized template). Certainly, the compiler has to be aware that these explicit instantiations exist, but I fail to see how this differs from the fact it has to be aware that the exported (unspecialized) template exists. What am I missing? Merci Hubert Holin --- [ 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 Ben Hutchings Guest Posted: Tue Oct 25, 2005 1:51 am    Post subject: Re: Can "export" be used with explicit specialization? Hubert Holin <Hubert.Holin (AT) lmd (DOT) polytechnique.fr> wrote: <snip> Quote: My belief that this represents a deficiency stems from the assumption that the "export" keyword exists to provide some amount of code modularity (in addition to potential code hiding): the user of the template need only know the methods of the template, not their implementation. I therefore find it odd that one can export general methods (the unspecialized template), but not specific ones (the explicitly specialized template). Certainly, the compiler has to be aware that these explicit instantiations exist, but I fail to see how this differs from the fact it has to be aware that the exported (unspecialized) template exists. What am I missing? In practice, "export" causes the compiler to generate information about a template definition in or alongside the object file that it produces, so that the template can be implicitly instantiated between the normal compilation process and linking (or by the linker calling the compiler when it recognises an undefined symbol as being satisfiable by such instantiation). I assume that the standard committee did not intend this to be done by default because it would bloat object files generated from the compilation of source files that use the existing model of including template definitions in every translation unit that requires them, and perhaps also because it could effectively reveal proprietary source code. For full specialisations, this issue does not arise. Linking to a full specialisation of a function template works much like linking to an ordinary function; callers only need the function's declaration and (at link time) its object code. -- Ben Hutchings For every complex problem there is a solution that is simple, neat, and wrong. --- [ 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 google@vandevoorde.com Guest Posted: Tue Oct 25, 2005 3:24 am    Post subject: Re: Can "export" be used with explicit specialization? Hubert Holin wrote: Quote: Somewhere in the E.U., le 24/10/2005 Bonjour In article <1129933108.040492.203720 (AT) z14g2000cwz (DOT) googlegroups.com>, [email]google (AT) vandevoorde (DOT) com[/email] wrote: Hubert Holin wrote: Somewhere in the E.U., le 21/10/2005 Bonjour Reading 14.7.3, it is not clear to me if it is possible to export an explicit specialization of a non-exported template function. [SNIP] Is the example legal or not? Since the optional "export" keyword is not listed in the grammar detailing explicit specialization, I would guess the program is perhaps, after all, not legal. Right. In that case, it would appear to be a deficiency of the standard. Can you elaborate? Why is it a deficiency? Daveed My belief that this represents a deficiency stems from the assumption that the "export" keyword exists to provide some amount of code modularity (in addition to potential code hiding): the user of the template need only know the methods of the template, not their implementation. I therefore find it odd that one can export general methods (the unspecialized template), but not specific ones (the explicitly specialized template). Certainly, the compiler has to be aware that these explicit instantiations exist, but I fail to see how this differs from the fact it has to be aware that the exported (unspecialized) template exists. What am I missing? Explicit template specializations are not templates and can therefore be separately compiled (or not) in the same way as their nonparameterized counterparts. For example: // File f.hpp: template<typename T> void f() {} template<> void f<int>(); // Declaration only // File f.cpp: #include "f.hpp" template<> void f<int>() {} The original intent was that template be separable in the exact same way, with no special new keyword required (although you would have been able to say "extern" as you can with ordinary functions and variables). The original Cfront implementation tried to fake this by making some assumptions about file organization. However, that scheme was not standardizable and for a long time the standard was unclear about template declaration/definition separation. Eventually a compromise was developed that required separated templates to be marked with "export". For nontemplates (including explicit specializations), that was not needed though. Daveed --- [ 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 Hubert Holin Guest Posted: Thu Oct 27, 2005 5:20 pm    Post subject: Re: Can "export" be used with explicit specialization? Somewhere in the E.U., le 27/10/2005 Bonjour In article <1130181540.805807.160460 (AT) g44g2000cwa (DOT) googlegroups.com>, [email]google (AT) vandevoorde (DOT) com[/email] wrote: [explanation of why "export" is not needed for explicit specialization of templates] I see now your point. It is therefore not a deficiency in the standard. Still some wording could be added (as a note, preferably), so as to explain (as you did) why it is thus and to prevent others from getting confused. Merci Hubert Holin --- [ 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 kanze Guest Posted: Tue Nov 01, 2005 4:31 am    Post subject: Re: Can "export" be used with explicit specialization? [email]google (AT) vandevoorde (DOT) com[/email] wrote: Quote: Hubert Holin wrote: Somewhere in the E.U., le 24/10/2005 In article <1129933108.040492.203720 (AT) z14g2000cwz (DOT) googlegroups.com>, [email]google (AT) vandevoorde (DOT) com[/email] wrote: Hubert Holin wrote: Somewhere in the E.U., le 21/10/2005 Reading 14.7.3, it is not clear to me if it is possible to export an explicit specialization of a non-exported template function. [SNIP] Is the example legal or not? Since the optional "export" keyword is not listed in the grammar detailing explicit specialization, I would guess the program is perhaps, after all, not legal. Right. In that case, it would appear to be a deficiency of the standard. Can you elaborate? Why is it a deficiency? My belief that this represents a deficiency stems from the assumption that the "export" keyword exists to provide some amount of code modularity (in addition to potential code hiding): the user of the template need only know the methods of the template, not their implementation. I therefore find it odd that one can export general methods (the unspecialized template), but not specific ones (the explicitly specialized template). Certainly, the compiler has to be aware that these explicit instantiations exist, but I fail to see how this differs from the fact it has to be aware that the exported (unspecialized) template exists. What am I missing? Explicit template specializations are not templates and can therefore be separately compiled (or not) in the same way as their nonparameterized counterparts. For example: // File f.hpp: template<typename T> void f() {} template<> void f<int>(); // Declaration only // File f.cpp: #include "f.hpp" template<> void f<int>() {} Does this mean that, as with normal functions, providing a definition in the .hpp file would result in undefined behavior (supposing that the file was included in more than one translation unit). -- 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 --- [ 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 google@vandevoorde.com Guest Posted: Wed Nov 02, 2005 3:51 pm    Post subject: Re: Can "export" be used with explicit specialization? kanze wrote: Quote: google (AT) vandevoorde (DOT) com wrote: [...] // File f.hpp: template<typename T> void f() {} template<> void f<int>(); // Declaration only // File f.cpp: #include "f.hpp" template<> void f<int>() {} Does this mean that, as with normal functions, providing a definition in the .hpp file would result in undefined behavior (supposing that the file was included in more than one translation unit). Indeed and it would in practice result in a duplicate symbol linker error. (Unless, of course, the specialization is an inline function.) Daveed --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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