On Sunday, 1 March 2015 at 15:28:56 UTC, Jakob Ovrum wrote:
On Sunday, 1 March 2015 at 01:40:40 UTC, Andrei Alexandrescu
wrote:
We have a few candidates for solutions, but wanted to open
with a good discussion first. So, how do you envision a way to
define and access mutable metadata for objects (including
immutable ones)?
Andrei
I don't think const or immutable intrusive-reference-counted
classes is a feasible idea.
I understand the motivation: we want to dynamically allocate a
class instance, initialize it and never mutate it again, and
pass it around freely; *without* using tracing GC. Having it
typed as immutable helps code readability and whatnot.
However, AFAICS, it comes with a serious problem. Immutable
objects are freely passable between threads, so surely an
immutable RC object would need atomic counting just like a
shared RC object, but unlike shared, immutable does not
necessarily express the intent of sharing between threads; the
immutable RC object could easily be counting atomically for
nothing.
Argh! I didn't think about this. Any chance we can deprecate this
behaviour? It's also an obstacle for the implementation of
thread-local heaps.
There might be other problems, such as problems regarding ROM.
Not if the RC wrapper allocated the memory in the first place. It
knows that it can't be in ROM.
