I just got it! (invariant/const)

[Georg Wrede]

Janice Caron wrote:
> On 09/04/2008, Denton Cockburn <diboss at hotmail.com> wrote:
> 
>> > Of course, the explicit cast necessary to
>> > create an invariant C in the first place is a bit ugly. Maybe we need
>> > "inew" to make new invariant objects?
>>
>>Couldn't the compiler insert the cast based on the declaration?
> 
> 
> No, because objects created by new are not necessarily transitively
> unique. For example
> 
>     class C
>     {
>         int * p;
> 
>         this()
>         {
>             p = &someGlobalVariable;
>         }
>     }
> 
>     invariant C c = cast(invariant) new C;
>     someGlobalVariable = 1;
> 
> Whoops! c just changed!
> 
> The explicit cast makes it the programmer's fault, not the compiler's!
> 
> Come to think of it, "inew" would suffer the exact same problem, so it
> doesn't solve anything. Looks like there's no easy way to make an
> invariant class instance.

It should be illegal to cast objects invariant unless the compiler can 
guarantee the invariantness. So the cast here should produce an error.