Making RCSlice and DIP74 work with const and immutable

via Digitalmars-d digitalmars-d at
Sun Mar 1 07:53:06 PST 2015

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.

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