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From: ka@felix.UUCP (Kenneth Almquist)
Newsgroups: comp.lang.ada
Subject: Re: Reference vs. copy semantics in passing parameters
Message-ID: <163815@felix.UUCP>
Date: 28 May 91 05:28:29 GMT
References: <2725@sparko.gwu.edu> <jls.666659373@yoda> <2742@sparko.gwu.edu> <1991May16.135103.1688@software.org>
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>In article <2742@sparko.gwu.edu> mfeldman@seas.gwu.edu () writes:
>> In C++, you can declare not only the pointer constant but the pointed
>> to construct constant as well. This allows passing by reference in a
>> read-only manner, which is NOT possible in Ada at present.

This feature has also been included in the ANSI C standard.

blakemor@software.org (Alex Blakemore) asks:
> This sounds like a nice safety feature but can callers really rely on it ?
> Even if the C++ language prevents updating the object if the pointer is
> declared appropriately, does it prevent assignment to a normal pointer
> which will allow the referenced object to be updated ?

The "constant" attribute is part of the type, so the type checking system
makes this secure *unless* the type system is circumvented by the use of
a type cast.  (By "type cast", I mean the C equivalent of Ada's
UNCHECKED_CONVERSION.)  The rule is that a pointer to a non-constant
object can be assigned to a pointer to a constant object, but not the
other way around.  (Assignment includes parameter passing.)

A problem with the "constant" attribute is that it is not inherited
properly.  Consider a routine that looks up an entry in a data
structure like a tree or a string, and returns a pointer to the
element found.  The returned pointer should point to a constant object
iff the data structure was declared to be constant, but the C type
system doesn't handle this.  For this reason, several routines in the
standard ANSI C library are defined in such a way that they cannot be
implemented without using type casts.
					Kenneth Almquist