From -9146027423652055940 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,17893fccd2b995cb,start X-Google-Attributes: gidf78e5,public From: Stephen.Clamage@Eng.Sun.COM (Steve Clamage) Subject: Re: Memory layout (Was: Re: Pointer details) Date: 1999/03/15 Message-ID: <7ci361$dg$1@engnews1.eng.sun.com>#1/1 X-Deja-AN: 455258315 Approved: Fergus Henderson References: <36e6ddf0@news.uni-rostock.de> <7c9kse$cqi$1@engnews1.eng.sun.com> <36EC0717.DF2@wanadoo.fr> X-Original-Date: 15 Mar 1999 04:43:45 GMT X-Complaints-To: news@news.unimelb.edu.au X-Trace: izvestia.its.unimelb.edu.au 921513967 5478 128.250.29.16 (15 Mar 1999 16:06:06 GMT) Organization: Sun Microsystems Inc., Mountain View, CA X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBFAgUANu0v1+EDnX0m9pzZAQGb3QF/cj40gnIAGoaprf0W9ghBjGfnRGJpcbEO FtXb61DRuEVseEqZFtmgkv2OUhpHVFoO =OkRT NNTP-Posting-Date: 15 Mar 1999 16:06:06 GMT Newsgroups: comp.std.c++ Valentin Bonnard writes: >Steve Clamage wrote: >> Note, however, that pointers to class objects have other concerns. >> A pointer to a base-class subobject is not necessarily a pointer >> to the entire (most-derived) object. In addition, a pointer to >> the entire class object is not necessarily a pointer to the first >> byte of the object. (For example, some implementations have put >> the vtable pointer at at negative offset.) >Hum... could you please elaborate ? >Do you mean that the following isn't garantied to pass: >void* pv = operator new (sizeof (T)); // #1 >T* pt = new (pv) T; // #2 >assert (pt == pv); If an implementation chooses to put the vpointer at a negative offset, it has to go to some trouble to make everything work right. In particular, a static_cast to or from void* must adjust the pointer value so that you can allocate and deallocate space with operator new and operator delete. Casting to void* must yield the address of the start of storage, and casting from void* must yield what is considered to be the object's address. In #1 above, the operator new returns a void*, assigned to a pv. That is the address of the beginning of a storage area. In #2, the new-expression returns the address of the T object. The address of T will be offset from the beginning of the storage area. The comparison compares a T* to a void*. Since the types are different, the T* must be converted (via an implicit static_cast) to a void*, adjusting the pointer value back to point to the beginning of storage. The pointers will then compare equal, just as you would expect. -- Steve Clamage, stephen.clamage@sun.com --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://reality.sgi.com/austern_mti/std-c++/faq.html ]