Nullable types

[Bill Baxter]

On Tue, Oct 21, 2008 at 9:40 AM, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org> wrote:
> Bill Baxter wrote:
>>
>> On Tue, Oct 21, 2008 at 9:11 AM, Lionello Lunesu <lio at lunesu.remove.com>
>> wrote:
>>>
>>> Bent Rasmussen wrote:
>>>>
>>>> Not true. It wraps the value type in a struct with a boolean field
>>>> expressing whether it is null or not.
>>>>
>>>> http://msdn.microsoft.com/en-us/library/1t3y8s4s(VS.80).aspx
>>>
>>> Indeed. Thanks for pointing that out.
>>>
>>> So C#'s 'foo?' syntax has even less to do with this compile-time nullness
>>> checking.
>>
>> Now it makes sense.  Yes the C# feature is apparently just a
>> convenient way to create a value type with a special
>> "none-of-the-above" value.
>> I guess this feature is driven by need to connect with databases that
>> often have nullable types.
>>
>> This chapter of a C# 2.0 book covering Nullable Types seems to agree
>> with that assessment :
>> http://www.springerlink.com/content/w2mh0571776t3114/
>
> Not a very clever design. If they took a hit of a Boolean field (= a word
> field when padding is accounted for), they might as well have added a
> reference to Exception. That way, you not only know the value is null, you
> may also have info on why.

Hmm, I was thinking storing a pointer to the value itself would make
more sense than a bool.
Along these lines:

struct Nullable(T)
{
    private T _value;
    private T* _ptr;  // maybe points to _value, maybe null

    T* ptr { return _ptr; }
    T value() { return *_ptr; }
    void value(T v) { _value = v; _ptr = &_value; }
    void value(T* v) { if (v is null) { _ptr = null; } else { _value =
*v; _ptr = &_value; } }
}

That way you just get a null pointer exception for free when you try
to use the nulled value.
But maybe that's not a workable approach in C#.

--bb