C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Reusing a deleted pointer.          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Alan Guest Posted: Sat Dec 18, 2004 5:39 pm    Post subject: Reusing a deleted pointer. Using VC++ (1998) compiler with PFE32 editor in Win2K Pro SP4. (DigitalMars CD on order. ) The program (below) instantiates a class and then deletes it. I would have thought that reusing the deleted pointer would have caused an exception, but it does not - or my coding or thinking is incorrect? #include <iostream> using namespace std; class A { int a; public: A(): a(0) { cout << " A::A()" << endl; } ~A() { cout << "~A::A()" << endl << endl; } void print() { cout << "a = " << a << endl; } void set(const int& aa) { a=aa; print(); } }; int main() { A* b = new A; // ptr A& c = *b; // ref b->set(10); // test ptr c.set(5); // test ref delete b; try { A& y = *b; // no problems! y.set(55); // causes system error (msg in Dos-box) } catch(...) { cout << "failed" << endl; exit(-1); } cout << "No failure" << endl; // always gets here return 0; } Back to top David Lindauer Guest Posted: Sat Dec 18, 2004 5:44 pm    Post subject: Re: Reusing a deleted pointer. Alan wrote: Quote: Using VC++ (1998) compiler with PFE32 editor in Win2K Pro SP4. (DigitalMars CD on order. ) The program (below) instantiates a class and then deletes it. I would have thought that reusing the deleted pointer would have caused an exception, but it does not - or my coding or thinking is incorrect? reusing a deleted pointer (or really any invalid pointer) simply causes 'undefined behavior'. Now undefined behaviour 'could' result in an exception, the same way it 'could' result in your computer taking a trip to mars... but neither is really necessary and if it seems to work ok that could be attributed to undefined behavior as well. David Back to top Jon Bell Guest Posted: Sat Dec 18, 2004 6:37 pm    Post subject: Re: Reusing a deleted pointer. In article <10s8qs23g9vbd17 (AT) news (DOT) supernews.com>, Alan <alan (AT) surfbest (DOT) net> wrote: Quote: The program (below) instantiates a class and then deletes it. I would have thought that reusing the deleted pointer would have caused an exception, but it does not - or my coding or thinking is incorrect? Deleting a pointer simply tells the operating system that it is free to allocate the memory that it points to, for something else, whenever the OS feels it is necessary. It does not actually clear that memory, or change the pointer itself, because that would be a waste of CPU cycles in many situations. Therefore there is a good chance that you will actually be able to follow that pointer to whatever it originally pointed to, for some time thereafter... but you can't predict how long this will work. -- Jon Bell <jtbellm4h (AT) presby (DOT) edu> Presbyterian College Dept. of Physics and Computer Science Clinton, South Carolina USA Back to top Dave Townsend Guest Posted: Sat Dec 18, 2004 7:32 pm    Post subject: Re: Reusing a deleted pointer. "Alan" <alan (AT) surfbest (DOT) net> wrote Quote: Using VC++ (1998) compiler with PFE32 editor in Win2K Pro SP4. (DigitalMars CD on order. ) The program (below) instantiates a class and then deletes it. I would have thought that reusing the deleted pointer would have caused an exception, but it does not - or my coding or thinking is incorrect? #include <iostream using namespace std; class A { int a; public: A(): a(0) { cout << " A::A()" << endl; } ~A() { cout << "~A::A()" << endl << endl; } void print() { cout << "a = " << a << endl; } void set(const int& aa) { a=aa; print(); } }; int main() { A* b = new A; // ptr A& c = *b; // ref b->set(10); // test ptr c.set(5); // test ref delete b; try { A& y = *b; // no problems! y.set(55); // causes system error (msg in Dos-box) } catch(...) { cout << "failed" << endl; exit(-1); } cout << "No failure" << endl; // always gets here return 0; } To follow up on the other replies, if you look more carefully at the contents of the object "b" you will see ( at least on VC++6) that after deletion, the memory used by b is written over and the original value a is destroyed! I tinkered a bit with this program, if you do a few more allocations of A's, you will eventually find that an object is allocated at the same address as b, very nasty, now modifying b modifies some other unrelated object! . Just goes to show, just because you don't crash, all may not be well. By the way, your object is too simple to cause a crash, the original address of b would almost always point to to valid memory so deferencing it would not cause a crash after you delete it - just put some virtual functions in your class and try calling one of those after deletion and then you'll crash for sure..! dave Back to top Alan Guest Posted: Sat Dec 18, 2004 8:33 pm    Post subject: Re: Reusing a deleted pointer. "Dave Townsend" <datownsend (AT) comcast (DOT) net> wrote Quote: "Alan" <alan (AT) surfbest (DOT) net> wrote in message news:10s8qs23g9vbd17 (AT) news (DOT) supernews.com... [snip] To follow up on the other replies, if you look more carefully at the contents of the object "b" you will see ( at least on VC++6) that after deletion, the memory used by b is written over and the original value a is destroyed! I tinkered a bit with this program, if you do a few more allocations of A's, you will eventually find that an object is allocated at the same address as b, very nasty, now modifying b modifies some other unrelated object! . Just goes to show, just because you don't crash, all may not be well. By the way, your object is too simple to cause a crash, the original address of b would almost always point to to valid memory so deferencing it would not cause a crash after you delete it - just put some virtual functions in your class and try calling one of those after deletion and then you'll crash for sure..! dave Thanks Dave, John and David. Back to top Alan Guest Posted: Mon Dec 20, 2004 11:58 am    Post subject: Re: Reusing a deleted pointer. "Dave Townsend" <datownsend (AT) comcast (DOT) net> wrote Quote: "Alan" <alan (AT) surfbest (DOT) net> wrote in message news:10s8qs23g9vbd17 (AT) news (DOT) supernews.com... [snip] To follow up on the other replies, if you look more carefully at the contents of the object "b" you will see ( at least on VC++6) that after deletion, the memory used by b is written over and the original value a is destroyed! I tinkered a bit with this program, if you do a few more allocations of A's, you will eventually find that an object is allocated at the same address as b, very nasty, now modifying b modifies some other unrelated object! . Just goes to show, just because you don't crash, all may not be well. By the way, your object is too simple to cause a crash, the original address of b would almost always point to to valid memory so deferencing it would not cause a crash after you delete it - just put some virtual functions in your class and try calling one of those after deletion and then you'll crash for sure..! dave I've put a virtual function in but it didn't fail until I introduced a static variable into the class for instance counting. I've cut and cut the original program.. The code below (I'm using VC++6 too) succeeds or fails, per user choice. What puzzles me is why one class fails and the other doesn't - as the differences appear small. #include <iostream> using namespace std; class Failure { int a; static int count; // count instances, display them public: Failure(): a(0) { cout << "constructor " << count++ << endl; } ~Failure() { cout << "destructor " << --count << endl; } virtual void set(const int& aa) { a=aa; } }; class Success { int a; static int count; // count instances, no display public: Success(): a(0) { count++; cout << "constructor ?" << endl; } ~Success() { --count; cout << "destructor ?" << endl; } virtual void set(const int& aa) { a=aa; } }; int Failure::count = 1; int Success::count = 1; int main(int argc) { // no command line arguments (succeeds) if( argc == 1 ) Success* s = new Success; delete s; cout << "class Success *s: try { s->set(1000) }" << endl; try { s->set(1000); } catch(...) { cout << "try failed" << endl; exit(-1); } } // one or more command line arguments (fails) else { Failure* f = new Failure; delete f; cout << "class Failure *f: try { f->set(1000) }" << endl; try { f->set(1000); } catch(...) { cout << "try failed" << endl; exit(-1); } } cout << "No failure" << endl; return 0; } Regards, Alan Back to top Alan Guest Posted: Mon Dec 20, 2004 12:38 pm    Post subject: Re: Reusing a deleted pointer. "Alan" <alan (AT) surfbest (DOT) net> wrote Quote: "Dave Townsend" <datownsend (AT) comcast (DOT) net> wrote "Alan" <alan (AT) surfbest (DOT) net> wrote in message news:10s8qs23g9vbd17 (AT) news (DOT) supernews.com... [snip] To follow up on the other replies, if you look more carefully at the contents of the object "b" you will see ( at least on VC++6) that after deletion, the memory used by b is written over and the original value a is destroyed! I tinkered a bit with this program, if you do a few more allocations of A's, you will eventually find that an object is allocated at the same address as b, very nasty, now modifying b modifies some other unrelated object! . Just goes to show, just because you don't crash, all may not be well. By the way, your object is too simple to cause a crash, the original address of b would almost always point to to valid memory so deferencing it would not cause a crash after you delete it - just put some virtual functions in your class and try calling one of those after deletion and then you'll crash for sure..! dave I've put a virtual function in but it didn't fail until I introduced a static variable into the class for instance counting. I've cut and cut the original program.. The code below (I'm using VC++6 too) succeeds or fails, per user choice. What puzzles me is why one class fails and the other doesn't - as the differences appear small. #include <iostream using namespace std; class Failure { int a; static int count; // count instances, display them public: Failure(): a(0) { cout << "constructor " << count++ << endl; } ~Failure() { cout << "destructor " << --count << endl; } virtual void set(const int& aa) { a=aa; } }; class Success { int a; static int count; // count instances, no display public: Success(): a(0) { count++; cout << "constructor ?" << endl; } ~Success() { --count; cout << "destructor ?" << endl; } virtual void set(const int& aa) { a=aa; } }; int Failure::count = 1; int Success::count = 1; int main(int argc) { // no command line arguments (succeeds) if( argc == 1 ) Success* s = new Success; delete s; cout << "class Success *s: try { s->set(1000) }" << endl; try { s->set(1000); } catch(...) { cout << "try failed" << endl; exit(-1); } } // one or more command line arguments (fails) else { Failure* f = new Failure; delete f; cout << "class Failure *f: try { f->set(1000) }" << endl; try { f->set(1000); } catch(...) { cout << "try failed" << endl; exit(-1); } } cout << "No failure" << endl; return 0; } Regards, Alan There is an opening brace missing after "if( argc == 1 )", should read "if(argc == 1) {" I can't explain it - it is there in the original file ;-) Back to top Tom Widmer Guest Posted: Mon Dec 20, 2004 3:20 pm    Post subject: Re: Reusing a deleted pointer. Alan wrote: Quote: I've put a virtual function in but it didn't fail until I introduced a static variable into the class for instance counting. I've cut and cut the original program.. The code below (I'm using VC++6 too) succeeds or fails, per user choice. What puzzles me is why one class fails and the other doesn't - as the differences appear small. That's the nature of undefined behaviour. Understanding why it has a certain effect often requires intimite knowledge of the compiler and its optimizations. The policy is to *never* write any code that has undefined behaviour, since even if such code appears to work here and now, as soon as you demo the system to an investor, it will almost certainly crash. Tom Back to top Alan Guest Posted: Mon Dec 20, 2004 8:56 pm    Post subject: Re: Reusing a deleted pointer. "Tom Widmer" <tom_usenet (AT) hotmail (DOT) com> wrote Quote: Alan wrote: I've put a virtual function in but it didn't fail until I introduced a static variable into the class for instance counting. I've cut and cut the original program.. The code below (I'm using VC++6 too) succeeds or fails, per user choice. What puzzles me is why one class fails and the other doesn't - as the differences appear small. That's the nature of undefined behaviour. Understanding why it has a certain effect often requires intimite knowledge of the compiler and its optimizations. The policy is to *never* write any code that has undefined behaviour, since even if such code appears to work here and now, as soon as you demo the system to an investor, it will almost certainly crash. If you could give me an example of the try..catch sequence (where the 'try' fails and the 'catch' catches it) in code that has no undefined behaviour - it would be most helpful. tia sincerely, -Alan Back to top Jonathan Mcdougall Guest Posted: Mon Dec 20, 2004 9:04 pm    Post subject: Re: Reusing a deleted pointer. Alan wrote: Quote: "Tom Widmer" <tom_usenet (AT) hotmail (DOT) com> wrote Alan wrote: I've put a virtual function in but it didn't fail until I introduced a static variable into the class for instance counting. I've cut and cut the original program.. The code below (I'm using VC++6 too) succeeds or fails, per user choice. What puzzles me is why one class fails and the other doesn't - as the differences appear small. That's the nature of undefined behaviour. Understanding why it has a certain effect often requires intimite knowledge of the compiler and its optimizations. The policy is to *never* write any code that has undefined behaviour, since even if such code appears to work here and now, as soon as you demo the system to an investor, it will almost certainly crash. If you could give me an example of the try..catch sequence (where the 'try' fails and the 'catch' catches it) in code that has no undefined behaviour - it would be most helpful. Try-catch blocks are used with throw. If nothing throws, a try-catch is useless. int main() { try { throw 2; } catch (int i) { std::cout << i << " was catched."; } } But I fail to see the point of your question. Using invalid pointers don't throw exceptions, it breaks your program. 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