C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  static class member variables Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Eric Guest Posted: Wed Nov 17, 2004 11:12 am    Post subject: static class member variables I have a static class member variable as follows: struct A { static void Set (int i) { v = i; } static int& Get () { return v; } static int v; }; int A::v; // define A::v in the cpp file A::v will have external linkage and there will only be one instance of this variable in the executable: However, let's say I don't want to define the variable in the cpp file (eg. it's a template class and I don't want users to have to define the variable). Is there anything wrong with defining it in a static member function which returns a reference to the variable? Will there also always only be one copy of the local static variable? Any other unforseen problems? Thanks in advance for any comments. struct A { static void Set (int i) { v() = i; } static int& Get () { return v(); } static int& v() { static v; return v; } }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maxim Yegorushkin Guest Posted: Thu Nov 18, 2004 12:09 am    Post subject: Re: static class member variables On 17 Nov 2004 06:12:08 -0500, Eric <shoongi (AT) yahoo (DOT) com> wrote: Quote: I have a static class member variable as follows: struct A { static void Set (int i) { v = i; } static int& Get () { return v; } static int v; }; int A::v; // define A::v in the cpp file A::v will have external linkage and there will only be one instance of this variable in the executable: However, let's say I don't want to define the variable in the cpp file (eg. it's a template class and I don't want users to have to define the variable). You can use a trick to place the static in *.h without getting multiple simbol definitions linker error - place that static in a template base class as follows: template<int dummy> struct statics { static int v; }; template<int dummy> int statics<dummy>::v; struct A : private statics<1234> { // Get/Set are the same as before }; Quote: Is there anything wrong with defining it in a static member function which returns a reference to the variable? Will there also always only be one copy of the local static variable? Any other unforseen problems? Thanks in advance for any comments. struct A { static void Set (int i) { v() = i; } static int& Get () { return v(); } static int& v() { static v; return v; } }; Probably, you forgot static keyword here. The code as it is written now returns a dangling reference to the already evaporated local variable. You can fix it as: static int& Get () { static int v; return v; } -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Hopkin Guest Posted: Fri Nov 19, 2004 12:50 pm    Post subject: Re: static class member variables "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> wrote Quote: You can use a trick to place the static in *.h without getting multiple simbol definitions linker error - place that static in a template base class as follows: template<int dummy> struct statics { static int v; }; template<int dummy> int statics<dummy>::v; struct A : private statics<1234 { // Get/Set are the same as before }; Just a slight tidy up: template template<int dummy> int statics<dummy>::v; struct A : private statics<> { // Get/Set are the same as before }; Quote: Is there anything wrong with defining it in a static member function which returns a reference to the variable? Will there also always only be one copy of the local static variable? Any other unforseen problems? Thanks in advance for any comments. struct A { static void Set (int i) { v() = i; } static int& Get () { return v(); } static int& v() { static v; <------------------ missing int return v; } }; Probably, you forgot static keyword here. The code as it is written now returns a dangling reference to the already evaporated local variable. You can fix it as: static int& Get () { static int v; return v; } Look again. The only mistake is a missing int on the line I marked (which of course will compile on older compilers, since the int was implied in pre-standard C++). (Also, Get should probably return by value, but that's a design point rather than an error.) James [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (ne Spange Guest Posted: Fri Nov 19, 2004 12:59 pm    Post subject: Re: static class member variables Hello Maxim Yegorushkin, Maxim Yegorushkin schrieb: Quote: On 17 Nov 2004 06:12:08 -0500, Eric <shoongi (AT) yahoo (DOT) com> wrote: Is there anything wrong with defining it in a static member function which returns a reference to the variable? Will there also always only be one copy of the local static variable? Any other unforseen problems? Thanks in advance for any comments. struct A { static void Set (int i) { v() = i; } static int& Get () { return v(); } static int& v() { static v; return v; } }; Probably, you forgot static keyword here. The code as it is written now returns a dangling reference to the already evaporated local variable. You can fix it as: static int& Get () { static int v; return v; } Your description is somewhat misleading. I think the actual error, the OP made was, that its declaration of the local static in v misses a type. This looks like an MSVC compiler: Its a real **shame** that even MSVC7.1 does allow implicit int declarations!! (In this case the compiler was taking the right "guess", but what about the next time??). Furtheron it seems reasonable that the OP's Get function signature should be changed to struct A { ... static int Get () { return v(); } ... }; because otherwise The Set function does not make very much sense... Last but not least: Your proposal to use the template helper class is a very cool idea, Maxim! Greetings from Bremen, Daniel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tokyo Tomy Guest Posted: Fri Nov 19, 2004 3:24 pm    Post subject: Re: static class member variables "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> wrote Quote: On 17 Nov 2004 06:12:08 -0500, Eric <shoongi (AT) yahoo (DOT) com> wrote: ... Is there anything wrong with defining it in a static member function which returns a reference to the variable? Will there also always only be one copy of the local static variable? Any other unforseen problems? Thanks in advance for any comments. struct A { static void Set (int i) { v() = i; } static int& Get () { return v(); } static int& v() { static v; return v; } }; Probably, you forgot static keyword here. Where? Quote: The code as it is written now returns a dangling reference to the already evaporated local variable. You can fix it as: static int& Get () { static int v; return v; } static int& v () { static int v; return v; } // ? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Renjith Mohan Guest Posted: Fri Nov 19, 2004 4:01 pm    Post subject: Re: static class member variables "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> wrote Quote: On 17 Nov 2004 06:12:08 -0500, Eric <shoongi (AT) yahoo (DOT) com> wrote: I have a static class member variable as follows: struct A { static void Set (int i) { v = i; } static int& Get () { return v; } static int v; }; int A::v; // define A::v in the cpp file A::v will have external linkage and there will only be one instance of this variable in the executable: However, let's say I don't want to define the variable in the cpp file (eg. it's a template class and I don't want users to have to define the variable). Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maxim Yegorushkin Guest Posted: Fri Nov 19, 2004 4:51 pm    Post subject: Re: static class member variables On 19 Nov 2004 07:59:16 -0500, Daniel Krügler (ne Spangenberg) <dsp (AT) bdal (DOT) de> wrote: [] Quote: Your description is somewhat misleading. I agree, that was not a well thought answer. Quote: Last but not least: Your proposal to use the template helper class is a very cool idea, Maxim! I've stolen the idea from Dinkum STL implementation where it's used for std::ios_base::flags (in, out, ate, etc.). -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Mang Guest Posted: Sat Nov 20, 2004 10:17 am    Post subject: Re: static class member variables "Renjith Mohan" <renjithmohan (AT) hotmail (DOT) com> schrieb im Newsbeitrag news:cd7223bc.0411180707.4fb20450 (AT) posting (DOT) google.com... Quote: "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> wrote in message news:<opshlwtry2ti5cme (AT) devlx007 (DOT) ipcb.net>... On 17 Nov 2004 06:12:08 -0500, Eric <shoongi (AT) yahoo (DOT) com> wrote: I have a static class member variable as follows: struct A { static void Set (int i) { v = i; } static int& Get () { return v; } static int v; }; int A::v; // define A::v in the cpp file A::v will have external linkage and there will only be one instance of this variable in the executable: However, let's say I don't want to define the variable in the cpp file (eg. it's a template class and I don't want users to have to define the variable). Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; Not a clever solution. v is not tied to class A by any means and can be accessed directly without any A object / operator ::. Also, try to use this with multiple translation units, each changing and reading v (via the functions provided in A). Surprise, surprise (at least when the semantic of the OP is intended). Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Seungbeom Kim Guest Posted: Sat Nov 20, 2004 4:23 pm    Post subject: Re: static class member variables Renjith Mohan wrote: Quote: Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; An unnamed namespace in a header file? Each of the translation units that includes it will have its own copy of v, then. :( -- Seungbeom Kim [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maxim Yegorushkin Guest Posted: Sat Nov 20, 2004 6:19 pm    Post subject: Re: static class member variables Renjith Mohan wrote: Quote: Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; I strongly disagree. I am not sure, but it seems to me, that you've got ODR violation here. The exact meaning of 'return v' is different in every TU because of the fact that an unnamed namespace does have a name, though hidden and distinct for every TU. Aside from (possible) ODR violation, there are other problems. This places a distinct instance of an object from the unnamed namespace in every TU that includes the header with the unnamed namespace, what results in quite different semantics because now every TU operates on an own instance of v. -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Mang Guest Posted: Sun Nov 21, 2004 5:00 am    Post subject: Re: static class member variables "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> schrieb im Newsbeitrag news:opshq9w3y9ti5cme (AT) wkcg6rirwp (DOT) .. Quote: Renjith Mohan wrote: Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; I strongly disagree. I am not sure, but it seems to me, that you've got ODR violation here. Don't think so. Since v has internal linkage.... Quote: Aside from (possible) ODR violation, there are other problems. This places a distinct instance of an object from the unnamed namespace in every TU that includes the header with the unnamed namespace, what results in quite different semantics because now every TU operates on an own instance of v. That IS a problem. Thomas [ 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: Sun Nov 21, 2004 11:10 pm    Post subject: Re: static class member variables In article <419fcbdb$0$24064$3b214f66 (AT) usenet (DOT) univie.ac.at>, Thomas Mang <nospam (AT) nospam (DOT) invalid> writes Quote: "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> schrieb im Newsbeitrag news:opshq9w3y9ti5cme (AT) wkcg6rirwp (DOT) .. Renjith Mohan wrote: Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; I strongly disagree. I am not sure, but it seems to me, that you've got ODR violation here. Don't think so. Since v has internal linkage.... It doesn't, it has external linkage but with an unutterable name. However that is not really relevant because there is a potential ODR violation which ever way you take it. If the above header file is included in more than one TU the definition of struct A will be an ODR issue. -- 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 Renjith Mohan Guest Posted: Sun Nov 21, 2004 11:16 pm    Post subject: Re: static class member variables "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> wrote Quote: Renjith Mohan wrote: Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; I strongly disagree. I am not sure, but it seems to me, that you've got ODR violation here. The exact meaning of 'return v' is different in every TU because of the fact that an unnamed namespace does have a name, though hidden and distinct for every TU. Aside from (possible) ODR violation, there are other problems. This places a distinct instance of an object from the unnamed namespace in every TU that includes the header with the unnamed namespace, what results in quite different semantics because now every TU operates on an own instance of v. I understand my misconception. Thanks for pointing this out. I seem to have been over enthusiastic about the clause 7.3.1.1 - Unnamed namespaces para 2 which says something like this "The use of the static keyword is deprecated when declaring objects in a namespace scope (see annex depr); the unnamed-namespace provides a superior alternative." As has been pointed out this applies to "namespace scope" members only and even with such variables this must be used with caution as the generated namespace name(for the unnamed namespace) in each TU is implementation dependent which may create binary differences of the executable which is beyond programmers control. But a doubt remains; since the standard does recommend it for "namespace scope" variables, does it violate ODR for such variables?. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Olsen Guest Posted: Sun Nov 21, 2004 11:30 pm    Post subject: Re: static class member variables Thomas Mang wrote: Quote: "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> schrieb im Newsbeitrag news:opshq9w3y9ti5cme (AT) wkcg6rirwp (DOT) .. Renjith Mohan wrote: Another solution is to use an unnamed namespace which is semantically equivalent to internal linkage and is a mcuh more simple and elegant solution. Thus in a header file you can do this. namespace { int v; } struct A { static void Set (int i) { v = i; } static int& Get () { return v; } }; I strongly disagree. I am not sure, but it seems to me, that you've got ODR violation here. Don't think so. Since v has internal linkage.... No, v does not have internal linkage. Things inside the unnamed namespace have external linkage. They just have a name that is known only to the current translation unit, making them inaccessible from other translation units. The ultimate effect is very similar to making v static, but there are some subtle differences that must be understood. The one-definition rule (ODR) violation is not with v, it is with A::Set and A::Get. Within each translation unit, A::Set and A::Get refer to the v that is unique to that translation unit. Therefore, A::Set and A::Get are not the same in all translation units, which is not allowed. -- David Olsen [email]qg4h9ykc5m (AT) yahoo (DOT) com[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Hopkin Guest Posted: Mon Nov 22, 2004 9:37 pm    Post subject: Re: static class member variables [email]renjithmohan (AT) hotmail (DOT) com[/email] (Renjith Mohan) wrote in message news:<cd7223bc.0411210606.50c8d68a (AT) posting (DOT) google.com>... Quote: But a doubt remains; since the standard does recommend it for "namespace scope" variables, does it violate ODR for such variables?. In this context, you get ODR violations using either (deprecated) namespace scope statics or a nameless namespace. Using a nameless namespace doesn't make it any more wrong :-) Point of interest: boost::bind uses a nameless namespace in a header to define _1, _2 etc. (the parameter placeholders). I've yet to get a grip on the pros and cons of this - not that it's something you have to understand to use bind itself. James [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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