Re: To Initialise or not

[kst@thomsoft.com (Keith Thompson)]

In <Dqp3HF.12t@world.std.com> bobduff@world.std.com (Robert A Duff) writes:
[...]
> No, sorry, I just meant the "if ... = null" as an *example*.  The
> general point is: If I explicitly initialize, then I plan to read that
> value, at least in some cases.  If I don't, then I promise I won't read
> that value.
[...]
> In other words, pretend that you're coding in a language that's just
> like Ada, except that there's no default initial value for pointers.
> Write the code accordingly.  Since this mythical language is a subset of
> Ada, this is safe.

I like the idea.  The only problem is that the compiler won't help
you enforce your promise not to read the value.  An explicit ":= null"
initialization is like a comment -- it's extremely useful, but it can
get out of sync with the actual code.

The real problem, I think, is that the access value null plays two
different roles: as an uninitialized (garbage) value, and as a legitimate
access value that doesn't designate any object.  If I were designing a
new language, I'd be tempted to define a second distinguished access
value called, say, "invalid".  The value null would act like it does
in Ada: it doesn't point to anything and an attempt to dereference it
raises Constraint_Error.  The value invalid would be the default initial
value of all access types; an attempt to dereference it *or to reference
it* would raise Constraint_Error.

For example:

    declare
        type Pointer_Type is access Some_Type;
        P1: Pointer_Type;       -- implicitly initialized to invalid
        P2: Pointer_Type := null;
        P3: Pointer_Type;
        Obj: Some_Type;
    begin
        Obj := P1.all;  -- raises Constraint_Error
        Obj := P2.all;  -- raises Constraint_Error
        P3 := P2;       -- ok, sets P3 to null
        P3 := P1;       -- raises Constraint_Error
    end;

The only cost would be a simple check on access assignments, which
probably could be optimized out in most cases.

Maybe for Ada 201Z?  8-)}

-- 
Keith Thompson (The_Other_Keith) kst@thomsoft.com <*>
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