C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  static template member          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Artem Alimarine Guest Posted: Thu Jun 22, 2006 4:01 pm    Post subject: static template member Hi all, The program below illustrates my problem. It compiles on VC8, also on GCC 3.4.4. (cygwin special) and on GCC 4.1.0 20060304 (Red Hat 4.1.0-3) However it does not link with GCCs: the static symbol is not found. I can make it link by uncommenting the second commented line, but then VC8 will refuse to work. I do not understand why one would need the two lines with template<> and template to make it work. Looks a bit shamanic to me. VC8 works as it is and also when the first commented line is used instead of template<> ... My questions are: Who is right: GCC or VC8? How do I write such a program in standard-compliant way that works with both compilers? ( I understand that good old ifdef will do but ... Artem //------------------------------------------------------------------- template<class T> struct foo { int method(int x) { return 0; } static foo<T> the_instance; }; struct bar { int method() { return foo<bar>::the_instance.method(1); } }; // foo<bar> foo<bar>::the_instance; //template foo<bar> foo<bar>::the_instance; template<> foo<bar> foo<bar>::the_instance; int main() { bar the_bar; the_bar.method(); } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Vladimir Marko Guest Posted: Fri Jun 23, 2006 3:03 pm    Post subject: Re: static template member Artem Alimarine wrote: Quote: Hi all, The program below illustrates my problem. It compiles on VC8, also on GCC 3.4.4. (cygwin special) and on GCC 4.1.0 20060304 (Red Hat 4.1.0-3) However it does not link with GCCs: the static symbol is not found. I can make it link by uncommenting the second commented line, but then VC8 will refuse to work. I do not understand why one would need the two lines with template<> and template to make it work. Looks a bit shamanic to me. VC8 works as it is and also when the first commented line is used instead of template<> ... My questions are: Who is right: GCC or VC8? How do I write such a program in standard-compliant way that works with both compilers? ( I understand that good old ifdef will do but ... Artem //------------------------------------------------------------------- template<class T struct foo { int method(int x) { return 0; } static foo<T> the_instance; }; struct bar { int method() { return foo<bar>::the_instance.method(1); } }; // foo<bar> foo<bar>::the_instance; //template foo<bar> foo<bar>::the_instance; template<> foo<bar> foo<bar>::the_instance; int main() { bar the_bar; the_bar.method(); } I'd suggest to define template <class T> foo<T> foo<T>::the_instance; but if you really want to use the explicit specialization you should read 14.7.3 very carefully. In particular, 14.7.3/15 says An explicit specialization of a static data member of a template is a definition if the declaration includes an initializer; otherwise, it is a declaration. [Note: ...] So the correct way to _define_ foo<bar>::the_instance is template<> foo<bar> foo<bar>::instance=foo<bar>(); (And should foo<bar> be non-CopyConstructible you'd need to find some workaround, say a constructor with a dummy parameter.) The other problem in your code is that you use foo<bar>::the_instance before you declare the explicit specialization. According to 14.7.3/6 this makes your code ill-formed without the need for a diagnostic. Thus, the correct way to write the program is to put the declaration of the explicit specialization before its first use and define it using the initializer. gcc 3.3 and 4.0 accept that and I hope the VC8 will accept it as well. Vladimir Marko [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Valentin Samko Guest Posted: Fri Jun 23, 2006 3:09 pm    Post subject: Re: static template member Artem Alimarine wrote: Quote: Hi all, The program below illustrates my problem. It compiles on VC8, also on GCC 3.4.4. (cygwin special) and on GCC 4.1.0 20060304 (Red Hat 4.1.0-3) However it does not link with GCCs: the static symbol is not found. I can make it link by uncommenting the second commented line, but then VC8 will refuse to work. I do not understand why one would need the two lines with template<> and template to make it work. Looks a bit shamanic to me. That looks like g++ bug. In any case, you can have template<class T> foo<T> foo<T>::the_instance; template foo<bar> foo<bar>::the_instance; which compiles with g++, comeau c++, vc7, vc8, intel c++ :) -- Valentin Samko - http://www.valentinsamko.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Greg Herlihy Guest Posted: Fri Jun 23, 2006 3:09 pm    Post subject: Re: static template member Artem Alimarine wrote: Quote: Hi all, The program below illustrates my problem. It compiles on VC8, also on GCC 3.4.4. (cygwin special) and on GCC 4.1.0 20060304 (Red Hat 4.1.0-3) However it does not link with GCCs: the static symbol is not found. I can make it link by uncommenting the second commented line, but then VC8 will refuse to work. I do not understand why one would need the two lines with template<> and template to make it work. Looks a bit shamanic to me. VC8 works as it is and also when the first commented line is used instead of template<> ... My questions are: Who is right: GCC or VC8? They are both right that the program is wrong. Quote: How do I write such a program in standard-compliant way that works with both compilers? ( By adding what is missing. Quote: //------------------------------------------------------------------- template<class T struct foo { int method(int x) { return 0; } static foo<T> the_instance; }; The program is missing the definition of foo's static variable, the_instance. Whether foo is a class template or not, foo::the_instance still has to be defined somewhere. It is not. So adding the missing definition after foo's declaration, fixes the problem: template <class T> foo<T> foo<T>::the_instance; Greg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top dan2online Guest Posted: Fri Jun 23, 2006 3:13 pm    Post subject: Re: static template member Artem Alimarine wrote: Quote: Hi all, The program below illustrates my problem. It compiles on VC8, also on GCC 3.4.4. (cygwin special) and on GCC 4.1.0 20060304 (Red Hat 4.1.0-3) However it does not link with GCCs: the static symbol is not found. I can make it link by uncommenting the second commented line, but then VC8 will refuse to work. I do not understand why one would need the two lines with template<> and template to make it work. Looks a bit shamanic to me. VC8 works as it is and also when the first commented line is used instead of template<> ... My questions are: Who is right: GCC or VC8? How do I write such a program in standard-compliant way that works with both compilers? ( I understand that good old ifdef will do but ... Artem //------------------------------------------------------------------- template<class T struct foo { int method(int x) { return 0; } static foo<T> the_instance; }; struct bar { int method() { return foo<bar>::the_instance.method(1); } }; // foo<bar> foo<bar>::the_instance; //template foo<bar> foo<bar>::the_instance; template<> foo<bar> foo<bar>::the_instance; template<class T> foo<T> foo<T>::the_instance; for both compilers. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael D. Carney Guest Posted: Sat Jun 24, 2006 2:36 pm    Post subject: Re: static template member In article <1151026564.153900.130230 (AT) r2g2000cwb (DOT) googlegroups.com>, Quote: The program is missing the definition of foo's static variable, the_instance. Whether foo is a class template or not, foo::the_instance still has to be defined somewhere. It is not. So adding the missing definition after foo's declaration, fixes the problem: template <class T foo<T> foo<T>::the_instance; Greg Be careful with that in VC8, however. If you link together a program that consists of several dlls, and the template is referenced in more than one of the dlls, each dll that references the template will have its own instance of the static member of the template. In linux (or any non-windows OS that I'm aware of), the instance will only exist once in the program, and each library will reference the same instance (which, I think, is the way its supposed to work according to the standard). The upshot of this is that if you're using the foo template above to implement a generic singleton, you won't get a singleton (i.e. one instance per process), but rather you'll get one instance per dll, which is definitely not what you want. If there's a way around this issue in VC8, please follow up here. The only way I found that works is to force the instance to be defined in only one dll by doing some macro tricks. - Mike. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gene Bushuyev Guest Posted: Sun Jun 25, 2006 6:46 pm    Post subject: Re: static template member "Michael D. Carney" <mcarney (AT) mathworks (DOT) com> wrote in message news:mcarney-380F53.09165723062006 (AT) news (DOT) mathworks.com... Quote: In article <1151026564.153900.130230 (AT) r2g2000cwb (DOT) googlegroups.com>, The program is missing the definition of foo's static variable, the_instance. Whether foo is a class template or not, foo::the_instance still has to be defined somewhere. It is not. So adding the missing definition after foo's declaration, fixes the problem: template <class T foo<T> foo<T>::the_instance; Greg Be careful with that in VC8, however. If you link together a program that consists of several dlls, and the template is referenced in more than one of the dlls, each dll that references the template will have its own instance of the static member of the template. In linux (or any non-windows OS that I'm aware of), the instance will only exist once in the program, and each library will reference the same instance (which, I think, is the way its supposed to work according to the standard). The standard doesn't say anything about dynamically linked libraries. Since dlls are shared by multiple processes, which may have nothing to do with each other, it only stands to reason that each one should get its own copy of static data, unless you explicitly put the data in shared segment. -- Gene Bushuyev (www.gbresearch.com) ---------------------------------------------------------------- To see what is in front of one's nose needs a constant struggle ~ George Orwell [ 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: Tue Jun 27, 2006 4:16 pm    Post subject: Re: static template member Michael D. Carney wrote: Quote: In article <1151026564.153900.130230 (AT) r2g2000cwb (DOT) googlegroups.com>, The program is missing the definition of foo's static variable, the_instance. Whether foo is a class template or not, foo::the_instance still has to be defined somewhere. It is not. So adding the missing definition after foo's declaration, fixes the problem: template <class T foo<T> foo<T>::the_instance; Be careful with that in VC8, however. If you link together a program that consists of several dlls, and the template is referenced in more than one of the dlls, each dll that references the template will have its own instance of the static member of the template. In linux (or any non-windows OS that I'm aware of), the instance will only exist once in the program, and each library will reference the same instance (which, I think, is the way its supposed to work according to the standard). According to the standard, as soon as you use dynamic linking, you have undefined behavior. An implementation can do whatever it wants. I'm not too familiar with Windows, but I know that under Solaris, you can get both behaviors, depending on what you want. (Most of the time, of course, you want to isolate the dynamiclyl linked objects as much as possible; the fact that one dynamically linked object happens to use some particular global variable in its implementation shouldn't interfere with any other dynamically linked object which might accidentally use a global object of the same type.) Quote: The upshot of this is that if you're using the foo template above to implement a generic singleton, you won't get a singleton (i.e. one instance per process), but rather you'll get one instance per dll, which is definitely not what you want. It depends, but I would say that most of the time, it is definitly what you want. The fact that my plugin happens to use some particular singleton (with a particular name) shouldn't interfere with other plugins, even if they accidentally use the same name. -- 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. 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