From 3186201000891745343 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,e94695845965b249 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 1992-12-21 10:46:39 PST Newsgroups: comp.std.c++ Path: sparky!uunet!munnari.oz.au!cs.mu.OZ.AU!munta.cs.mu.OZ.AU!fjh From: fjh@munta.cs.mu.OZ.AU (Fergus James HENDERSON) Subject: Re: Zero-length structures and pointer comparisons Message-ID: <9235623.26370@mulga.cs.mu.OZ.AU> Sender: news@cs.mu.OZ.AU Organization: Computer Science, University of Melbourne, Australia References: <1992Dec18.192202.17023@ucc.su.OZ.AU> <1992Dec18.204337.3084@lth.se> <1992Dec19.202043.20366@ucc.su.OZ.AU> Date: Mon, 21 Dec 1992 12:02:43 GMT Lines: 57 maxtal@extro.ucc.su.OZ.AU (John MAX Skaller) writes: >However, lets consider that Unix file names are 'pointers' to files, >and that links are allowed. > >Then the average program cant tell if two file names point to >the same file or not, since computing the inode number is the only >way to do that, and only super users can do this calculation. >(Imagine I'm right even if this isnt the case. Is it the case?) No. Computing the inode number is not a privileged operation. >Would this make such a system 'unworkable'? Not if the system had mandatory (as opposed to advisory) locks. >The issue is whether pointers must provide any more functionality >than the ability to access an object, that is, whether object >identity can be established by pointer comparisons. Like it or not (in many ways I don't!), it seems that this principle is firmly entrenched in C++. Consider the requirement empty classes must have non-zero sizes, etc. >I suggest that whole classes of software can be written that >do not rely on the ability to test if two pointers refer >to the same object or not, other types of software require >this information. > >Perhaps we can conceive two types of pointers, type 1 >provides no comparisons at all, type 2 allows equality test. > >Most of the code I've written uses type 1, and most uses >of type 2 could be rewritten as type 1. Equality tests are often needed in assignment operators, to check for the special case of x=x. >In those cases where type 2 pointers are desirable, it >seems reasonable to restrict them to pointers >to objects created directly by me, and not the operating >system. > >Thus I would like to see the ARM == rewritten so the >results of == on externally derived or otherwise mangled >pointers are implementation defined. As I said in a previous post, I think that this is already the case for C, and I expect it to be the same for C++ when the ANSI or ISO C++ standard finally comes out. Once you have invoked the operating system, all bets are off. -- Fergus Henderson fjh@munta.cs.mu.OZ.AU This .signature virus is a self-referential statement that is true - but you will only be able to consistently believe it if you copy it to your own .signature file!