Re: private types

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Maciej Sobczak <no.spam@no.spam.com> writes:

> Robert A Duff wrote:
> 
> >>>Only a very few C/C++ programmers
> >>>actually master the language.
> >>
> >>As is true for any language that is at least moderately useful.
> > True.  Language designers ought to take that into account -- whenever
> > possible, try to design features subtle misunderstandings are unlikely
> > to cause bugs.
> 
> True, I don't have any doubts about it.
> 
> > Suppose an Ada programmer and a C++ programmer both think that null/NULL
> > must be represented as all-zero-bits.  Both programmers are confused.
> 
> Both programmers take assumptions which are equally wrong.

Right.

> > But which programmer is more likely to write code that won't work
> > on a machine where the assumption turns out to be false?
> 
> Naither of those programmer should have any problems with their
> programs, because pointers are, well, pointers, not integers. Neither of
> them should have any temptation to interpret pointers as numbers, so
> that the actual "value" of the pointer never really matters.

You say "should".  But we're talking about confused/wrong programmers.

But the C++ programmer has been taught to use "0" as a value of type
char* (which pointer value, of course, is not necessarily represented as
all-zero-bits).  See other posts in this thread, which recommend that
style.  It's not far from there, to (wrongly) believing that casting an
integer zero to pointer will result in null.

>... Even the
> question "is the NULL pointer zero?" makes no sense in this light,
> unless you target specific piece of silicon (see my replies to Martin
> Krischik).

I agree that the question "is the NULL pointer zero?" makes no sense,
but the question "is the NULL pointer represented as zero (in some or
all implementations of a certain language)?" makes sense.

It's not usually a hardware ("silicon") issue, though.
Ada compilers choose to represent null as zero because compilers
for other languages do that, and because operating systems arrange
for page zero to trap.  Also, I think there are some minor efficiency
advantages.

> In practice, in the world of pointers it's not the NULL pointer
> representation which is causing problems, but rather unconstrained
> pointer arithmetics.

Certainly pointer arithmetic adds extra trouble.  But even without that,
dereferencing of null pointers is a plentiful source of bugs.

- Bob