I just got it! (invariant/const)

[Georg Wrede]

Steven Schveighoffer wrote:
> "Georg Wrede" wrote
> 
>>Janice Caron wrote:
>>
>>>On 09/04/2008, Denton Cockburn 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.
> 
> This cannot be guaranteed by the compiler.  It is depending on you to ensure 
> that the object stays invariant. From 
> http://www.digitalmars.com/d/2.0/const3.html
> 
> "The second way (to create invariant data) is to cast data to invariant. 
> When doing so, it is up to the programmer to ensure that no other mutable 
> references to the same data exist."

My mistake.