Re: not null

[Adam Beneschan <adam@irvine.com>]

On Mar 4, 12:38 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> Recursive types is not the single case where references are used. For smart
> pointers not null constraint is very useful.

Right, I didn't say a "not null" constraint isn't useful; I'm just
saying that it's also useful to have access objects that *can* be
null, if needed.  Actually, I'm not clear on whether anyone disagrees
with that.  This points to a curiosity in Hoare's abstract: first he
implies that inventing "null references" was a big mistake, and then
he says "declarations for non-null references" is the solution.  If
allowing null references is a mistake, then the solution is to
disallow them---not to allow the programmer to declare a reference
that may not be null, right?  I think maybe Hoare misstated the
problem.  The mistake wasn't inventing null references; the mistake
was not providing a way for the programmer to disallow selected
reference types or variables (objects) from being null.    This
misstatement may have led to some confusion on this thread.

                                 -- Adam