C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  querry related to STL map..          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message ravips Guest Posted: Wed Sep 27, 2006 8:52 pm    Post subject: querry related to STL map.. I have a map<char *,int> it does not work consistently. I have used similar sort of implementation as below: map<char *,int> m1; map<char *,int>::iterator i; m1["January"] = 31; m1["February"] = 28; m1["March"] = 31; m1["April"] = 30; m1["May"] = 31; m1["June"] = 30; m1["July"] = 31; This is similar to the map that i have used. Now accessing this yields different results as below: Option1: char *p = "May"; int it = m1[p]; // Works properly returns 31. Option2: char *p = new char[3]; strcpy(p, "May"); int it = m1[p]; // returns 0 .. instead of 31 .... Can anybody help me to resolve this issue? Thanks. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Wed Sep 27, 2006 10:55 pm    Post subject: Re: querry related to STL map.. ravips wrote: Quote: I have a map<char *,int> it does not work consistently. I have used similar sort of implementation as below: map<char *,int> m1; map<char *,int>::iterator i; m1["January"] = 31; m1["February"] = 28; m1["March"] = 31; m1["April"] = 30; m1["May"] = 31; m1["June"] = 30; m1["July"] = 31; This is similar to the map that i have used. Now accessing this yields different results as below: Option1: char *p = "May"; int it = m1[p]; // Works properly returns 31. Option2: char *p = new char[3]; strcpy(p, "May"); int it = m1[p]; // returns 0 .. instead of 31 .... Can anybody help me to resolve this issue? You cross-posted to comp.lang.c++ and received a number of responses answering your question. Here's a link to your post and the responses: http://tinyurl.com/qex8v Best regards, Tom -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Wed Sep 27, 2006 10:58 pm    Post subject: Re: querry related to STL map.. { extraneous linebreaks removed. -mod } "ravips" <ravips123 (AT) gmail (DOT) com> writes: Quote: I have a map<char *,int> it does not work consistently. I have used similar sort of implementation as below: map<char *,int> m1; map<char *,int>::iterator i; m1["January"] = 31; Side note: The type of "January" is 'array of 8 char const'. The implicit conversion of a string literal to 'pointer to char' (non-const!) that this code relies on is deprecated. Better make the key type of your map char const *. Quote: m1["February"] = 28; m1["March"] = 31; m1["April"] = 30; m1["May"] = 31; m1["June"] = 30; m1["July"] = 31; This is similar to the map that i have used. Now accessing this yields different results as below: Option1: char *p = "May"; int it = m1[p]; // Works properly returns 31. Option2: char *p = new char[3]; strcpy(p, "May"); This copies 4 char objects to an array that can only hold 3 char objects. Your program has undefined behavior. Quote: int it = m1[p]; // returns 0 .. instead of 31 .... That's one instance of undefined behavior. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Harald Luessen Guest Posted: Wed Sep 27, 2006 10:59 pm    Post subject: Re: querry related to STL map.. On 27 Sep 2006 "ravips" wrote: Quote: Option2: char *p = new char[3]; strcpy(p, "May"); int it = m1[p]; // returns 0 .. instead of 31 .... Can anybody help me to resolve this issue? Why do you believe that the 4 chars of "May" fit in the 3 chars of p[]? Harald -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top RenjithMohan Guest Posted: Wed Sep 27, 2006 11:24 pm    Post subject: Re: querry related to STL map.. Quote: char *p = new char[3]; Here you are wrong. In order to have "May" you need size 4 to accomodate the null terminator. Quote: strcpy(p, "May"); int it = m1[p]; // returns 0 .. instead of 31 .... Basically your comparator for the map is wrong. string literals have the same type and may be ordered according to the < operator. For char* , the default less<> comparator is no longer acceptable.. The correct way to order the keys, if it is char* is to have a custom comparator for the map. template<class _Ty> struct comp_function : public binary_function<_Ty, _Ty, bool> { // functor for operator< bool operator()(const _Ty& _Left, const _Ty& _Right) const { // apply strcmp to operands return (strcmp(_Left, _Right)< 0); } }; //Notice how we have given our comparator map<char *,int, comp_function<char*> > m1; map<char *,int, comp_function<char*> >::iterator i; m1["January"] = 31; m1["February"] = 28; m1["March"] = 31; m1["April"] = 30; m1["May"] = 31; m1["June"] = 30; m1["July"] = 31; Now the second example(as well as the first) yields the correct result. char *p = new char[4]; strcpy(p, "May"); int it = m1[p]; //now gives 31. Quote: Can anybody help me to resolve this issue? Thanks. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Thu Sep 28, 2006 12:27 am    Post subject: Re: querry related to STL map.. ravips wrote: Quote: I have a map<char *,int> it does not work consistently. I have used similar sort of implementation as below: You've gotten several replies poining out that the code opies too many characters into the allocated array. That's true, and that's a potential problem, but it's not the cause of the match failures. The problem is that map doesn't do anything special with a char* key. That is, two such keys are equal if the values of the pointers are the same. So when the code copies "May" into an allocated array, the address of that copy is different from the address of the original, and the two keys do not match. Further, there's no guarantee that two string literals with the same text (i.e. "May" and "May") will have the same address, so it was just luck that the first version of the code seemed to work. The solution is to either provide a comparator object that calls strcmp to compare two char*s for equality, or to use std::string instead of char*. The latter is simpler. -- -- Pete Author of "The Standard C++ Library Extensions: a Tutorial and Reference." For more information about this book, see www.petebecker.com/tr1book. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu Sep 28, 2006 6:31 pm    Post subject: Re: querry related to STL map.. Thomas Maeder wrote: Quote: "ravips" <ravips123 (AT) gmail (DOT) com> writes: I have a map<char *,int> it does not work consistently. It does. You just don't like the semantics it has. Quote: I have used similar sort of implementation as below: map<char *,int> m1; I can think of very, very few cases where this map might make sense. Do you really want to map addresses to int's? More likely, you want to map C-style strings to ints (judging from the code which follows). In that case: 1. you need to provide a comparison function or functional object, with its type as the third instantiation argument of map, and 2. you need to do something to ensure that no one modifies the C style string once it is in the map---using char const* is the easiest solution. In practice, I generally find it a lot easier to just map std::string to whatever. That takes care of all of the above issues, plus any memory management issues which crop up using the map. Quote: map<char *,int>::iterator i; m1["January"] = 31; Side note: The type of "January" is 'array of 8 char const'. The implicit conversion of a string literal to 'pointer to char' (non-const!) that this code relies on is deprecated. Better make the key type of your map char const *. Agreed, but I think you'd agree that making it std::string would be even better. Quote: m1["February"] = 28; m1["March"] = 31; m1["April"] = 30; m1["May"] = 31; m1["June"] = 30; m1["July"] = 31; This is similar to the map that i have used. Now accessing this yields different results as below: Option1: char *p = "May"; int it = m1[p]; // Works properly returns 31. Whether it returns 0 or 31 is not specified. The implementation may merge identical string literals or not. And since the index is the address of the data, and not the data itself, you get different results depending on whether the string literals have been merged or not. Quote: Option2: char *p = new char[3]; strcpy(p, "May"); This copies 4 char objects to an array that can only hold 3 char objects. Your program has undefined behavior. int it = m1[p]; // returns 0 .. instead of 31 .... That's one instance of undefined behavior. Actually, even correcting the undefined behavior (which isn't actually likely to cause a problem in the case of "May", because most memory allocators cannot allocated an odd number of bytes exactly, and round up), the standard requires 0 here. There's no way that the string literal used to initialize the entry could possibly have the same address as dynamically allocated memory. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu Sep 28, 2006 6:32 pm    Post subject: Re: querry related to STL map.. RenjithMohan wrote: Quote: char *p = new char[3]; Here you are wrong. In order to have "May" you need size 4 to accomodate the null terminator. strcpy(p, "May"); int it = m1[p]; // returns 0 .. instead of 31 .... Basically your comparator for the map is wrong. string literals have the same type and may be ordered according to the < operator. For char*, the default less<> comparator is no longer acceptable.. The correct way to order the keys, if it is char* is to have a custom comparator for the map. template<class _Ty struct comp_function : public binary_function<_Ty, _Ty, bool { // functor for operator bool operator()(const _Ty& _Left, const _Ty& _Right) const { // apply strcmp to operands return (strcmp(_Left, _Right)< 0); } }; Just one question: why on earth do you make this a template, since it will fail to instantiate unless _Ty is char* or char const*?j Quote: //Notice how we have given our comparator map<char *,int, comp_function<char*> > m1; And there is still one problem left: m1.begin()[ 0 ] = 'z' ; Looks fine to me, and the compiler won't complain. But supposing the initializations below, I rather doubt that m1["zanuary"] will return 31 after this. The obvious solution is to simply write: map< std::string, int > m1 ; and be done with it. If you really insist on C style strings, then at the very least: map< char const*, int, comp_function > m1 ; -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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