Nullable or Optional? Or something else?

[Andrei Alexandrescu]

Rainer Deyke wrote:
> Steven Schveighoffer wrote:
>> Sure, do whatever you want for your *private data members*.  I see too
>> much value in Optional!T being aliased to the underlying T to go for the
>> completely generic solution which requires special syntax to get at the T.
> 
> And I see no value at all, but a lot of danger.

The value is in the fact that you can use an Optional!T as a substitute 
for a T. Actually that's quite what happens today with null references - 
we know they can't be used, but they are substitutable for valid objects.

>  Even ignoring, what
> about this:
> 
>   void f(ref int v) {
>     // Do something...
>   }
> 
>   void g() {
>     Optional!int x = 5;
>     f(x);
>   }

Such code should not compile. An Optional!T should be at most 
substitutable for an rvalue of type T.

> Or this:
> 
>   void report_error(int);
> 
>   void f(T)(out T v) {
>     if (error_condition) throw new SomeException();
>     v = something;
>   }
> 
>   void g() {
>     Optional!int v = someCalculation();
>     if (!isNull(v)) {
>       try {
>         f(v);
>       } catch (SomeException) {
>         report_error(v); // Oops, invalid: 'v' turned into null.
>       }
>     }
>   }
> 
> Note how in the second example, the semantics of the function 'f' subtly
> change depending on whether the argument is an 'int' or and 'Optional!int'.

If it's an int, you assign the int. If it's an Optional!int, you assign 
the optional, which assigns the int and makes the Optional non-null if 
it was null. I might be missing something, but I don't think there's 
much danger for confusion here.

> Proxies, especially proxies that add functionality to their base type,
> cannot act exactly like the type they are proxying.  Using a proxy
> therefore requires the significant mental overhead of keeping track of
> all the corner cases in which the proxy does not act like the type it is
> proxying, as well as the hassle of working around those limitation
> whenever they come up.

I agree to some extent.


Andrei