Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!brutus.cs.uiuc.edu!lll-winken!ames!pacbell!att!cbnewsc!lgm
From: lgm@cbnewsc.ATT.COM (lawrence.g.mayka)
Newsgroups: comp.lang.misc
Subject: Re: Relationship between C and C++
Message-ID: <14539@cbnewsc.ATT.COM>
Date: 21 Mar 90 02:25:11 GMT
References: <5200048@m.cs.uiuc.edu>
Reply-To: lgm@cbnewsc.ATT.COM (lawrence.g.mayka,ihp,)
Organization: AT&T Bell Laboratories
Lines: 55

In article <5200048@m.cs.uiuc.edu> robison@m.cs.uiuc.edu writes:
>Most C implementations do not do bounds checking on pointers, 
>but I fail to see why pointer arithmetic is inherently evil.

My own objection to pointer arithmetic in a purportedly
object-oriented language such as C++ is that pointer arithmetic
assumes that the storage cells actually referenced by the pointer
at run time have the precise size denoted by the pointer's
compile-time declaration.  Thus, if I construct an array of
objects of class Derived (derived from Base), and pass that array
as argument to a function declared to accept a pointer to a Base -
a perfectly legal and automatic coercion - and the function
attempts to index into the array/off the pointer, the result is
random garbage.  If Derived has virtual functions, I might even
overwrite a pointer to its virtual function table.  The next
virtual function call then gives "Memory fault, core dumped." An
attempt to delete the array of Derived through a Base pointer can
cause less debuggable havoc - repeatedly calling the Base
destructor with incorrect pointers - and this is even if one
remembers to specify the size of the array (which the requestor of
the deletion is expected to know, but whose omission is quite
legal and prevents the necessary calling of the destructor on each
array element).

In short, the C array is a poor substitute for an indexable
collection type, and the C pointer is a poor iterator for such a
type.

>Can anyone clue me as to the basis for pointer paranoia?

Programming languages such as Common Lisp and Smalltalk support an
abstraction of storage (memory) itself, in which memory consists
exclusively of typed objects, and those objects are referenced by
names (bindings).  Assignment is essentially a rebinding; physical
copying is a rather unusual operation, performed only when one
specifically wishes to preserve a mutable object's current state
in the face of possible future alteration by others.  Pointers
vitiate this abstraction by encouraging the more primitive view of
memory as merely an enormous array of untyped machine words, to be
used or abused in whatever way the programmer desires.  This loss
of abstraction is particularly acute when pointer casting is
permitted and encouraged, and especially when pointers are often
coerced automatically and silently by the compiler.

Many languages outright encourage a pointer implementation in
which an integer or other datum can have the same bit pattern as a
pointer.  This almost completely rules out truly reliable garbage
collection, further invalidating the objects/bindings abstraction.


	Lawrence G. Mayka
	AT&T Bell Laboratories
	lgm@ihlpf.att.com

Standard disclaimer.