C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Clarification for std::vector and std::map          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Jonathan Mcdougall Guest Posted: Wed Jul 30, 2003 9:00 am    Post subject: Re: Clarification for std::vector and std::map On Wed, 30 Jul 2003 18:13:48 -0400, "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote: Quote: "Sir_Ciph3r" <Sir_Ciph3r (AT) nospam (DOT) com> wrote... Can someone explain what is happening in the following code? Undefined behaviour. #include <iostream #include #include I was just wondering if map<char*, int> m; m["TEST"] = 4; m["TEST"] = 6; is guaranteed to work, knowing that "TEST" is a pointer to a c-style string. To me, it seems that a compiler is allowed to have two different "TEST" strings and to map the values to the pointer address, no? Jonathan Back to top Sir_Ciph3r Guest Posted: Wed Jul 30, 2003 10:03 pm    Post subject: Clarification for std::vector and std::map Can someone explain what is happening in the following code? #include <iostream> #include <vector> #include <map> using namespace std; int main() { vector<int> v; map<char*,int*> m; v.push_back(99); m["TEST"]=&v[0]; cout<<"Vec size: "< v.erase(&v[0]); cout<<"Map: "<<*m["TEST"]< cout<<"Vec size: "< return 0; } I don't understand why I am getting "99" from this line of code cout<<"Map: "<<*m["TEST"]< I erased the vector element and the map only contained the address to that vector element...yet I am still getting the value I pushed_back. What is happening behind the scenes? Is this code safe? Any help would be appreciated Back to top Victor Bazarov Guest Posted: Wed Jul 30, 2003 10:13 pm    Post subject: Re: Clarification for std::vector and std::map "Sir_Ciph3r" <Sir_Ciph3r (AT) nospam (DOT) com> wrote... Quote: Can someone explain what is happening in the following code? Undefined behaviour. Quote: #include <iostream #include #include You're saving a pointer to an object that may vanish into thin air at any point and the map won't know about it. That's called "asking for trouble". Quote: cout<<"Vec size: "< v.erase(&v[0]); There is no 'erase' member that takes a pointer to int. If your implementation _happens_ to have vector::iterator the same as a pointer to the contained type, it doesn't mean it will work on any other computers/compilers/platforms. You have to pass an iterator to 'erase'. Quote: cout<<"Map: "<<*m["TEST"]< After the 'erase' (which probably worked by some weird chance) you're trying to dereference a pointer that doesn't exist any longer. The behaviour is undefined. You get your 99, but you could as well get flying nasal demons. Quote: cout<<"Vec size: "< return 0; } I don't understand why I am getting "99" from this line of code cout<<"Map: "<<*m["TEST"]< I erased the vector element and the map only contained the address to that vector element...yet I am still getting the value I pushed_back. What is happening behind the scenes? Impossible to say. Your code causes undefined behaviour. Quote: Is this code safe? Absolutely not. Victor Back to top ES Kim Guest Posted: Wed Jul 30, 2003 10:42 pm    Post subject: Re: Clarification for std::vector and std::map "Sir_Ciph3r" <Sir_Ciph3r (AT) nospam (DOT) com> wrote Quote: Can someone explain what is happening in the following code? #include <iostream #include #include A container of raw pointers is usually not a good idea. Use string and iterator instead of char* and int*. int main() { vector map<string, vector m; v.push_back(99); m["TEST"]=v.begin(); cout<<"Vec size: "< cout<<"Map: "<<*m["TEST"]< v.erase(v.begin()); // cout<<"Map: "<<*m["TEST"]< // v.erase() invalidates m["TEST"] cout<<"Vec size: "< return 0; } -- ES Kim Back to top Default User Guest Posted: Wed Jul 30, 2003 11:02 pm    Post subject: Re: Clarification for std::vector and std::map Sir_Ciph3r wrote: Quote: I erased the vector element and the map only contained the address to that vector element...yet I am still getting the value I pushed_back. What is happening behind the scenes? Is this code safe? You did all kinds of undefined behavior. Iterators and pointers into vectors are invalidated as soon as you do any kind of operation on the vector that make cause it to resize memory. Erase() certainly could have done that. What probably happened with that the vector saw that you were erasing the last element, so it just backed its valid end pointer by one, and decreased the size so that the element you are pointing to will be overwritten the next time. What are you trying to accomplish? Brian Rodenborn Back to top Sir_Ciph3r Guest Posted: Thu Jul 31, 2003 12:41 am    Post subject: Re: Clarification for std::vector and std::map On Wed, 30 Jul 2003 18:13:48 -0400, "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote: Thanks for the help guys, I am slowing learning. With this advice -> Quote: There is no 'erase' member that takes a pointer to int. If your implementation _happens_ to have vector::iterator the same as a pointer to the contained type, it doesn't mean it will work on any other computers/compilers/platforms. You have to pass an iterator to 'erase'. Would this be a safe way to erase an element from a vector? std::vector<CWindowKeeperWindow*>::iterator IVec; // for(UINT i=0;i<uiSize;i++) for(IVec=m_vecInstalledTimers.begin(); IVec!=m_vecInstalledTimers.end() if((*IVec)->get_window_id()==hTreeItem) { // m_vecInstalledTimers.erase(&m_vecInstalledTimers[i]); IVec=m_vecInstalledTimers.erase(IVec); #ifdef __CWINDOWKEEPER_DEBUG__ ConPrintf(_T("OnDeleteWindow() - erased timer elementn")); #endif } else ++IVec; } 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.