NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/lang/cplus/13454

Text File | 1992-09-09 | 1.5 KB | 55 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!stanford.edu!lucid.com!lucid.com!jss
From: [e-mail] (Jerry Schwarz)
Subject: Re: zero-length datatype
Message-ID: <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail] (Jerry Schwarz)
Organization: Lucid, Inc.
References:  <[e-mail]>
Distribution: comp
Date: Thu, 10 Sep 92 00:37:26 GMT
Lines: 41

In article <[e-mail]>, [e-mail] (Thomas M. Breuel) writes:
|> C++ seems to be lacking in a data type that is guaranteed to be zero
|> length.


This understates the situation.  C++ guarantees that no object
will have size 0.  For example

    struct Z { } ;

may not have size 0.

One reason for this constraint is to ensure that no two objects
have the same address.  In particular the elements of an array
are always at different addresses.

    Z a[10] ;

Without that constrain a[0],...,a[10] would all compare equal.
(I know a[10] isn't a real member, but ANSI C guarantees that
it is legal to take its address).

Another is that  new Z will never return the same adddress.
Of course you could add this as an additional constraint on
operator new if you wanted to.

The constraint predates templates. Maybe it should be reconsidered.
However that would require some consideration of the effect
on existing code, and on common programming idioms.  For example
something like

    Z a[10] ;
    ...
    count = sizeof(a)/sizeof(Z)

suddenly becomes a divde by zero.

  -- Jerry Schwarz