C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  stl c++ & map          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message lmanchur. Guest Posted: Sat Jan 29, 2005 9:17 pm    Post subject: stl c++ & map Hi, I am implementing a version of the C++ STL map class for a school assignment and have a question... what is the difference between const_iterator and just iterator??... If I already have an iterator implemented, can I just copy that code and change return types to const and would that be sufficient?? I am lost 'cuz I don't know this definition! Thanks in advance! Back to top Mike Wahler Guest Posted: Sat Jan 29, 2005 9:32 pm    Post subject: Re: stl c++ & map "lmanchur." <lmanchur (AT) gmail (DOT) com> wrote Quote: Hi, I am implementing a version of the C++ STL map class for a school assignment and have a question... what is the difference between const_iterator and just iterator??... 'iterator' should allow modification of the object to which it refers. 'const_iterator' should disallow modification (you can do via 'help' from the language's 'const' qualifier, or simply implement the prevention in your logic). Quote: If I already have an iterator implemented, can I just copy that code The code will be very similar, yes. Quote: and change return types to const and would that be sufficient?? It might be, it might not. That depends upon the details of your implementation. Quote: I am lost 'cuz I don't know this definition! The best explanation I've seen is in this book: www.josuttis.com/libbook You can also get some ideas from looking at your standard library header files. E.g. look at header <map> -Mike Back to top Gianni Mariani Guest Posted: Sat Jan 29, 2005 9:33 pm    Post subject: Re: stl c++ & map lmanchur. wrote: Quote: Hi, I am implementing a version of the C++ STL map class for a school assignment and have a question... what is the difference between const_iterator and just iterator??... If I already have an iterator implemented, can I just copy that code and change return types to const and would that be sufficient?? I am lost 'cuz I don't know this definition! This is when a little test code might help... #include <iostream> #include <ostream> #include <map> int main() { typedef std::map<int,int> T; T l_map; T::const_iterator l_iter; T::iterator l_iter2; l_map[1] = 1; l_map[2] = 2; l_iter2 = l_map.begin(); l_iter = l_iter2; l_iter2->second = 3; l_iter->second = 4; } Back to top Matthew Schaefer Guest Posted: Sat Jan 29, 2005 9:38 pm    Post subject: Re: stl c++ & map See Scott Meyers' article, "Three Guidelines for Effective Iterator Use". You can find it on his website: http://www.aristeia.com/publications_frames.html. Back to top lmanchur@gmail.com Guest Posted: Sun Jan 30, 2005 2:49 pm    Post subject: Re: stl c++ & map Thanks so much to all who responded but especially to Gianni!!! That really helped illustrate the point and gave me a basis for which to test my own. Thanks again!!!! Back to top lmanchur@gmail.com Guest Posted: Sun Jan 30, 2005 3:01 pm    Post subject: Re: stl c++ & map Actually if I can ask another question.... i thought i had it working but it turns out I only partially have it working.... for some reason it's not figuring out which begin() method to use... I have the following methods: iterator begin(); const_iterator begin() const; And if I rename the second one to something like cbegin() it works exactly as the std::map::const_iterator begin(); does. But if I leave it as defined above I get the following compile error: StrIntMap.cpp: In function `static void StrIntMap::test()': StrIntMap.cpp:57: no match for `StrIntMap::const_iterator & = StrIntMap::iterator' StrIntMap.hh:150: candidates are: class StrIntMap::const_iterator & StrIntMap::const_iterator::operator =(const StrIntMap::const_iterator &) gmake: *** [StrIntMap.o] Error 1 Any thoughts?? Thanks alot, Lee Back to top Gianni Mariani Guest Posted: Sun Jan 30, 2005 4:12 pm    Post subject: Re: stl c++ & map [email]lmanchur (AT) gmail (DOT) com[/email] wrote: Quote: Actually if I can ask another question.... i thought i had it working but it turns out I only partially have it working.... for some reason it's not figuring out which begin() method to use... I have the following methods: iterator begin(); const_iterator begin() const; And if I rename the second one to something like cbegin() it works exactly as the std::map::const_iterator begin(); does. But if I leave it as defined above I get the following compile error: StrIntMap.cpp: In function `static void StrIntMap::test()': StrIntMap.cpp:57: no match for `StrIntMap::const_iterator & = StrIntMap::iterator' StrIntMap.hh:150: candidates are: class StrIntMap::const_iterator & StrIntMap::const_iterator::operator =(const StrIntMap::const_iterator &) gmake: *** [StrIntMap.o] Error 1 Any thoughts?? The error says it can't convert an "iterator" to a "const_iterator &" (note the reference .. &) ! This is a different problem. In this case it looks like you have somthing like (speculating): M::const_iterator & foo() { M::iterator x; return x; } A reference is kind of a special thing in C++, it like a pointer but looks like an object. The speculation code above tries to return a reference (pointer) to an object that it needs to create (temporary) but since it's returning from a function, there is no way to allocate one without it being destroyed. Try returning by value. M::const_iterator foo() { M::iterator x; return x; } Now, you can make a copy of the iterator. You could also generate a cons_iterator (by calling begin() const) but you still have a problem with returning temporaries. (look in the FAQ). #include <iostream> #include <ostream> #include <map> int main() { typedef std::map<int,int> T; T l_map; const T & l_const_map; l_map[1] = 1; l_map[2] = 2; T::const_iterator l_iter3 = l_const_map.begin(); T::iterator l_iter3 = l_const_map.begin(); // illegal. } Back to top lmanchur@gmail.com Guest Posted: Sun Jan 30, 2005 5:12 pm    Post subject: Re: stl c++ & map No, that's what I have. Again the error is: StrIntMap.cpp: In function `static void StrIntMap::test()': StrIntMap.cpp:136: no match for `StrIntMap::const_reverse_iterator & = StrIntMap::iterator' StrIntMap.hh:318: candidates are: class StrIntMap::const_reverse_iterator & StrIntMap::const_reverse_iterator::operator =(const StrIntMap::const_reverse_iterator &) gmake: *** [StrIntMap.o] Error 1 obelix[93]% But my code is this: StrIntMap::const_iterator StrIntMap::lower_bound(const StrIntMap::key_type& k) const{ StrIntMap::const_iterator it; for(it=this->cbegin(); it!=this->cend(); it++) if((it->first)>=k) return it; return it; } I already have a method called lower_bound that returns an iterator. If I rename the above method to something else like clower_bound it works perfectly as required. Any additional thoughts? 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