C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Unresolved externals on static members in nested templates          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Mikael Andersson Guest Posted: Sat Aug 12, 2006 1:28 am    Post subject: Unresolved externals on static members in nested templates I'm using nested templates in an access-time optimisation storage class and I'm trying to use static members to store commonly data which values are calcualable at compile time. However, static members in nested templates consitently result in unresolved external link errors when compiling with Visual Studio 2005's compiler (I have not tried any other compiler). Boiling down the problem to the bones leaves me with this structure: template<typename T> struct Outer { static int outer_member; template<int TT> struct Inner; template<> struct Inner<1> { static int inner_member; }; }; template<typename T> int Outer<T>::outer_member = 0; template<typename T> template<int TT> int Outer<T>::Inner<1>::inner_member = 0; int main() { Outer<void> foo; Outer<void>::Inner<1> bar; // ok foo.outer_member = 1; //unresolved external bar.inner_member = 1; //unresolved external Outer<void>::Inner<1>::inner_member = 1; } Not using a templated Inner class, or using templated but not specialised Inner classes, makes the program compile. Thus, the question is: what am I doing wrong? Or is it a compiler issue? Regards, Michael Andersson [ 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: Sat Aug 12, 2006 4:29 am    Post subject: Re: Unresolved externals on static members in nested templat Mikael Andersson wrote: Quote: I'm using nested templates in an access-time optimisation storage class and I'm trying to use static members to store commonly data which values are calcualable at compile time. However, static members in nested templates consitently result in unresolved external link errors when compiling with Visual Studio 2005's compiler (I have not tried any other compiler). Boiling down the problem to the bones leaves me with this structure: template<typename T struct Outer { static int outer_member; template<int TT> struct Inner; template<> struct Inner<1 { static int inner_member; }; }; It's not legal to specialize an inner class template without specializing each enclosing class template. And although VC may allow it, it's not standard C++. Quote: template<typename T> int Outer<T>::outer_member = 0; This definition is OK. Quote: template<typename T> template<int TT> int Outer<T>::Inner<1>::inner_member = 0; This definition is not. As noted above, the Inner class template cannot be specialized without the Outer class template being specialized as well. One way of solving this problem would be to "reverse" the Inner and Outer class templates. Moreover, since the concepts of "inner" and "outer" when applied to templates more aptly refer to levels of template instantiation rather layers of nested classes - it would make sense to use an instantiation of one class template to instantiatate the other. So implementing these suggestions would produce a program like the following: // Outer class template template<typename T> struct Outer { static int outer_member; }; // Outer::outer_member definition template <class T> int Outer<T>::outer_member = 0; // Inner class template template <class T, int N> struct Inner; // Inner class template partial specialization template <class T> struct Inner<Outer<T>, 1> { static int inner_member; }; // Inner::inner_member definition (partial specialization) template <class T> int Inner<Outer<T>, 1>::inner_member = 0; int main() { Outer<void> foo; foo.outer_member = 1; Inner<Outer<void>, 1>::inner_member = 1; } Greg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mikademus Guest Posted: Sun Aug 13, 2006 4:29 pm    Post subject: Re: Unresolved externals on static members in nested templat Quote: One way of solving this problem would be to "reverse" the Inner and Outer class templates. Moreover, since the concepts of "inner" and "outer" when applied to templates more aptly refer to levels of template instantiation rather layers of nested classes - it would make sense to use an instantiation of one class template to instantiatate the other. Thank you. I followed your advice and rewrote the class hierarchy along your suggestion. Nesting the classes was conventíent for reducing namespece cluttering, but that is a small price to pay for more efficiently working structure. Quote: It's not legal to specialize an inner class template without specializing each enclosing class template. And although VC may allow it, it's not standard C++. Is that something that might be revised in later specifications of the Standard? It seems of a similar nature to alias typedefs on templates, which apparently is something C++0x will include. Regards, Michael Andersson [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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