From -5646045910529402144 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,8366734ec6aad121 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 1994-06-18 19:55:33 PST Newsgroups: comp.std.c++ Path: nntp.gmd.de!Germany.EU.net!EU.net!howland.reston.ans.net!news.moneng.mei.com!uwm.edu!msuinfo!harbinger.cc.monash.edu.au!news.cs.su.oz.au!metro!news From: maxtal@physics.su.OZ.AU (John Max Skaller) Subject: Re: new foo[0] Message-ID: Sender: news@ucc.su.OZ.AU Nntp-Posting-Host: physics.su.oz.au Organization: School of Physics, University of Sydney, Australia References: <9417005.12163@mulga.cs.mu.OZ.AU> Date: Sun, 19 Jun 1994 02:37:10 GMT Lines: 44 In article jason@cygnus.com (Jason Merrill) writes: >> foo& r = myfoo0[0]; > >Certainly such usage would yield undefined behavior, if 'r' were ever >actually used for anything, and consequently an implementation can give a >diagnostic for such code. What do you mean by 'use'? Its a sadly abused term. In general for class types "use" means "call a virtual member function virtually" or "select a data member of scalar type in a context requiring loading a value" or "enquire as to the Run Time Type" (and a few other things like conversions of pointers to virtual bases). Everything else is just address calculations not requiring access to the object. Even calling a non-virtual member is just address calculations: only virtual function calls actually require an object to exist. Address calculations should be legal if, and only if, they compute valid addresses: binding a reference may be thought of as the same as initialising a pointer. Thus above, computing the address of a non-static member by say: r.mem is invalid (its "more" than one past the end of the array :-) But forming the reference 'r' is not. Such an address calculation is perfect fine, however even if the object does not exist, provided the underlying memory does -- ONLY attempts to read memory should yield undefined behaviour (including use of virtual bases or functions and reading the value of a scalar member subobject.) -- JOHN (MAX) SKALLER, INTERNET:maxtal@suphys.physics.su.oz.au Maxtal Pty Ltd, CSERVE:10236.1703 6 MacKay St ASHFIELD, Mem: SA IT/9/22,SC22/WG21 NSW 2131, AUSTRALIA