Nullable or Optional? Or something else?

[Steven Schveighoffer]

On Thu, 03 Sep 2009 16:20:41 -0400, Rainer Deyke <rainerd at eldwood.com>  
wrote:

> Steven Schveighoffer wrote:
>> I think Andrei is right here, for any type T that already is nullable
>> (including pointer and reference types), Optional!T should alias to T.
>
> Your conclusion is backwards here.  In order for Optional!T to be
> useful, it must be able to hold all possible of T plus a distinct null
> value.  If Optional!T cannot do that for T = Optional!T2, then
> Optional!T is useless in generic programming, bordering on useless in
> general.  I think my example program demonstrated this amply.

I disagree, how many ways do you want to say something is unset?  I think  
one case -- is this thing null -- is plenty for 99% of cases.  Your case,  
well, I had trouble understanding it, but I think the nuance of how  
something is unset is not usually important.

I think that what you will find with a solution that *does* allow this,  
you will be sacrificing something else -- such as syntax clarity or a  
member function.

If you have specific requirements for "is this set to nulla or nullb,"  
then I think that's a more complicated problem to solve, one which  
requires more complicated structures than what is being proposed here.  I  
don't think we should cater a feature like this to the one time somebody  
needs 8 different names for snow...err null.

What I would *love* to see is a way to intercept these statements, and  
handle them specially:

x is null;
x = null;

That doesn't fit your problem at all, since there is only one keyword  
named null.

-Steve