C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Koenig lookup Problem - quite arcane!          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message pongba@gmail.com Guest Posted: Fri Sep 23, 2005 1:20 pm    Post subject: Koenig lookup Problem - quite arcane! The code is just simple, as: namespace My { struct X{}; } using namespace My; ostream& operator<<(ostream& os,X const& p) { return os; } int main() { vector cout< copy(vp.begin(),vp.end(), ostream_iterator } However,when I copy 'ostream_iterator' to my own namespace and use that fake ostream_iterator, everything just go fine! Is it about koenig lookup? Why even comeau issues an error??? I don't know why, you guys plz help me,thanks in advance! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gianluca Silvestri Guest Posted: Sat Sep 24, 2005 2:13 am    Post subject: Re: Koenig lookup Problem - quite arcane! <pongba (AT) gmail (DOT) com> ha scritto nel messaggio news:1127416666.110879.41570 (AT) f14g2000cwb (DOT) googlegroups.com... Quote: The code is just simple, as: namespace My { struct X{}; } using namespace My; ostream& operator<<(ostream& os,X const& p) { return os; } int main() { vector cout< copy(vp.begin(),vp.end(), ostream_iterator } However,when I copy 'ostream_iterator' to my own namespace and use that fake ostream_iterator, everything just go fine! Is it about koenig lookup? Why even comeau issues an error??? I don't know why, you guys plz help me,thanks in advance! Yes, it's an ADL problem, but also a design fault. You can read this article by Herb Sutter upon these things: http://www.gotw.ca/publications/mill08.htm HTH Gianluca [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tpochep@mail.ru Guest Posted: Sat Sep 24, 2005 2:17 am    Post subject: Re: Koenig lookup Problem - quite arcane! This is result of two-phase name-lookup in templates. One way to fix the proble - put operator << in your namespace (you know it). Another (tested with como online, but I'm not shure about it) namespace My { struct X{}; } #include using namespace My; std::ostream &operator<<(std::ostream &os, X const& p) { //you can put this definition after inclusion of need //full definitions return os; } #include <iostream> #include <vector> #include <iterator> int main() { std::vector<X> vp; std::copy(vp.begin(),vp.end(), std::ostream_iterator<X>(std::cout)); } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top msalters Guest Posted: Sat Sep 24, 2005 2:19 am    Post subject: Re: Koenig lookup Problem - quite arcane! [email]pongba (AT) gmail (DOT) com[/email] schreef: Quote: The code is just simple, as: namespace My { struct X{}; } using namespace My; ostream& operator<<(ostream& os,X const& p) { return os; } int main() { vector cout< copy(vp.begin(),vp.end(), ostream_iterator } I'm missing some headers and some std:: qualifications. It's a bit hard to answer questions about namespaces if you don't show us the uses of one of the namespaces involved! HTH, Michiel Salters [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bo Persson Guest Posted: Sat Sep 24, 2005 2:38 am    Post subject: Re: Koenig lookup Problem - quite arcane! <pongba (AT) gmail (DOT) com> skrev i meddelandet news:1127416666.110879.41570 (AT) f14g2000cwb (DOT) googlegroups.com... Quote: The code is just simple, as: namespace My { struct X{}; } using namespace My; This makes the content of namespace My visible here. It doesn't make this part of the namespace. Quote: ostream& operator<<(ostream& os,X const& p) { return os; } This operator is outside namespace My. Quote: int main() { vector cout< copy(vp.begin(),vp.end(), ostream_iterator You are using types from namespaces std and My, so the compiler looks for a suitable operator<< in those namespaces. It will find lots of them, but none that is suitable - error! Quote: } However,when I copy 'ostream_iterator' to my own namespace and use that fake ostream_iterator, everything just go fine! Of course - now the interface to type X is all in its namespace, so it's found by the lookup. Quote: Is it about koenig lookup? Why even comeau issues an error??? I don't know why, you guys plz help me,thanks in advance! Try reading "Namespaces and the Interface Principle" by Herb Sutter http://www.gotw.ca/publications/mill08.htm Bo Persson [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Sat Sep 24, 2005 8:16 pm    Post subject: Re: Koenig lookup Problem - quite arcane! [email]pongba (AT) gmail (DOT) com[/email] writes: Quote: The code is just simple, as: namespace My { struct X{}; } using namespace My; ostream& operator<<(ostream& os,X const& p) { return os; } int main() { vector cout< copy(vp.begin(),vp.end(), ostream_iterator } However,when I copy 'ostream_iterator' to my own namespace and use that fake ostream_iterator, everything just go fine! Yes, because then the operator is in an associated namespace of X. Quote: Is it about koenig lookup? Yes, that and 2-phase name lookup Quote: Why even comeau issues an error??? Because your code is ill-formed. Quote: I don't know why, you guys plz help me,thanks in advance! 14.6.4 Dependent name resolution 1 In resolving dependent names, names from the following sources are considered: --- Declarations that are visible at the point of definition of the template. --- Declarations from namespaces associated with the types of the function arguments both from the instantiation context (14.6.4.1) and from the definition context. The point of definition of std::copy occurs long before the compiler ever sees your declaration of operator<<. You could start playing games with #include order to change that, but that's extremely fragile approach. HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frank Chang Guest Posted: Sat Sep 24, 2005 8:18 pm    Post subject: Re: Koenig lookup Problem - quite arcane! When I do this it compiles okay: namespace My { struct X1{}; ostream& operator<<(ostream&os,const X1& p) { return os; } }; As you say this is Koenig lookup(i.e argument dependent lookup) , allowing the compiler to find the operator << in namespace my without having to resort to exotic namespace qualification. Thank you. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Sat Sep 24, 2005 8:22 pm    Post subject: Re: Koenig lookup Problem - quite arcane! [email]pongba (AT) gmail (DOT) com[/email] wrote: Quote: The code is just simple, as: namespace My { struct X{}; } using namespace My; ostream& operator<<(ostream& os,X const& p) { return os; } int main() { vector cout< copy(vp.begin(),vp.end(), ostream_iterator } However,when I copy 'ostream_iterator' to my own namespace and use that fake ostream_iterator, everything just go fine! Is it about koenig lookup? Why even comeau issues an error??? I don't know why, you guys plz help me,thanks in advance! Yes it's all about Koenig lookup (aka argument dependent lookup or ADL) and two-phase name binding. As std::copy is in namespace std, the only operators << that are considered are: a) At the point of definition of std::copy (first phase) 1) all operator<< in the global namespace 2) all operator<< in namespace std b) At the point of instantiation, i.e. just before main (second phase, using ADL) 3) all operator<< in namespace std (again) 4) all operator<< in namespace My The problem is that as you put operator<< in the global namespace, it cannot be found by any of those rules. (it cannot be found by rule 1 because it's not visibile at the point of definition of std::copy!) Yes, putting the operator in the global namespace is *preventing* ADL to kick in. Just move your operator<< in same namespace as X so that it can be found by rule 4 and everything will be ok. Copying the ostream_iterator in namespace My achieve the same result (for a different reason, though), but why make things complicated by duplicating library code, where there's a much simpler solution? Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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