From 3264196964805777404 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,db4807a3f0a1e742 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 1994-07-18 06:43:25 PST Path: bga.com!news.sprintlink.net!redstone.interpath.net!ddsw1!panix!MathWorks.Com!europa.eng.gtefsd.com!howland.reston.ans.net!usenet.ins.cwru.edu!news.ecn.bgu.edu!psuvax1!news.pop.psu.edu!news.cac.psu.edu!newsserver.jvnc.net!yale.edu!xlink.net!scsing.switch.ch!swidir.switch.ch!newsfeed.ACO.net!Austria.EU.net!EU.net!uunet!news.centerline.com!news!immel From: immel@chord.centerline.com (Mark Immel) Newsgroups: comp.std.c++ Subject: Re: Multiple inheritance and delete Date: 18 Jul 1994 13:22:30 GMT Organization: CenterLine Software, Inc. Lines: 64 Message-ID: References: NNTP-Posting-Host: chord.centerline.com In-reply-to: immel@chord.centerline.com's message of 15 Jul 1994 15:09:50 GMT C++ gurus and standard committee -- I asked about the following code a few days ago. The problem is that the call to delete b calls free with a bad address; this is a particularly nasty silent failure -- often the program will crash much later. Many of you commented that giving B a virtual destructor will fix the problem; that's true. However, the WP 5.3.5 (26 May, 1994 version) implies that my code is legal: struct A {}; struct B {}; struct C: A, B {}; void foo() { B* b = new C; delete b; } 5.3.5 paragraph 2: "The value of the operand of delete must be a pointer to a non-array object created by a new-expression without a new-placement specification, or a pointer to a subobject representing a base class of such an object." 5.3.5 paragraph 3: "If the static type of the operand is different from its dynamic type *AND THE CLASS OF THE COMPLETE OBJECT HAS A DESTRUCTOR*, the static type must have a virtual destructor, or the result is undefined." (emphasis mine) Mr. Hartmut Kocher has been courteous enough to send me his thoughts on the issue -- he would like to see the WP altered to remove the emphasized type, I believe (if I have misstated his position, I apologize). Then, B would have to have a virtual destructor and all would be well. Another possibility is: "If the static type of the operand is different from its dynamic type, and the class of the complete object has multiple inheritance anyplace, the static type must have a virtual destructor, or the result is undefined." However, neither of these is easy to statically check (BTW neither is the correct WP wording, and this is a nasty bug that I would like my compiler to nail me for). Compilers could generate warnings, most of which would be spurious, about deleting classes without virtual destructors; we would ignore them all and miss the few that were real trouble. I think there are two reasonable options: 1) Require a compiler diagnostic if multiple inheritance is used and at least one direct or indirect base class does not have a virtual destructor. 2) (Preferably) use the RTTI information mandated in the standard to convert the B* to a C* before it goes off to delete; the language now mandates that the type information be around anyhow. This may be the intention in the WP; but it should say so explicitly so that compiler implementors realize this. -- Mark Immel immel@centerline.com (You may email me if convenient -- I will summarize)