C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Seg fault when delete a pointer to vector<int>          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Piotr Guest Posted: Mon Jan 30, 2006 5:00 am    Post subject: Seg fault when delete a pointer to vector<int> I have a class 'Statistics' which has a private attribute ' vector<int>* _x;' And in the destructor of the Statistics, I have this code to free the memory: Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } But I get a Seg. Fault when I execute the program. Can you please tell me what did I do wrong? Thank you. (gdb) bt #0 0x00248eaa in __gnu_cxx::__pool<true>::_M_reclaim_block () from /usr/lib/libstdc++.so.6 #1 0x0250da84 in __gnu_cxx::__mt_alloc<int, __gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, true> >::deallocate (this=0x9107474, __p=0xdadadada, __n=152471256) at mt_allocator.h:746 #2 0x0250daac in std::_Vector_base<int, std::allocator<int> Quote: ::_M_deallocate (this=0x9107474, __p=0x91686d8, __n=0) at stl_vector.h:123 #3 0x0250dacd in ~_Vector_base (this=0x91686d8) at stl_vector.h:109 #4 0x0250db0b in ~vector (this=0x9107474) at stl_vector.h:273 #5 0x0250caf2 in ~Statistics (this=0xbfd85ad8) at Statistics.cpp:54 Back to top Luke Meyers Guest Posted: Mon Jan 30, 2006 6:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Piotr wrote: Quote: I have a class 'Statistics' which has a private attribute ' vector<int>* _x;' And in the destructor of the Statistics, I have this code to free the memory: Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } But I get a Seg. Fault when I execute the program. Can you please tell me what did I do wrong? Looks like _x isn't valid at the point where you try to delete it. Would need to see the rest of the code dealing with _x to know how that happened. Are you setting it? Attempting conversions? Deleting it twice? Quote: (gdb) bt #0 0x00248eaa in __gnu_cxx::__pool<true>::_M_reclaim_block () from /usr/lib/libstdc++.so.6 #1 0x0250da84 in __gnu_cxx::__mt_alloc<int, __gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, true> >::deallocate (this=0x9107474, __p=0xdadadada, __n=152471256) at mt_allocator.h:746 Note the suspicious hex address. __p is likely to be the value of _x, by my guess. Use your debugger to trace what's going on. Luke Back to top Piotr Guest Posted: Mon Jan 30, 2006 6:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Thank you. Can you please explain what you meant by 'Attempting conversions?' And I have set it somewhere, otherwise, the code won't execute, right? Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } Back to top Luke Meyers Guest Posted: Mon Jan 30, 2006 7:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Piotr wrote: Quote: Thank you. Can you please explain what you meant by 'Attempting conversions?' Argh... i wrote up a whole reply and lost it to the bit bucket... sigh. Anyway, conversions. I mean mucking about with explicit casts, or any of the various forms of implicit conversions. Quote: And I have set it somewhere, otherwise, the code won't execute, right? Uh, no. If you haven't initialized _x, its contents are undefined, which means it points to a random location of memory. What do you suppose happens when you try to delete a pointer like that? Quote: Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } Yup. What's with the "if(_x)", by the way? Can x_ become null somehow? If so, I'd reconsider your design not to have two destruction paths for x_. Luke Back to top Luke Meyers Guest Posted: Mon Jan 30, 2006 7:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Damn, I guess I'm just having a Bad Usenet Day. Back to top Piotr Guest Posted: Mon Jan 30, 2006 7:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Can you please tell me what is the right way to write the destructor if i have a private attribute of ' vector<int>* _x;' in my class? class Statistic { private: vector<int>* _x; }; Back to top Luke Meyers Guest Posted: Mon Jan 30, 2006 7:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Piotr wrote: Quote: Thank you. Can you please explain what you meant by 'Attempting conversions?' Mucking about with typecasts, conversion operators, single-argument constructors, operator=, etc. Basically just stuff that would make it easier to mess up some fancy pointer hocus-pocus. Quote: And I have set it somewhere, otherwise, the code won't execute, right? Uh, no. If you never initialize _x, your program will (hopefully) seg fault because you are attempting to delete memory pointed to by an uninitialized address. Quote: Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } What do you mean "if(_x)"? Don't you know whether it's null? Is it possible that _x became null at some point? Is it really necessary for there to be two destruction paths for _x and its referent? If you want more help, you should post, I say again, the rest of the code dealing with _x. Luke Luke Back to top Kai-Uwe Bux Guest Posted: Mon Jan 30, 2006 8:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Piotr wrote: Quote: Can you please tell me what is the right way to write the destructor if i have a private attribute of ' vector<int>* _x;' in my class? class Statistic { private: vector<int>* _x; }; You are asking for "the right way"? There is no such thing. It depends. The most important question here is: ?? Can a pointee be shared among different objects of this class ?? a) If so, you will either need to be *very* careful or use a smart pointer instead of a raw pointer. The reason is that you will want to delete the pointer only if there are no other references to the same vector<int> object left (rule: the last pointer pointing there has to kill it). b) If not, you could and should just have a vector<int> member. In this case, the default destructor of Statistic would take care of the proper destruction of the member. Note: Even if you did not intend the pointee to be shared, you may have implemented the copy constructor or assignment operator in a way that shares the pointee. The default copy constructor and assignment operator, for instance, will happily copy the pointer so that afterwards you have two Statistic objects whose _x members point to the same vector<int>. Rule of thumb: avoid pointer members. They just cause a headache. If you need them some reason, consider using something like tr1::shared_ptr (formerly known as boost::shared_ptr). Best Kai-Uwe Bux Back to top Jacek Dziedzic Guest Posted: Mon Jan 30, 2006 9:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> Piotr wrote: Quote: I have a class 'Statistics' which has a private attribute ' vector<int>* _x;' And in the destructor of the Statistics, I have this code to free the memory: Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } But I get a Seg. Fault when I execute the program. Can you please tell me what did I do wrong? First of all, the 'if' part is unnecessary -- delete works fine for NULL pointers. Second of all -- I'd bet that either a) you don't allocate _x with new, b) you had already deleted _x earlier, c) you have corrupted _x. Try inserting some 'cerr << "I am here" << endl;' constructs into sensitive places, like the destructor or the place where you actually new your _x. HTH, - J. Back to top David Harmon Guest Posted: Mon Jan 30, 2006 10:00 am    Post subject: Re: Seg fault when delete a pointer to vector<int> On 29 Jan 2006 22:08:13 -0800 in comp.lang.c++, "Piotr" <rasputin.piotr (AT) gmail (DOT) com> wrote, Quote: Can you please tell me what is the right way to write the destructor if i have a private attribute of ' vector<int>* _x;' in my class? The right way? The right way is to change it to vector<int> x; Then let the vector destructor handle it. Back to top Daniel T. Guest Posted: Mon Jan 30, 2006 2:00 pm    Post subject: Re: Seg fault when delete a pointer to vector<int> In article <1138596844.462403.146100 (AT) o13g2000cwo (DOT) googlegroups.com>, "Piotr" <rasputin.piotr (AT) gmail (DOT) com> wrote: Quote: I have a class 'Statistics' which has a private attribute ' vector<int>* _x;' And in the destructor of the Statistics, I have this code to free the memory: Statistics::~Statistics() { if (_x) { delete _x; // Seg fault here... } } But I get a Seg. Fault when I execute the program. Can you please tell me what did I do wrong? Thank you. (gdb) bt #0 0x00248eaa in __gnu_cxx::__pool<true>::_M_reclaim_block () from /usr/lib/libstdc++.so.6 #1 0x0250da84 in __gnu_cxx::__mt_alloc<int, __gnu_cxx::__common_pool_policy<__gnu_cxx::__pool, true> >::deallocate (this=0x9107474, __p=0xdadadada, __n=152471256) at mt_allocator.h:746 #2 0x0250daac in std::_Vector_base<int, std::allocator<int ::_M_deallocate (this=0x9107474, __p=0x91686d8, __n=0) at stl_vector.h:123 #3 0x0250dacd in ~_Vector_base (this=0x91686d8) at stl_vector.h:109 #4 0x0250db0b in ~vector (this=0x9107474) at stl_vector.h:273 #5 0x0250caf2 in ~Statistics (this=0xbfd85ad8) at Statistics.cpp:54 My guess is that at some point in your code, you assign one Statistics object to another. Here are a few things to try: First: Make sure _x is default initialized properly. Statistics::Statistics():_x(0) { } or Statistics::Statistics():_x(new vector<int>) { } Second: Make sure you aren't passing the pointer to other objects accidentally: class Statistics { // don't implement the two functions below Statistics( const Statistics& ); Statistics& operator=( const Statistics& ); // other privates public: // public stuff }; I have a feeling that when you put the two function definitions above in, your code won't compile anymore. That tells you that you had multiple Statistics objects sharing the same vector<int> object, the first one to go out of scope deleted the vector<int> but the second one doesn't know that so it tries to delete it again. -- 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. 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