C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Problem with Boost iterators          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Guest Posted: Wed Nov 01, 2006 10:10 am    Post subject: Problem with Boost iterators Hi all, I am running into the compilation error (gcc 4.0.2 Suse Linux) while trying to compile the following simple program: #include <boost/iterator/iterator_facade.hpp> struct Pixel { int _abc;}; class PixelIterator : public boost::iterator_facade< PixelIterator, Pixel, boost::random_access_traversal_tag > { public: friend class boost::iterator_core_access; explicit PixelIterator() : _prPixel() {} explicit PixelIterator(Pixel * iprPixel) : _prPixel(iprPixel) {} void increment(){++_prPixel;} void decrement(){--_prPixel;} bool equal(const PixelIterator& iOther) const{ return _prPixel == iOther._prPixel; } Pixel& dereference() const{return *_prPixel;} void advance(int iDist){_prPixel += iDist;} uint distance_to(const PixelIterator& iOther) const{return iOther._prPixel - _prPixel;} private: Pixel * _prPixel; }; int main() { Pixel p[10]; PixelIterator it(p); it[1]._abc = 1; } The error message is 'class boost::detail::operator_brackets_proxy<PixelIterator>' has no member named '_abc The Boost website is currently unavailable to look-up the documentation but I am pretty match sure that the semantic of operator[] (int dist) is *(it +dist). And indeed if I replace the last line in main with (*(it+1))._abc = 1; the error goes away. Any help is greatly appreciated. Mike -- [ 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: Fri Nov 03, 2006 10:10 am    Post subject: Re: Problem with Boost iterators In article <1162348295.249095.231880 (AT) e64g2000cwd (DOT) googlegroups.com>, <michael.alexeev (AT) gmail (DOT) com> wrote: Quote: Hi all, I am running into the compilation error (gcc 4.0.2 Suse Linux) while trying to compile the following simple program: #include <boost/iterator/iterator_facade.hpp struct Pixel { int _abc;}; class PixelIterator : public boost::iterator_facade< PixelIterator, Pixel, boost::random_access_traversal_tag { public: friend class boost::iterator_core_access; explicit PixelIterator() : _prPixel() {} explicit PixelIterator(Pixel * iprPixel) : _prPixel(iprPixel) {} void increment(){++_prPixel;} void decrement(){--_prPixel;} bool equal(const PixelIterator& iOther) const{ return _prPixel == iOther._prPixel; } Pixel& dereference() const{return *_prPixel;} void advance(int iDist){_prPixel += iDist;} uint distance_to(const PixelIterator& iOther) const{return iOther._prPixel - _prPixel;} private: Pixel * _prPixel; }; int main() { Pixel p[10]; PixelIterator it(p); it[1]._abc = 1; } The error message is 'class boost::detail::operator_brackets_proxy<PixelIterator>' has no member named '_abc The Boost website is currently unavailable to look-up the documentation but I am pretty match sure that the semantic of operator[] (int dist) is *(it +dist). And indeed if I replace the last line in main with (*(it+1))._abc = 1; the error goes away. Any help is greatly appreciated. Mike The docs do state that if the proxy solution is not needed as in your case your class can define operator [] and it will be used instead of the operator[] supplied by iterator_facade. So if you add Pixel & operator [] (int n) {return *(_prPixel+n);} to Pixelterator it will work since nothing is really destroyed here... -- [ 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: Sun Nov 05, 2006 9:15 am    Post subject: Re: Problem with Boost iterators Alberto Ganesh Barbati <AlbertoBarbati (AT) libero (DOT) it> writes: Quote: shunsuke ha scritto: michael.alexeev (AT) gmail (DOT) com wrote: int main() { Pixel p[10]; PixelIterator it(p); it[1]._abc = 1; } The error message is 'class boost::detail::operator_brackets_proxy<PixelIterator>' has no member named '_abc The Boost website is currently unavailable to look-up the documentation but I am pretty match sure that the semantic of operator[] (int dist) is *(it +dist). And indeed if I replace the last line in main with (*(it+1))._abc = 1; the error goes away. The standard requires 'operator[]' to return a type which is *convertible* to 'Pixel'. That's correct in C++03, but in the latest draft of the standard the requirement has changed: the return type of operator[] is required to be convertible to const Pixel&, not Pixel. The intent is clearly to disallow the use of the return value to modify the referred object. No, it's not intended to disallow modification; it's only intended to not require modifiability. An operator[] that returns Pixel& is conforming, since Pixel& is convertible to Pixel const&. Quote: So the assignment "it[1].abc = 1;" might compile but is non-portable. With it being an unknown iterator type, yes. -- 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 Alberto Ganesh Barbati Guest Posted: Mon Nov 06, 2006 3:56 am    Post subject: Re: Problem with Boost iterators David Abrahams ha scritto: Quote: Alberto Ganesh Barbati <AlbertoBarbati (AT) libero (DOT) it> writes: shunsuke ha scritto: The standard requires 'operator[]' to return a type which is *convertible* to 'Pixel'. That's correct in C++03, but in the latest draft of the standard the requirement has changed: the return type of operator[] is required to be convertible to const Pixel&, not Pixel. The intent is clearly to disallow the use of the return value to modify the referred object. No, it's not intended to disallow modification; it's only intended to not require modifiability. An operator[] that returns Pixel& is conforming, since Pixel& is convertible to Pixel const&. Oh! I understand. Thanks for correcting me. By the way, why isn't modifiability required for mutable iterators? That seems quite reasonable. As I'm pretty sure this case has been considered, there must be some obvious rationale against it that I'm failing to see. Could you please enlighten me? Will there be changes in this area if the "new iterators concepts" proposal gets accepted? TIA, Ganesh -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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