C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  delete[] of POD          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message MaxMax Guest Posted: Tue Oct 25, 2005 9:29 am    Post subject: delete[] of POD int *p; p = new int[10]; void *q = (void*)p; delete[] q; Is this valid if p is guaranteed to be a POD? (don't ask why... there is a reason I need this. Clearly I will create p in function, use p in another function that know what type p is and delete p in a function that for simplicity won't know what p is) Reading the MSDN it seems that "The global operator delete function, if declared, takes a single argument of type void *, which contains a pointer to the object to deallocate. The return type is void (operator delete cannot return a value). Two forms exist for class-member operator delete functions:" so it should be ok But this is the MSDN guide :-) --- bye Back to top Ferdi Smit Guest Posted: Tue Oct 25, 2005 9:37 am    Post subject: Re: delete[] of POD MaxMax wrote: Quote: int *p; p = new int[10]; void *q = (void*)p; delete[] q; Is this valid if p is guaranteed to be a POD? (don't ask why... there is a reason I need this. Clearly I will create p in function, use p in another function that know what type p is and delete p in a function that for simplicity won't know what p is) Reading the MSDN it seems that "The global operator delete function, if declared, takes a single argument of type void *, which contains a pointer to the object to deallocate. The return type is void (operator delete cannot return a value). Two forms exist for class-member operator delete functions:" so it should be ok But this is the MSDN guide :-) --- bye operator delete[] has a void* as parameter in the standard, so you're fine. -- Regards, Ferdi Smit (M.Sc.) Email: [email]Ferdi.Smit (AT) cwi (DOT) nl[/email] Room: C0.07 Phone: 4229 INS3 Visualization and 3D Interfaces CWI Amsterdam, The Netherlands Back to top Alf P. Steinbach Guest Posted: Tue Oct 25, 2005 9:46 am    Post subject: Re: delete[] of POD * Ferdi Smit: Quote: MaxMax wrote: int *p; p = new int[10]; void *q = (void*)p; delete[] q; Is this valid if p is guaranteed to be a POD? (don't ask why... there is a reason I need this. Clearly I will create p in function, use p in another function that know what type p is and delete p in a function that for simplicity won't know what p is) Reading the MSDN it seems that "The global operator delete function, if declared, takes a single argument of type void *, which contains a pointer to the object to deallocate. The return type is void (operator delete cannot return a value). Two forms exist for class-member operator delete functions:" so it should be ok But this is the MSDN guide :-) operator delete[] has a void* as parameter in the standard, so you're fine. Sorry, that's an incorrect inference. The standard defines this as Undefined Behavior, and in case somebody should not see that implication, it states that explicitly for void* in note 73: [context: delete array] "... an object cannot be deleted using a pointer of type void* because ...". -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? Back to top Ferdi Smit Guest Posted: Tue Oct 25, 2005 10:08 am    Post subject: Re: delete[] of POD Alf P. Steinbach wrote: Quote: operator delete[] has a void* as parameter in the standard, so you're fine. Sorry, that's an incorrect inference. The standard defines this as Undefined Behavior, and in case somebody should not see that implication, it states that explicitly for void* in note 73: [context: delete array] "... an object cannot be deleted using a pointer of type void* because ...". I think that refers to an array of void which is impossible (size 0, a similar reason to why an (empty) instantiated object has size at least 1); and thus you can't delete[] a true void*. But in max's case the underlying data is in fact of another type (int*). The operator delete[] takes a void*, so whatever pointer you pass it, it will be cast to a void* first before the operator runs anyway. -- Regards, Ferdi Smit (M.Sc.) Email: [email]Ferdi.Smit (AT) cwi (DOT) nl[/email] Room: C0.07 Phone: 4229 INS3 Visualization and 3D Interfaces CWI Amsterdam, The Netherlands Back to top Alf P. Steinbach Guest Posted: Tue Oct 25, 2005 10:30 am    Post subject: Re: delete[] of POD * Ferdi Smit: Quote: Alf P. Steinbach wrote: operator delete[] has a void* as parameter in the standard, so you're fine. Sorry, that's an incorrect inference. The standard defines this as Undefined Behavior, and in case somebody should not see that implication, it states that explicitly for void* in note 73: [context: delete array] "... an object cannot be deleted using a pointer of type void* because ...". I think that refers to an array of void which is impossible (size 0, a similar reason to why an (empty) instantiated object has size at least 1); and thus you can't delete[] a true void*. There is no such thing as a "true void*" Except the nullpointer of void* type, in which case you have no object to be deleted. Quote: But in max's case the underlying data is in fact of another type (int*). The operator delete[] takes a void*, so whatever pointer you pass it, it will be cast to a void* first before the operator runs anyway. That doesn't matter. The compiler can't infer the size or type of the array elements statically. The standard couldn't be more clear on this issue: it's Undefined Behavior. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? Back to top John Carson Guest Posted: Tue Oct 25, 2005 11:05 am    Post subject: Re: delete[] of POD "Ferdi Smit" <Ferdi.Smit (AT) cwi (DOT) nl> wrote Quote: I think that refers to an array of void which is impossible (size 0, a similar reason to why an (empty) instantiated object has size at least 1); and thus you can't delete[] a true void*. But in max's case the underlying data is in fact of another type (int*). The operator delete[] takes a void*, so whatever pointer you pass it, it will be cast to a void* first before the operator runs anyway. You need to distinguish between delete[] and operator delete[], which are easily confused. delete[] does two things: 1. it calls the destructor for the objects in the array, and then 2. it calls operator delete[] to deallocate memory. Once the destructor has run, the object no longer exists and hence it is not surprising that operator delete[] is given a void* pointer. To see the effect of using delete[] with a void* pointer, run the following: #include <iostream> class Test { public: ~Test() { std::cout << "Destructor calledn"; } }; int main() { Test *ptest = new Test[4]; void * ptr = (void*)ptest; delete [] ptr; return 0; } The result of this is undefined, but I predict with reasonable confidence that the destructor will not be called. If, by contrast, you call delete[] on ptest, then the destructor will be called. Of course, if your array members don't have destructors, then you may get away with calling delete[] on a void* pointer. There is no guarantee, however. The behaviour is undefined. -- John Carson Back to top Ferdi Smit Guest Posted: Tue Oct 25, 2005 11:23 am    Post subject: Re: delete[] of POD John Carson wrote: Quote: You need to distinguish between delete[] and operator delete[], which are easily confused. delete[] does two things: 1. it calls the destructor for the objects in the array, and then 2. it calls operator delete[] to deallocate memory. Once the destructor has run, the object no longer exists and hence it is not surprising that operator delete[] is given a void* pointer. To see the effect of using delete[] with a void* pointer, run the following: #include class Test { public: ~Test() { std::cout << "Destructor calledn"; } }; int main() { Test *ptest = new Test[4]; void * ptr = (void*)ptest; delete [] ptr; return 0; } The result of this is undefined, but I predict with reasonable confidence that the destructor will not be called. If, by contrast, you call delete[] on ptest, then the destructor will be called. Of course, if your array members don't have destructors, then you may get away with calling delete[] on a void* pointer. There is no guarantee, however. The behaviour is undefined. Ah, alright. Then this time I stand corrected (arrays are evil anyway ) -- Regards, Ferdi Smit (M.Sc.) Email: [email]Ferdi.Smit (AT) cwi (DOT) nl[/email] Room: C0.07 Phone: 4229 INS3 Visualization and 3D Interfaces CWI Amsterdam, The Netherlands Back to top MaxMax Guest Posted: Sat Oct 29, 2005 12:37 pm    Post subject: Re: delete[] of POD Quote: Of course, if your array members don't have destructors, then you may get away with calling delete[] on a void* pointer. There is no guarantee, however. The behaviour is undefined. So, if I know I'm working with a POD, can I directly use the operator delete? POD don't have destructors. --- bye Back to top Ivan Vecerina Guest Posted: Sat Oct 29, 2005 2:02 pm    Post subject: Re: delete[] of POD "MaxMax" <none (AT) none (DOT) com> wrote : >> Of course, if your array members don't have destructors, then you may get : >> away with calling delete[] on a void* pointer. There is no guarantee, : >> however. The behaviour is undefined. : So, if I know I'm working with a POD, can I directly use the operator : delete? POD don't have destructors. Not if you want your code to be portable and standards-compliant. Let me restate what was written above about this being undefined behavior: you can do it for a POD, and it is even likely to work as you expect on your platform/compiler. However, code that uses this trick will not be portable, and will behave unexpectedly on some other platform, or when you port your code to a new version of your compiler. In other words: don't do it. Allocated memory always must be disposed in a way that strictly matches the allocation method. -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form Brainbench MVP for C++ <> http://www.brainbench.com Back to top Jim Langston Guest Posted: Mon Oct 31, 2005 10:54 am    Post subject: Re: delete[] of POD "Ivan Vecerina" <INVALID_use_webform (AT) ivan (DOT) vecerina.com> wrote Quote: "MaxMax" <none (AT) none (DOT) com> wrote in message news:n2K8f.37014$Pe2.675133 (AT) twister2 (DOT) libero.it... : >> Of course, if your array members don't have destructors, then you may get : >> away with calling delete[] on a void* pointer. There is no guarantee, : >> however. The behaviour is undefined. : So, if I know I'm working with a POD, can I directly use the operator : delete? POD don't have destructors. Not if you want your code to be portable and standards-compliant. Let me restate what was written above about this being undefined behavior: you can do it for a POD, and it is even likely to work as you expect on your platform/compiler. However, code that uses this trick will not be portable, and will behave unexpectedly on some other platform, or when you port your code to a new version of your compiler. In other words: don't do it. Allocated memory always must be disposed in a way that strictly matches the allocation method. Seeing how this is POD, couldn't you just call delete q; as the destructors don't need to be called? OP question: int *p; p = new int[10]; void *q = (void*)p; delete[] q; Is this valid if p is guaranteed to be a POD? ... Back to top Ron Natalie Guest Posted: Tue Nov 01, 2005 1:50 pm    Post subject: Re: delete[] of POD Jim Langston wrote: Quote: Seeing how this is POD, couldn't you just call delete q; as the destructors don't need to be called? The standard makes no such exception for POD types. Quote: OP question: int *p; p = new int[10]; void *q = (void*)p; delete[] q; Is this valid if p is guaranteed to be a POD? ... It's never valid. You must pass delete[] the exact same type and value you got from new []. The only time you can pass something to delete that's a different type than what was new is for the single object case where you have a base class pointer with a derived destructor. Even then you can't use void*. It has to be a base class, not any pointer you converted. 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