On Tuesday, 12 July 2016 at 18:45:00 UTC, Jacob Carlborg wrote:
On 2016-07-12 07:33, Andrei Alexandrescu wrote:
The solution (very ingenious, due to dicebot) in fact does not
quite
cast immutability away. Starting from a possibly immutable
pointer, it
subtracts an offset from it. At that point the memory is not
tracked by
the type system, but known to the allocator to contain metadata
associated with the pointer that had been allocated with it.
After the
subtraction, the cast exposes the data which is mutable without
violating the immutability of the object proper. As I said,
it's quite
an ingenious solution.
What if the immutable data is stored in ROM [1]? I assume it's
not possible to have an offset to a completely different memory
storage.
Not sure if this is important enough to care about.
[1] http://dlang.org/const-faq.html#invariant
The idea is that in general, you cannot cast away immutability
because of this. But the allocator knows that the data at
(ptr-offset) is not in ROM (because it didn't allocate it from
ROM), and thus can cast away immutability without any issue.
And the allocator is not violating any type system invariant,
because there cannot exist any immutable reference to
(ptr-offset), given that this area was not given away by the
allocator.
