C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  yet another Q: template+friend !!          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Samee Zahur Guest Posted: Thu Apr 28, 2005 6:04 pm    Post subject: yet another Q: template+friend !! Here's the code ... just wouldn't compile! (not under g++ at least) I'm just trying to make test<Y>::mem() a friend of test<X> for all Y and X. I found quite a lot of very similar things googling around, but not quite what I needed...not feeling lucky I guess :( ------------------------------Code start---------------------------------- template <class X> class test { X a; public: test(X b):a(b) {} friend void fun<>(test); //Compiles fine template <class Y> void mutatewith(test<Y> b) {a=b.a;} template <class Y> template <class Z> friend void test<Y>::mutatewith(test<Z>); //Compiles fine void mem(); template <class Y> friend void test<Y>::mem(); //ERROR!! }; template <class X> void fun(test<X> par){par.a=7;} template class test<double>; //What if I wanted only the test<Y>::mutatewith(test<X>) to be a friend of // test<X> for all Y? Right now, mutatewith has friendship to all test<Z> // for all Z !! -----------------------------code end---------------------------------------- Samee Back to top Victor Bazarov Guest Posted: Fri Apr 29, 2005 1:27 am    Post subject: Re: yet another Q: template+friend !! Samee Zahur wrote: Quote: Here's the code ... just wouldn't compile! (not under g++ at least) I'm just trying to make test<Y>::mem() a friend of test<X> for all Y and X. I found quite a lot of very similar things googling around, but not quite what I needed...not feeling lucky I guess :( ------------------------------Code start---------------------------------- template <class X> class test { X a; public: test(X b):a(b) {} friend void fun<>(test); //Compiles fine Actually doesn't with Comeau C++ (online 'test drive'). Quote: template <class Y> void mutatewith(test<Y> b) {a=b.a;} template <class Y> template <class Z friend void test void mem(); template <class Y> friend void test<Y>::mem(); //ERROR!! Actually no error with Comeau C++. Quote: }; template <class X> void fun(test<X> par){par.a=7;} template class test<double>; I have no suggestion for you, just thought I'd point out the difference in results for those two compilers. V Back to top Bushido Hacks Guest Posted: Fri Apr 29, 2005 2:11 am    Post subject: Re: yet another Q: template+friend !! Actually, you don't need to use "template" twice. template <class Y,class Z> friend void test<Y>::mutatewith(test<Z>); The other thing I recommend is to put the function on the next line. It seems that is the best solution. template <class Y> friend void test<Y>::mem(); Your other alternative is to try friend template <class Y> void test<Y>::mem(); Back to top Samee Zahur Guest Posted: Sat Apr 30, 2005 10:22 am    Post subject: Re: yet another Q: template+friend !! Quote: I have no suggestion for you, just thought I'd point out the difference in results for those two compilers. Actually, when I saw that the online test drive gave errors, I assumed that they were the same as g++ - felt too lazy to examine what the errors actually said! Thanks for pointing that out. In fact, comeau compiles the ENTIRE code just fine ... all I need are forward declarations of fun! So, which is standard-compliant in this case? Should it compile and gcc has a bug? Or should it NOT compile and comeau has an extension/bug ? Samee Back to top Samee Zahur Guest Posted: Sat Apr 30, 2005 10:23 am    Post subject: Re: yet another Q: template+friend !! I'm sorry to say this, but NONE of these work (nor are they supposed to). And wait, since when was introducing line-breaks a language requirement? C++ treats ALL whitespaces as identical! A space and a line-break has NO difference whatsoever anywhere in the language! And I DO need to use "template" twice! Samee Back to top Victor Bazarov Guest Posted: Sat Apr 30, 2005 6:45 pm    Post subject: Re: yet another Q: template+friend !! Samee Zahur wrote: Quote: [..] C++ treats ALL whitespaces as identical! A space and a line-break has NO difference whatsoever anywhere in the language! That's simply incorrect. A newline ("line-break") is not allowed inside a string literal. V Back to top Samee Zahur Guest Posted: Mon May 02, 2005 3:37 am    Post subject: Re: yet another Q: template+friend !! Ok yes, my mistake thanks. Actually, since I never took a look at the actual copy of the standard, I can't always come up with the exceptional cases in my mind. Yes, a newline is not allowed inside a string literal (but...not sure though...I do remember seeing a case where a backslash followed by a newline is allowed to represent a continuous string, but is that legal?) Anyway, my original problem still stays ... there seems to be no way to control friendships here. I guess that shouldn't really be a problem since I am the one writing all versions of the template! 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