Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!think!yale!cmcl2!lanl!lambda!jlg
From: jlg@lambda.UUCP (Jim Giles)
Newsgroups: comp.lang.misc
Subject: Re: Relationship between C and C++
Message-ID: <14286@lambda.UUCP>
Date: 23 Mar 90 01:02:41 GMT
References: <1990Mar22.181947.27026@maths.nott.ac.uk>
Lines: 22

From article <1990Mar22.181947.27026@maths.nott.ac.uk>, by anw@maths.nott.ac.uk (Dr A. N. Walker):
> [...]                          Yes, aliassing is a problem for optimising
> compilers, and if used casually is a problem for program-proving;  but
> exactly the same problems arise with arrays, [...]

NO!  The same problem does NOT arise with arrays.  Two different arrays
are two different objects.  Period.  Two different pointers might both
point to the SAME object.  This behaviour of pointers is not only legal
in all languages, it is encouraged.

Yes, it is true that most implementations don't check arrays to make sure
that no aliasing has occurred.  But, the implementor can optimize array
usage with a clear conscience - after all, only illegal programs get
broken.  Pointer usage cannot be optimized this way.  Further, arrays
CAN be tested to make sure illegal aliasing has not occurred - it costs
about the same as bounds checking.  Pointers cannot be tested in this manner
because it is _LEGAL_ for them to be aliased!

[I know - still beating the same dead horse.  This issue NEVER seems to
go away.]

J. Giles