NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/std/cplus/2113

Text File | 1993-01-25 | 1.6 KB | 38 lines

Newsgroups: comp.std.c++
Path: sparky!uunet!titan!lpi!pkt
From: [e-mail] (Scott Turner)
Subject: Re: pointer comparisons
Message-ID: <[e-mail]>
Sender: pkt@lpi (Scott Turner)
Organization: Liant Software Corporation
References: <[e-mail]> <[e-mail]>
Date: Mon, 25 Jan 1993 15:15:16 GMT
Lines: 26

In article <[e-mail]>, [e-mail] (Hendrik Boom) writes:
> In article <[e-mail]>, [e-mail] (Scott Turner) writes:
> > If we have objects of a type which requires no state, e.g.
> >        struct B{} a, b;
> > then in order for them to have distinct identities an implementation
> > might allocate addresses that don't point to valid memory.  Again, no
> > storage is required.
>
> But if nonempty class C inherits from B, you have trouble comparing
> pointers to B with pointers to C that have been converted to B,
> unless you are careful that addresses of B's are outside of usable
> memory.

The trouble is negligible almost all of the time.  For example,
    struct C : B { int i; };
an implementation could convert "pointer to C" to "pointer to B" using the
same representation for both values.  If some pointers to B are pointing to
invalid memory, the comparison will work out just right.

In unusual cases such as Johan Bengtsson's example, the compiler can "waste"
a byte for the base class.
--
Prescott K. Turner, Jr.
Liant Software Corp. (developers of LPI languages)
959 Concord St., Framingham, MA 01701 USA    [phone]
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