C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why can't I use an Iterator with a Reference Parameter?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message cppaddict Guest Posted: Fri Aug 27, 2004 11:38 pm    Post subject: Why can't I use an Iterator with a Reference Parameter? My initial method sig was: bool NativeOcr::testPoints(Point positionCoords, std::map<Point, COLORREF> testPnts) const ; I changed it to: bool NativeOcr::testPoints(const Point& positionCoords, const std::map<Point, COLORREF>& testPnts) const ; Whereas before it compiled, now it errors on the line marked below (excerpt from method body): std::map<Point, COLORREF>::iterator iter; int x,y; //ERROR ON NEXT LINE for (iter = testPnts.begin(); iter != testPnts.end(); ++iter) { //do stuff } I thought you could use refs just like the regular variable. According the FAQ at: http://www.parashift.com/c++-faq-lite/references.html "please do not think of a reference as a funny looking pointer to an object. A reference is the object. It is not a pointer to the object, nor a copy of the object. It is the object." Why does this not apply here? Thanks for an explanation, cpp -----------------ERROR MESSAGE--------------- Error E2034 NativeOcr.cpp 494: Cannot convert '__rwstd::__rb_tree<Point,std::pai r Point,unsigned long>,Point>,std::less<Point>,std::allocator<std::pair Point,unsigned lon g> > >::const_iterator' to '__rwstd::__rb_tree<Point,std::pair Point,unsig ned long>,__rwstd::__select1st<std::pair long>,Point>,std:: less<Point>,std::allocator<std::pair Quote: ::iterator' i n function NativeOcr::testPoints(const Point &,const std::map<Point,unsigned lon g,std::less Quote: &) co nst Error E2094 NativeOcr.cpp 494: 'operator!=' not implemented in type '__rwstd::__ rb_tree<Point,std::pair long>,__rwstd::__select1st<std::pai r long>,Point>,std::less<Point>,std::allocator<std::pair nst Point,unsigned long> > >::iterator' for arguments of type '__rwstd::__rb_tre e<Point,std::pair long>,__rwstd::__select1st<std::pair t Point,unsigned long>,Point>,std::less<Point>,std::allocator<std::pair int,unsigned long> > >::const_iterator' in function NativeOcr::testPoints(const Point &,const std::map<Point,unsigned long,std::less pair > &) const *** 2 errors in Compile *** make: *** [NativeOcr.obj] Error 1 Back to top Andrey Tarasevich Guest Posted: Fri Aug 27, 2004 11:51 pm    Post subject: Re: Why can't I use an Iterator with a Reference Parameter? cppaddict wrote: Quote: My initial method sig was: bool NativeOcr::testPoints(Point positionCoords, std::map<Point, COLORREF> testPnts) const ; I changed it to: bool NativeOcr::testPoints(const Point& positionCoords, const std::map<Point, COLORREF>& testPnts) const ; Whereas before it compiled, now it errors on the line marked below (excerpt from method body): std::map<Point, COLORREF>::iterator iter; int x,y; //ERROR ON NEXT LINE for (iter = testPnts.begin(); iter != testPnts.end(); ++iter) { //do stuff } I thought you could use refs just like the regular variable. In this case it is not about references, it is about const-correctness. You made your 'std::map<>' parameter const-qualified. For this reason in the reference-based version the container's 'begin()' method returns 'std::map<>::const_iterator'. A 'const_iterator' cannot be assigned to (or used to initialize) an 'iterator'. That's what's causing the error. Change the iterator declaration to std::map<Point, COLORREF>::const_iterator iter; and the code should compile. -- Best regards, Andrey Tarasevich Back to top cppaddict Guest Posted: Fri Aug 27, 2004 11:57 pm    Post subject: Re: Why can't I use an Iterator with a Reference Parameter? Quote: In this case it is not about references, it is about const-correctness. You made your 'std::map<>' parameter const-qualified. For this reason in the reference-based version the container's 'begin()' method returns 'std::map<>::const_iterator'. A 'const_iterator' cannot be assigned to (or used to initialize) an 'iterator'. That's what's causing the error. Change the iterator declaration to std::map<Point, COLORREF>::const_iterator iter; and the code should compile. That was it. Thanks very much. cpp 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.