C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  deleting this          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Abhijeet Guest Posted: Tue Apr 20, 2004 10:25 pm    Post subject: deleting this I was just toying around idea of deleting this from a member function. Was expecting that any acess to member variable or function after deleting sould give me dump.Cause now I am trying to access freed memory. Is my assumption correct? I tried following code --------------------------------------------------- #include <iostream.h> class A { public: A():_i(0){} A(int i_):_i(i_){} void killself(){cout<<"deleting this"< test = ((A*)t his);test=0;_i = 5;} int getI(){return _i;} void setI(int i_=0){_i= i_;} private: int _i; }; int main() { A a; a.setI(10); cout< a.killself(); cout< A* pa = NULL; pa = new A(10); cout< cout<<"pointer value"< pa->killself(); cout< cout<<"pointer value"< delete pa; pa = NULL; return 0; } ------------------------------------------ As I was expecting msvc6 gave dump at statement "a.killself();".But if I comment this line I get dump at "delete pa;". I am unable to understand the behaviour.Can somebody help me understand what's going on? I think there was thread on similar topic, but I m unable to locate it.Would even appreciate if get link for same. Regards, Abhijeet Ps. - If I comiple it with "gcc version 3.3.1 (cygming special)" code runs perfectly fine without any dump. Same is case with Sun workshop 5.6.Which is confusing me more. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stephen Ludin Guest Posted: Wed Apr 21, 2004 9:24 am    Post subject: Re: deleting this Abhijeet wrote: Quote: I was just toying around idea of deleting this from a member function. Was expecting that any acess to member variable or function after deleting sould give me dump.Cause now I am trying to access freed memory. Is my assumption correct? I tried following code --------------------------------------------------- #include <iostream.h class A { public: A():_i(0){} A(int i_):_i(i_){} void killself(){cout<<"deleting this"< test = ((A*)t his);test=0;_i = 5;} int getI(){return _i;} void setI(int i_=0){_i= i_;} private: int _i; }; int main() { A a; a.setI(10); cout< a.killself(); cout< A* pa = NULL; pa = new A(10); cout< cout<<"pointer value"< pa->killself(); cout< cout<<"pointer value"< delete pa; pa = NULL; return 0; } You are playing around in the world of 'undefined' behavior. Toying with freed memory will not guarantee a crash or a seg fault - that just happens to be the likely result. The first a.killself() tries to delete a variable on the stack. This will usually crash. The second killall ( pa->killself() ), with the exception of assigning to _i is valid, though "bad". Observe: void killself() { cout << "deleting this" < delete this; A* test = ((A*)this); // ok, test points to the now deleted this test=0; // ok - test points to 0 _i = 5; // very iffy - assigning to free'd memory - might crash } Unless the process returned the page containing this to the system via sbrk(), unmapping virtual memory, or otherwise marking the page as unwritable, the assignment of 5 to this->_i will usually succeed without complaint. Assuming the page remains valid pa->getl() after killself() might succeed as well. Now, when you call delete pa after calling pa->killself() we are in a double free situation. Similar to 'deleting' the stack variable this is often, though certainly not always caught by the allocator. Might, maybe, often, not always - these are the results of mishandling memory. The only guarantee you get is from Murphy's Law which when applied says it _won't_ crash, at least until your most important client tries to use it. Hope that helps, -stephen [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Heinz Ozwirk Guest Posted: Wed Apr 21, 2004 2:00 pm    Post subject: Re: deleting this "Abhijeet" <abhi_10dulkr (AT) indiatimes (DOT) com> schrieb im Newsbeitrag news:f0bf477c.0404200221.2ae7aaa9 (AT) posting (DOT) google.com... Quote: I was just toying around idea of deleting this from a member function. Was expecting that any acess to member variable or function after deleting sould give me dump.Cause now I am trying to access freed memory. Is my assumption correct? I tried following code --------------------------------------------------- #include <iostream.h class A { public: A():_i(0){} A(int i_):_i(i_){} void killself(){cout<<"deleting this"< test = ((A*)t his);test=0;_i = 5;} int getI(){return _i;} void setI(int i_=0){_i= i_;} private: int _i; }; int main() { A a; a.setI(10); cout< a.killself(); cout< A* pa = NULL; pa = new A(10); cout< cout<<"pointer value"< pa->killself(); cout< cout<<"pointer value"< delete pa; pa = NULL; return 0; } ------------------------------------------ As I was expecting msvc6 gave dump at statement "a.killself();".But if I comment this line I get dump at "delete pa;". I am unable to understand the behaviour.Can somebody help me understand what's going on? You are calling delete pa twice, first in pa->killself() and then explicitly in main. That is undefined behaviour (or unspecified?). Quote: Ps. - If I comiple it with "gcc version 3.3.1 (cygming special)" code runs perfectly fine without any dump. Same is case with Sun workshop 5.6.Which is confusing me more. Undefined behaviour includes behaving as expected by some users. HTH Heinz [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Wed Apr 21, 2004 3:47 pm    Post subject: Re: deleting this [email]abhi_10dulkr (AT) indiatimes (DOT) com[/email] (Abhijeet) wrote in message news:<f0bf477c.0404200221.2ae7aaa9 (AT) posting (DOT) google.com>... Quote: I was just toying around idea of deleting this from a member function. Was expecting that any acess to member variable or function after deleting sould give me dump. Cause now I am trying to access freed memory. It is undefined behavior. It might dump. It might work. It might reformat your hard disk. My experience is that read-only access will usually work, unless it is an important demo in front of your principal customer, and that write access will usually work, but will screw up the free space arena, causing an almost impossible to find error later. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr 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 Michiel Salters Guest Posted: Wed Apr 21, 2004 3:50 pm    Post subject: Re: deleting this [email]abhi_10dulkr (AT) indiatimes (DOT) com[/email] (Abhijeet) wrote in message news:<f0bf477c.0404200221.2ae7aaa9 (AT) posting (DOT) google.com>... Quote: I was just toying around idea of deleting this from a member function. Have you read chapter [16] of the FAQ? The footer contains a link. Quote: V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Richter Guest Posted: Wed Apr 21, 2004 7:35 pm    Post subject: Re: deleting this Hi, Quote: I was just toying around idea of deleting this from a member function. Typically, not a good idea except for very special purposes. Quote: Was expecting that any acess to member variable or function after deleting sould give me dump. C++ does not give you this guarantee. The "this" pointer still points to something, and *if you're lucky* you can mess with the memory it points to. Just that you cannot expect that this memory is in any consistent state. It is likely to be reused for objects newly created. It may throw, but there's no need for throwing. Quote: Is my assumption correct? No. Quote: a.killself(); cout< Of course a bad idea trying to delete an automatic object. Anything could happen. Quote: A* pa = NULL; pa = new A(10); cout< cout<<"pointer value"< pa->killself(); cout< cout<<"pointer value"< delete pa; pa = NULL; return 0; As I was expecting msvc6 gave dump at statement "a.killself();".But if I comment this line I get dump at "delete pa;". You're releasing the memory pointed to by "pa" twice. Once in killself, a second time in the explicit delete call. The memory is obviously not returned to the Os immediately so you're still holding a pointer to "valid memory" in pa after a "killself". Just not to a valid A object. Quote: Ps. - If I comiple it with "gcc version 3.3.1 (cygming special)" code runs perfectly fine without any dump. Same is case with Sun workshop 5.6.Which is confusing me more. It doesn't run "perfectly fine". It may or may not crash. If it doesn't crash, then you're at best lucky. C++ does not require that this program must dump. So long, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Abhijeet Guest Posted: Fri Apr 23, 2004 12:22 am    Post subject: Re: deleting this Thank you guys, Actually was trying to explain same thing to an interviewer.But seems like she was stuck up on thing that it has to give segmentation violation.She didn't accept that its a undefined behaviour. So asked to check if i was heading in some wrong direction. Thanks & Regards, Abhijeet Thomas Richter <thor (AT) cleopatra (DOT) math.tu-berlin.de> wrote Quote: Hi, I was just toying around idea of deleting this from a member function. Typically, not a good idea except for very special purposes. Was expecting that any acess to member variable or function after deleting sould give me dump. C++ does not give you this guarantee. The "this" pointer still points to something, and *if you're lucky* you can mess with the memory it points to. Just that you cannot expect that this memory is in any consistent state. It is likely to be reused for objects newly created. It may throw, but there's no need for throwing. Is my assumption correct? No. a.killself(); cout< Of course a bad idea trying to delete an automatic object. Anything could happen. A* pa = NULL; pa = new A(10); cout< cout<<"pointer value"< pa->killself(); cout< cout<<"pointer value"< delete pa; pa = NULL; return 0; As I was expecting msvc6 gave dump at statement "a.killself();".But if I comment this line I get dump at "delete pa;". You're releasing the memory pointed to by "pa" twice. Once in killself, a second time in the explicit delete call. The memory is obviously not returned to the Os immediately so you're still holding a pointer to "valid memory" in pa after a "killself". Just not to a valid A object. Ps. - If I comiple it with "gcc version 3.3.1 (cygming special)" code runs perfectly fine without any dump. Same is case with Sun workshop 5.6.Which is confusing me more. It doesn't run "perfectly fine". It may or may not crash. If it doesn't crash, then you're at best lucky. C++ does not require that this program must dump. So long, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] 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 (Moderated) 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.