C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Seg Fault Help          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message subaruwrx88011 Guest Posted: Wed Feb 15, 2006 2:06 pm    Post subject: Seg Fault Help I am using a map that takes an string as the key and a structure that is being stored. struct StateType { int i; float f; char* s; } typedef std::map<std::string, StateType> ItemStateMap; ItemStateMap m_itemMap; StateType state; state.s = "HelloWorld"; m_itemMap["FirstPosition"] = state; This is a really watered down version of what I am doing but this is exactly what its doing in a nutshell. I am getting a segmentation fault on m_itemMap["FirstPosition"] = state. Can anyone tell me what I am doing wrong? I tried to use m_itemMap.insert("FirstPosition", state) but that wouldn't compile. Thanks in advance subaruwrx88011 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ralf Fassel Guest Posted: Wed Feb 15, 2006 6:06 pm    Post subject: Re: Seg Fault Help * "subaruwrx88011" <subaruwrx88011 (AT) gmail (DOT) com> | This is a really watered down version of what I am doing but this is | exactly what its doing in a nutshell. I am getting a segmentation | fault Use a debugger to find the cause of that segmentation fault. Or provide a compilable example which reproduces the problem. The code you show does not provide a compilable example, and if I turn it into one, it runs w/o segfault. R' [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Paul Floyd Guest Posted: Thu Feb 16, 2006 2:06 am    Post subject: Re: Seg Fault Help On 15 Feb 2006 08:26:45 -0500, subaruwrx88011 <subaruwrx88011 (AT) gmail (DOT) com> wrote: Quote: I am using a map that takes an string as the key and a structure that is being stored. struct StateType { int i; float f; char* s; } A class with a pointer but without a copy assignment operator. Probably not a good idea, and a fairly likely cause of your problem; A bientot Paul -- Paul Floyd http://paulf.free.fr (for what it's worth) Surgery: ennobled Gerald. [ 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: Sat Feb 18, 2006 1:06 am    Post subject: Re: Seg Fault Help Paul Floyd ha scritto: Quote: A class with a pointer but without a copy assignment operator. Probably not a good idea, and a fairly likely cause of your problem; May be you're right in the specific (i think the example has been "watered down" too much) but telling that a class with a pointer but without a copy constructor is a likely source of problems seems to me too much. Something nicer that's easy to remember is the rule of "the three amigos" (as named in "C++ FAQs"): if your class has ANY of destructor, assignment operator or copy constructor then probably it needs indeed ALL three of them. This rule covers cases where the indirect ownership is not based on a pointer (but for example on an handle, or indirectly on registration) and avoids false positives on classes that logically point out without owenership. In the shown case there is no destructor, no copy constructor and no assignment operator, so I don't know why you thought that there was ownership of the string involved (indeed I think that the destructor was lost in the simplification process before posting). Surely in general at least a line of comment saying that no explicit copy constructor is required because the default generated one is OK would be nice, at least to inform a future reader that it wasn't something that was simply forgot. Andrea [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Sat Feb 18, 2006 1:06 am    Post subject: Re: Seg Fault Help Quote: subaruwrx88011 wrote: I am using a map that takes an string as the key and a structure that is being stored. struct StateType { int i; float f; char* s; } Paul Floyd wrote: Quote: A class with a pointer but without a copy assignment operator. Probably not a good idea, and a fairly likely cause of your problem; "Fairly likely," okay. But not definitely. The OP didn't explain what is being done with this pointer... But consider the fact that struct StateType doesn't have a destructor, either. So there's no danger of double-delete. It could be that s only ever points to string literals, which should NEVER be deleted. Any undefined behavior here? Note that there's nothing being explicitly created or deleted from heap... #include <iostream> #include <map> #include <ostream> #include <string> struct StateType { int i; float f; char* s; }; typedef std::map<std::string, StateType> ItemStateMap; int main() { ItemStateMap m_itemMap; StateType state; state.i = 1; state.f = 123.4f; state.s = "HelloWorld"; m_itemMap["FirstPosition"] = state; state.i = 2; state.f = 246.8f; state.s = "GoodbyeWorld"; m_itemMap["LastPosition"] = state; state = m_itemMap["FirstPosition"]; std::cout << "First Position: " << state.i << ' ' << state.f << ' ' << state.s << std::endl; state = m_itemMap["LastPosition"]; std::cout << "Last Position: " << state.i << ' ' << state.f << ' ' << state.s << std::endl; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top eden Guest Posted: Sat Feb 18, 2006 1:06 am    Post subject: Re: Seg Fault Help subaruwrx88011 wrote: Quote: StateType state; state.s = "HelloWorld"; This is the problem. You didn't allocate a memory for that pointer. You can't just assign a value to it. Why you don't use std::string also for the s? Like: struct StateType { int i; float f; std::string s; } Goran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sat Feb 18, 2006 2:06 am    Post subject: Re: Seg Fault Help In article <slrndv78dd.ot.root (AT) bisanne (DOT) netpratique.fr>, Paul Floyd <root (AT) 127 (DOT) 0.0.1> writes Quote: On 15 Feb 2006 08:26:45 -0500, subaruwrx88011 subaruwrx88011 (AT) gmail (DOT) com> wrote: I am using a map that takes an string as the key and a structure that is being stored. struct StateType { int i; float f; char* s; } A class with a pointer but without a copy assignment operator. Probably not a good idea, and a fairly likely cause of your problem; I doubt it. We need to distinguish between pointers that own a resource (which need to handled with care and usually some form of smart-pointer is part of the solution) and pointers that just reference a resource. As the above struct lacks any member functions it would seem that s is likely to be of the latter type. If it isn't because s is used to hang on to a dynamically created array of char then there is a lot more wrong with the C++ design which would have to cope with exceptions (that is th reason that the C design solution is inappropriate in C++) -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/ youcandoit/projects [ 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: Mon Feb 20, 2006 11:06 am    Post subject: Re: Seg Fault Help eden wrote: Quote: subaruwrx88011 wrote: StateType state; state.s = "HelloWorld"; This is the problem. You didn't allocate a memory for that pointer. You can't just assign a value to it. Sure he can, as long as the value is a pointer (and the compiler won't allow any other type). In this case, the implicit conversion char const[] to char const* is taking place, and there is no problem. What he can't do, of course, is dereference the pointer before assigning something to it. Quote: Why you don't use std::string also for the s? Like: struct StateType { int i; float f; std::string s; } While generally a good idea, there are exceptions. In cases where all of the string values will be literals, for example, I still use char const*. And when order of initialization issues raise their head, using primitive types has definite advantages. (Not knowing what he is doing with StateType, I cannot say whether either of these issues affect this class -- and except for the order of initialization issues, std::string is never wrong, and usually a superior choice.) At any rate, there is no problem in the code he actually posted. There are two possible explinations for his problem: -- He simplified too much. If the string in question isn't really a string literal, but e.g. allocated on the heap, and freed in the destructor, then the lack of an explicit copy constructor would be a real problem. -- He has a problem elsewhere, which only shows up at this place. Errors which corrupt the heap have a nasty propensity of not being immediatly apparent, but causing the program to crash on some later, unrelated allocation or free (and of course, creating a new element in a map requires allocation on the part of the map). The usual solution in a professional environment for the latter problem is to run Purify on the program. As far as I know, however, Purify has no "amateur" ou "student" licenses, and it's normal license is priced outside the bounds of a student or an amateur, so this option isn't normally available unless your company is paying. -- 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 eden Guest Posted: Mon Feb 20, 2006 1:06 pm    Post subject: Re: Seg Fault Help kanze wrote: Quote: eden wrote: subaruwrx88011 wrote: StateType state; state.s = "HelloWorld"; This is the problem. You didn't allocate a memory for that pointer. You can't just assign a value to it. Sure he can, as long as the value is a pointer (and the compiler won't allow any other type). In this case, the implicit conversion char const[] to char const* is taking place, and there is no problem. What he can't do, of course, is dereference the pointer before assigning something to it. Well, I must admit I was too fast in my conclusion that he can't assign string literals to the pointer. I was assuming that he is using some char arrays on the stack that could be out of scope later when the map is referenced, but of course, this is assuming too much. goran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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