C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  template func in class question          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Active8 Guest Posted: Sun Feb 27, 2005 7:34 am    Post subject: template func in class question I have an STL vector I pass to a function and I need to know what type data is in the vector when I'm in the fuction. class MyClass { template class<T> int Myfunc(T&){int i = sizeof(T);// oops - I want sizeof(float) } }; vector<float> vec; MyClass mc; mc.Myfunc(vec) Is there any way to do this without writing separate functions for each type of data the vector might contain? And without a 2 parameter template *class* like "template class<V, T>" ? Thanks in advance. -- Best Regards, Mike Back to top Jonathan Turkanis Guest Posted: Sun Feb 27, 2005 7:50 am    Post subject: Re: template func in class question Active8 wrote: Quote: I have an STL vector I pass to a function and I need to know what type data is in the vector when I'm in the fuction. class MyClass { template class<T int Myfunc(T&) { typedef typename T::value_type type; // Works if you know T will always be an STL container } Quote: }; vector MyClass mc; mc.Myfunc(vec) Jonathan Back to top Active8 Guest Posted: Sun Feb 27, 2005 8:44 am    Post subject: Re: template func in class question On Sun, 27 Feb 2005 00:50:00 -0700, Jonathan Turkanis wrote: Quote: Active8 wrote: I have an STL vector I pass to a function and I need to know what type data is in the vector when I'm in the fuction. class MyClass { template class int Myfunc(T&) { typedef typename T::value_type type; ^^^^^^^ I don't understand the use of "typename" above. I would've said: typedef T::value_type type; Quote: // Works if you know T will always be an STL container } }; I wish I could find my Osbourne C++ ref. -- Best Regards, Mike Back to top Rolf Magnus Guest Posted: Sun Feb 27, 2005 10:28 am    Post subject: Re: template func in class question Active8 wrote: Quote: On Sun, 27 Feb 2005 00:50:00 -0700, Jonathan Turkanis wrote: Active8 wrote: I have an STL vector I pass to a function and I need to know what type data is in the vector when I'm in the fuction. class MyClass { template class int Myfunc(T&) { typedef typename T::value_type type; ^^^^^^^ I don't understand the use of "typename" above. It tells the compiler that T::value_type is the name of a type. In C++, it's sometimes hard for the compiler to find out whether a name that depends on a template is actually a type or not. Therefore, the compiler assumes it's a value unless you use the typename keyword. Quote: I would've said: typedef T::value_type type; That would've been wrong, and the compiler should bail out with an error. Back to top Active8 Guest Posted: Sun Feb 27, 2005 5:02 pm    Post subject: Re: template func in class question On Sun, 27 Feb 2005 11:28:40 +0100, Rolf Magnus wrote: Quote: Active8 wrote: On Sun, 27 Feb 2005 00:50:00 -0700, Jonathan Turkanis wrote: Active8 wrote: I have an STL vector I pass to a function and I need to know what type data is in the vector when I'm in the fuction. class MyClass { template class int Myfunc(T&) { typedef typename T::value_type type; ^^^^^^^ I don't understand the use of "typename" above. It tells the compiler that T::value_type is the name of a type. In C++, it's sometimes hard for the compiler to find out whether a name that depends on a template is actually a type or not. Therefore, the compiler assumes it's a value unless you use the typename keyword. I would've said: typedef T::value_type type; That would've been wrong, and the compiler should bail out with an error. It doesn't (VC++ 6.0) and it works, but thanks the same. That makes sense. Maybe if I turn the warnings back on... -- Best Regards, Mike Back to top Jonathan Turkanis Guest Posted: Sun Feb 27, 2005 5:49 pm    Post subject: Re: template func in class question Active8 wrote: Quote: Rolf Magnus wrote: Active8 wrote: I would've said: typedef T::value_type type; That would've been wrong, and the compiler should bail out with an error. It doesn't (VC++ 6.0) and it works, but thanks the same. That makes sense. Maybe if I turn the warnings back on... There's little connection between what a compiler should do and what VC++ 6.0 does. ;-) Jonathan Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.