C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Wierd Map Behavior          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message subaruwrx88011 Guest Posted: Tue Feb 21, 2006 12:06 am    Post subject: Wierd Map Behavior I am very new to c++ and very new to the idea of maps. Below is the source of my program. I am getting wierd output from the print and I actually think it might be inserting wrong. Can you guys help me. #include <string> #include <iostream> #include <map> #include <sstream> class CCSI_Env { public: int setItemState(std::string item, std::string state); void print(); private: struct StateStruct { std::string type; union { int m_integer; char* m_string; float m_float; } stateUnion; }; //Map with string for the key and StateType for Value typedef std::map<std::string, StateStruct> ItemStateMap; //Item/State value pair of the enviroment static ItemStateMap m_itemMap; }; CCSI_Env::ItemStateMap CCSI_Env::m_itemMap = ItemStateMap(); int CCSI_Env::setItemState(std::string p_item, std::string p_state) { StateStruct itemState; //Store state as union itemState.type = "STRING"; itemState.stateUnion.m_string = (char*)p_state.c_str();//Convert to char* //Store item with state in map m_itemMap[p_item] = itemState; return 0; } void CCSI_Env::print() { ItemStateMap::iterator mapIterator; std::string message; std::cout<<"******"<<std::endl; for(mapIterator = m_itemMap.begin(); mapIterator != m_itemMap.end(); mapIterator++) { if(mapIterator->second.type == "STRING") { message = mapIterator->first + " = " + mapIterator->second.stateUnion.m_string; std::cout << message << std::endl; } } std::cout<<"******"<<std::endl; } int main() { CCSI_Env m_env; int status; std::string s,t; s = "ONE"; t = "Online"; status = m_env.setItemState(s,t); m_env.print(); s = "TWO"; t = "Online"; status = m_env.setItemState(s,t); m_env.print(); s = "THREE"; t = "Offline"; status = m_env.setItemState(s,t); m_env.print(); s = "FOUR"; t = "Offline"; status = m_env.setItemState(s,t); m_env.print(); } The output should be: ****** ONE = Online ****** ****** ONE = Online TWO = Online ****** ****** ONE = Online THREE = Offline TWO = Online ****** ****** FOUR = Offline ONE = Online THREE = Offline TWO = Online ****** [ 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: Tue Feb 21, 2006 11:06 am    Post subject: Re: Wierd Map Behavior subaruwrx88011 wrote: Quote: I am very new to c++ and very new to the idea of maps. Below is the source of my program. I am getting wierd output from the print and I actually think it might be inserting wrong. Can you guys help me. #include <string #include <iostream #include <map #include <sstream class CCSI_Env { public: int setItemState(std::string item, std::string state); void print(); private: struct StateStruct { std::string type; union { int m_integer; char* m_string; float m_float; } stateUnion; }; //Map with string for the key and StateType for Value typedef std::map<std::string, StateStruct> ItemStateMap; //Item/State value pair of the enviroment static ItemStateMap m_itemMap; }; CCSI_Env::ItemStateMap CCSI_Env::m_itemMap = ItemStateMap(); You don't need the = ... here. An std::map has a non-trivial constructor, which will be called during program start up. Quote: int CCSI_Env::setItemState(std::string p_item, std::string p_state) { StateStruct itemState; //Store state as union itemState.type = "STRING"; itemState.stateUnion.m_string = (char*)p_state.c_str();//Convert to char* Be very careful here. string::c_str() returns a pointer to internal data, which is only guaranteed to be valid until the next non-const function which is called on the string. In general, in StateStruct, you should probably use std::string, and not char*. I would use char const* if all of the strings were known to actually be literals, or for order of initialization issues (not an issue here), but that's about it. And I can't think of any case where I would use char* in a class; if I'm going to modify the string, I definitly want to use std::string, so as not to have to manage the memory myself. Of course, this means that you cannot use a union, but big deal. Using a struct here won't kill you. Quote: //Store item with state in map m_itemMap[p_item] = itemState; return 0; After this statement, the pointer returned by p_state.c_str() is invalid. Since you left a copy of it in the map, your program now has undefined behavior. Anything may happen. Quote: } -- 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 Daniel T. Guest Posted: Tue Feb 21, 2006 11:06 am    Post subject: Re: Wierd Map Behavior In article <1140465890.840553.265600 (AT) f14g2000cwb (DOT) googlegroups.com>, "subaruwrx88011" <subaruwrx88011 (AT) gmail (DOT) com> wrote: Quote: I am very new to c++ and very new to the idea of maps. Below is the source of my program. I am getting wierd output from the print and I actually think it might be inserting wrong. Can you guys help me. int CCSI_Env::setItemState(std::string p_item, std::string p_state) { StateStruct itemState; //Store state as union itemState.type = "STRING"; itemState.stateUnion.m_string = (char*)p_state.c_str();/*Convert to char* */ //Store item with state in map m_itemMap[p_item] = itemState; return 0; } The function above will not work. You store the result of string::c_str() in a char*, but its lifetime is not guaranteed. What you need to do is copy the result of the c_str into memory that you manage. Personally, I would dump the whole idea of a union. Store the data as a string, if you need to convert to an integer or float, do it on the fly. -- Magic depends on tradition and belief. It does not welcome observation, nor does it profit by experiment. On the other hand, science is based on experience; it is open to correction by observation and experiment. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Schüle Daniel Guest Posted: Tue Feb 21, 2006 11:06 am    Post subject: Re: Wierd Map Behavior subaruwrx88011 wrote: Quote: #include <string #include <iostream #include <map #include <sstream class CCSI_Env { public: int setItemState(std::string item, std::string state); better int setItemState(std::string const & item, std::string const & state); think why notice const & Quote: void print(); private: struct StateStruct { std::string type; union { int m_integer; char* m_string; float m_float; } stateUnion; }; //Map with string for the key and StateType for Value typedef std::map<std::string, StateStruct> ItemStateMap; //Item/State value pair of the enviroment static ItemStateMap m_itemMap; }; CCSI_Env::ItemStateMap CCSI_Env::m_itemMap = ItemStateMap(); CCSI_Env::ItemStateMap CCSI_Env::m_itemMap; is sufficient Quote: int CCSI_Env::setItemState(std::string p_item, std::string p_state) { StateStruct itemState; //Store state as union itemState.type = "STRING"; itemState.stateUnion.m_string = (char*)p_state.c_str();//Convert to char* bad idea do you have the guarantie that string, when it gets bigger and bigger won't reallocate itself to some other memory location? leaving you with wild pointer pointing to nirvana Regards, Daniel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrea Griffini Guest Posted: Tue Feb 21, 2006 11:06 am    Post subject: Re: Wierd Map Behavior Quote: int CCSI_Env::setItemState(std::string p_item, std::string p_state) { StateStruct itemState; //Store state as union itemState.type = "STRING"; itemState.stateUnion.m_string = (char*)p_state.c_str();//Convert to char* //Store item with state in map m_itemMap[p_item] = itemState; return 0; } This function has a problem with lifetimes. When you call "c_str()" the returned value is a pointer to the first char of a NUL-terminated C-string but, and this is a most important point, you do not OWN the returned string. In the specific the returned string is a property of the object instance you're asking it to, namely p_state. What happens in the code is that you're taking this pointer to char and storing it in the map where then pointer will live longer than the pointed to sequence of characters; when the function exits the parameter p_state will be destroyed and after that time you're no more allowed to look to what c_str() returned. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kupoo Guest Posted: Thu Mar 09, 2006 11:06 am    Post subject: Re: Wierd Map Behavior subaruwrx88011 wrote: Quote: I am very new to c++ and very new to the idea of maps. Below is the source of my program. I am getting wierd output from the print and I actually think it might be inserting wrong. Can you guys help me. Use question marks. You say you are new to maps, and it's "inserting wrong" which leads me to think you think maps are ordered. It's a common mistake for people new to maps, so disregard this if you already know about it. When you relate two types say for instance: --- A a; B b; map<A,B> r; r[a] = b; --- You are not setting the order of the map's keys. That is done behind the scenes, and is not really predictable. The standard C++ says you should never depend on the order of a map's keys. Thus when you go from map.begin() to map.end(), you can't tell which key will come up first. You didn't provide a copy of the actual weird output, but I imagine it is something like this: --- ****** ONE = Online ****** ****** ONE = Online TWO = Online ****** ****** THREE = Offline ONE = Online TWO = Online ****** ****** TWO = Online FOUR = Offline ONE = Online THREE = Offline ****** --- ....with an order to the keys different from the order in which you added them. There may be an auxiliary std:: type that functions as an ordered map, but what I usually do when the keys need to stay in order is: pair together a map, and a std::queue or std::stack (depending on FIFO or LIFO). Upon adding a key,value pair to the map, I add a new entry to its corresponding key vector! Clumsy, but it can be encapsulated, and thankfully for most all applications of maps we do not need the keys to stay in a certain order. [ 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: Fri Mar 10, 2006 2:06 pm    Post subject: Re: Wierd Map Behavior kupoo wrote: Quote: subaruwrx88011 wrote: I am very new to c++ and very new to the idea of maps. Below is the source of my program. I am getting wierd output from the print and I actually think it might be inserting wrong. Can you guys help me. Use question marks. You say you are new to maps, and it's "inserting wrong" which leads me to think you think maps are ordered. If he's using std::map, it is ordered. In TR1, I think, there is an unordered_map, but the standard map is ordered. Quote: It's a common mistake for people new to maps, so disregard this if you already know about it. When you relate two types say for instance: --- A a; B b; map<A,B> r; r[a] = b; --- You are not setting the order of the map's keys. The order of the map's keys is defined by the ordering relationship of the map -- std::less, by default, but the user can specify what he wants. Quote: That is done behind the scenes, and is not really predictable. The standard C++ says you should never depend on the order of a map's keys. Where did you learn this? It is very definitly wrong; the order is strictly defined. Quote: Thus when you go from map.begin() to map.end(), you can't tell which key will come up first. You must definitly can. -- 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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