Null references redux

[Yigal Chripun]

On 28/09/2009 12:05, Jeremie Pelletier wrote:
> Nick Sabalausky wrote:
>> "Jeremie Pelletier" <jeremiep at gmail.com> wrote in message
>> news:h9mmre$1i8j$1 at digitalmars.com...
>>> Ary Borenszweig wrote:
>>>> Object is not-nullable, Object? (or whatever syntax you like) is
>>>> nullable. So that line is a compile-time error: you can't cast a
>>>> null to an Object (because Object *can't* be null).
>>>>
>>> union A {
>>> Object foo;
>>> Object? bar;
>>> }
>>>
>>> Give me a type system, and I will find backdoors :)
>>>
>>
>> Unions are nothing more than an alternate syntax for a reinterpret
>> cast. And it's an arguably worse syntax because unlike casts, uses of
>> it are indistinguishable from normal safe code, there's nothing to
>> grep for. As such, unions should never be considered any more safe
>> than cast(x)y. The following is just as dangerous as your example
>> above and doesn't even touch the issue of nullability/non-nulability:
>>
>> union A {
>> int foo;
>> float bar;
>> }
>>
>
> Yet it's the only way I know of to do bitwise logic on floating points
> in D to extract the exponent, sign and mantissa for example.
>
> And yes they are much, much more than a simple reinterpret cast, a
> simple set of casts will not set the size of the union to its largest
> member. Unions make for elegant types which can have many valid
> representations:
>
> union Vec3F {
> struct { float x, y, z; }
> float[3] v;
> }
>
> I just can't picture D without unions :)

here's a type-safe alternative
note: untested

struct Vec3F {
   float[3] v;
   alias v[0] x;
   alias v[1] y;
   alias v[2] z;
}

D provides alignment control for structs, why do we need to have a 
separate union construct if it is just a special case of struct alignment?

IMO the use cases for union are very rare and they all can be redesigned 
in a type safe manner.
when software was small and simple, hand tuning code with low level 
mechanisms (such as unions and even using assembly) made a lot of sense. 
Today's software is typically far more complex and is way to big to risk 
loosing safety features for marginal performance gains.

micro optimizations simply doesn't scale.