How to break const

[Artur Skawina]

On 06/19/12 15:29, deadalnix wrote:
> Le 19/06/2012 14:30, Artur Skawina a écrit :
>>> Due to D concept of weak purity, this doesn't ensure the required what we need here.
>>
>> Actually, it does - if it can be proved that the delegate can't alter the object
>> via the context pointer (eg because whatever it points to is not mutable) then
>> even D's "pure" is enough. Because the delegate would need to be passed a mutable
>> ref to be able to alter the object, which then could hardly be constructed as
>> "breaking" constness.
>>
>> But such a limit (const/immutable context) would be a problem for the cases where
>> the delegate needs to alter some state (like export the result of some operation),
>> but does _not_ modify the object it's embedded in. Note that the current object may
>> very well be reachable (and mutable) from the delegate - but at some point you have
>> to trust the programmer. Sure, fixing this hole would be great, but /how/ - w/o
>> incurring unacceptable collateral damage?
>>
> 
> This isn't a problem as long as the delegate isn't a member of the object. If it is, transitivity is broken, which is something you don't want.
> 
> Relying on the trust on the programmer is a dumb idea. Human do mistake, way more than computers. The basic behavior MUST be a safe one.
> 
> Transitivity has been introduced for good reason and language already provide a way to break it via cast. So it is unacceptable to rely on programmer on that point.
> 
>>> It is possible to get the error when trying to call the delegate instead of preventing to make it const, as I said in the post you quote. It is probably a better solution.
>>
>> Any delegate?
>>
> 
> No, any delegate that have type that isn't covariant with the expected delegate type.

   struct S {
      int i; this(int i) { this.i = i; }
      T* p; 
      void f(int i) { this.i = i; /*p.i++;*/ }
   }
   struct T {
      int i; this(int i) { this.i = i; }
      void delegate(int i) f;
   }

   void main() {
      auto t = new T(42);
      auto s = new S(17);
      s.p = t;
      t.f = &s.f;
      f(t);
   }

   void f(const (T)* t) {
      t.f(t.i*2);
   }

You're proposing to make the last 'f' function illegal, just because the
commented out part could happen. I'm saying that this is unlikely to happen
*by accident*, and of course would still be possible by casting away the
constness. 
But banning "unsafe" delegates would result in casts *when using "safe" ones*
- which is not a real improvement because this would make the "bad" casts much
harder to spot.

artur