C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  for every overloaded new we need to declate a delete with si          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Rajaram S. Gaunker Guest Posted: Wed Dec 17, 2003 10:13 am    Post subject: for every overloaded new we need to declate a delete with si hi all for every overloaded "operator new" we need to overload "operator delete" with similar signature. I've read in D&E that this is not the case but in Scott Mayers book "Effective c++" i read that u need to do that. Is it a new change in standrads. Please clarify. rajaram [ 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: Wed Dec 17, 2003 4:58 pm    Post subject: Re: for every overloaded new we need to declate a delete wit In article <e3c5fdc7.0312162042.75abff25 (AT) posting (DOT) google.com>, Rajaram S. Gaunker <rajaram (AT) emuzed (DOT) com> writes Quote: hi all for every overloaded "operator new" we need to overload "operator delete" with similar signature. I've read in D&E that this is not the case but in Scott Mayers book "Effective c++" i read that u need to do that. Is it a new change in standrads. Please clarify. The issue is more complicated. Whenever we write an overload for operator new we must consider whether we should also write an operator delete. If we are writing a global (often called placement) overload for operator new we have no choice, because there is no mechanism for overloading operator delete at that level (hence the advice in D&E). But when we write a class scope operator new we would normally also write a corresponding class scope operator delete. If we do not, the global one will be used instead, and that would often do the wrong thing (hence the advice given in EC++) -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit or http://www.robinton.demon.co.uk [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dhruv Guest Posted: Wed Dec 17, 2003 7:13 pm    Post subject: Re: for every overloaded new we need to declate a delete wit On Wed, 17 Dec 2003 05:13:50 -0500, Rajaram S. Gaunker wrote: Quote: hi all for every overloaded "operator new" we need to overload "operator delete" with similar signature. I've read in D&E that this is not the case but in Scott Mayers book "Effective c++" i read that u need to do that. Is it a new change in standrads. It is not that we NEED to strictly define operator delete for operator new defined by us, but consider the case where you operator new does something else apart from memory allocation, or perhaps operator new allocated memory from some other heap, or some other place that the default operator new does not know about. Then, how is default operator delete supposed to know that memory was allocated from here? So, to keep things sane, we should do that too. Regards, -Dhruv. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rajaram S. Gaunker Guest Posted: Thu Dec 25, 2003 7:12 am    Post subject: Re: for every overloaded new we need to declate a delete wit " Quote: hi all for every overloaded "operator new" we need to overload "operator delete" with similar signature. I've read in D&E that this is not the case but in Scott Mayers book "Effective c++" i read that u need to do that. Is it a new change in standrads. It is not that we NEED to strictly define operator delete for operator new defined by us, but consider the case where you operator new does something else apart from memory allocation, or perhaps operator new allocated memory from some other heap, or some other place that the default operator new does not know about. Then, how is default operator delete supposed to know that memory was allocated from here? So, to keep things sane, we should do that too. Hi Thanx I understood why we have to declare a delete but .If we declare 2 operator new, while calling delete how it will be known that which new is used to do the allocation. for example we may pass two the object allocate din two different ways to same function which deletes them in this case how it will be know. please explain [ 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: Thu Dec 25, 2003 2:41 pm    Post subject: Re: for every overloaded new we need to declate a delete wit In message <e3c5fdc7.0312242236.2021de88 (AT) posting (DOT) google.com>, Rajaram S. Gaunker <rajaram (AT) emuzed (DOT) com> writes Quote: Hi Thanx I understood why we have to declare a delete but .If we declare 2 operator new, while calling delete how it will be known that which new is used to do the allocation. for example we may pass two the object allocate din two different ways to same function which deletes them in this case how it will be know. please explain The delete expression always uses the delete operator whose declaration is delete(void*, size_t). If no such function is supplied by the class then the global one will be used. The other delete operators are supplied for the special case when a dynamically constructed object is being destroyed as part of the process resulting from a failed construction. At that point the code 'knows' which operator new was used and selects the matching operator delete if there is one, else it uses the default. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit or http://www.robinton.demon.co.uk Happy Xmas, Hanukkah, Yuletide, Winter/Summer Solstice to all. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dhruv Guest Posted: Thu Dec 25, 2003 5:39 pm    Post subject: Re: for every overloaded new we need to declate a delete wit On Thu, 25 Dec 2003 02:12:16 -0500, Rajaram S. Gaunker wrote: Quote: " hi all for every overloaded "operator new" we need to overload "operator delete" with similar signature. I've read in D&E that this is not the case but in Scott Mayers book "Effective c++" i read that u need to do that. Is it a new change in standrads. It is not that we NEED to strictly define operator delete for operator new defined by us, but consider the case where you operator new does something else apart from memory allocation, or perhaps operator new allocated memory from some other heap, or some other place that the default operator new does not know about. Then, how is default operator delete supposed to know that memory was allocated from here? So, to keep things sane, we should do that too. Hi Thanx I understood why we have to declare a delete but .If we declare 2 operator new, while calling delete how it will be known that which new is used to do the allocation. for example we may pass two the object allocate din two different ways to same function which deletes them in this case how it will be know. please explain Two versions of operator new overloaded or rather decalared by the user can not have the exact same signature. So, you cannot have: void *operator new (size_t); ___[1] void *operator new (size_t); In the same program. Furthermore, an operator delete having matching parameters should be present for the deallocation to happen correctly. Considering [1] as the allocation function, this will be the corresponding deallocation function: void operator delete (void*); Similarly, you can have parameterised operator new and operator delete: struct Foo { }; void *operator new (size_t, Foo *fptr); void operator delete (void*, Foo*); Now, you can call parameterized operator delete to select which deallocation function to use. Regards, -Dhruv. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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