C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  placing restrictions in template class creation          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message @velu.invalid Guest Posted: Tue Jun 28, 2005 10:40 am    Post subject: placing restrictions in template class creation Hi , Can i restrict the types for which the class can be created. say I have a generic class Cfoo and i would like to create only Cfoo<int> , Cfoo<float> & i dont want to allow Cfoo<double> , Is there any way to place a constraint like this ? Regds, @velu [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stanimir Kabaivanov Guest Posted: Tue Jun 28, 2005 12:05 pm    Post subject: Re: placing restrictions in template class creation Hi Velu, Quote: Can i restrict the types for which the class can be created. say I have a generic class Cfoo and i would like to create only Cfoo<int> , Cfoo<float> & i dont want to allow Cfoo<double> , Is there any way to place a constraint like this ? Yes, you can place restriction on your template types, and its a common question what you're asking. I'd recommend to read Bjarne Stroustrup technical FAQ, since this is explained very well there: http://www.research.att.com/~bs/bs_faq2.html Check out "Why can't I define constraints for my template parameters?" answer (don't worry - you can ). In particular the example provided in the FAQ for Can_copy template will work with your float, int and double example. But at least it will produce compiler warning for precission loss. Hope this one helps, Best regards, Stanimir Kabaivanov [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Tue Jun 28, 2005 12:07 pm    Post subject: Re: placing restrictions in template class creation In article <1119948377.943106.83220 (AT) g49g2000cwa (DOT) googlegroups.com>, <""@velu".invalid"> wrote: Quote: Hi , Can i restrict the types for which the class can be created. say I have a generic class Cfoo and i would like to create only Cfoo<int> , Cfoo<float> & i dont want to allow Cfoo<double> , Is there any way to place a constraint like this ? Regds, @velu simplest if classes are not big,,, template <class T> struct foo_base; template <> struct foo_base<int> {}; template <> struct foo_base<float>{}; template <class T> class foo:foo_base<T> { //.... }; see boost::enable_if and boost::mpl for a general solution... template <class T> class foo:boost::enable_if < boost::mpl::contains < boost::mpl::vector T Quote: , mpl::identity ::type { //... } come to mind not tested!!! [ 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: Tue Jun 28, 2005 1:10 pm    Post subject: Re: placing restrictions in template class creation In article <1119948377.943106.83220 (AT) g49g2000cwa (DOT) googlegroups.com>, ""@velu".invalid" <thirumalaivelu (AT) gmail (DOT) com> writes Quote: Hi , Can i restrict the types for which the class can be created. say I have a generic class Cfoo and i would like to create only Cfoo<int> , Cfoo<float> & i dont want to allow Cfoo<double> , Is there any way to place a constraint like this ? Declare the template but do not define it. Now provide specialisations for the types you want to use. For those that are curious, the only way of 'overloading' a class name is via a class template. -- 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 chris jefferson Guest Posted: Tue Jun 28, 2005 3:52 pm    Post subject: Re: placing restrictions in template class creation "@velu".invalid wrote: Quote: Hi , Can i restrict the types for which the class can be created. say I have a generic class Cfoo and i would like to create only Cfoo<int> , Cfoo<float> & i dont want to allow Cfoo<double> , Is there any way to place a constraint like this ? My personal favourite way to do something like that is like: template<class T> struct is_valid_for_class { static const int value = false; }; template<class T, bool check = is_valid_for_class struct my_class; template<class T> struct my_class<T,true> { ... } Then overload is_valid_for_class for each type where it is valid. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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