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Reference counting is still not garbage collection. ARC causes the compiler to implicitly insert calls to retain and release, that’s it. Everything is still 100

Reference counting is still not garbage collection. ARC causes the compiler to implicitly insert calls to retain and release, that’s it. Everything is still 100% deterministic at runtime.

jolux 6y ago
It’s not tracing GC, but it’s well-known as a garbage collection algorithm: https://en.wikipedia.org/wiki/Reference_counting#Garbage_collection https://en.wikipedia.org/wiki/Reference_counting#Garbage_col...
PaulDavisThe1st 6y ago
It's not deterministic because you don't know the timing relationship between threads which may drop references. The only sense in which it is deterministic is that "when the reference count reaches zero, the object will be destroyed". That's a long way from the determinism of manual/explicit memory management. It gets even worse if there are per-thread pools in use, though to be fair, this is mostly an application design issue rather than a language one.
pcwalton 6y ago
Reference counting is a form of garbage collection [1]. Any automatic memory management is garbage collection. There is in fact a continuum of memory management strategies between full reference counting on one end and full stop-the-world mark/sweep on the other. Malloc heuristics also mean that it is not true that everything is 100% deterministic at runtime. It's easy to forget that malloc is simply an abstraction like any other. When do free memory blocks get reshuffled between threads in a thread-caching malloc? When does unused memory get returned to the OS? These are all things that cannot be predicted. Even retain and release are not 100% deterministic, as they can and will be optimized away by the compiler. [1]: https://www.memorymanagement.org/glossary/g.html#term-garbage-collection https://www.memorymanagement.org/glossary/g.html#term-garbag...
- cat199 6y ago
  reference counting is reference counting. a reference counting gc is a form of garbage collection. not sure what applies here, as am not versed in objc, but, the terms are distinct - in theory one can have an object system that refcounts objects so that they are not mis-deleted if some reference still exists and the object is accidentally deleted by some code, but still allow for mostly-manual memory management.
  - jolux 6y ago
    We're discussing ARC here, i.e. Automatic Reference Counting, which is a garbage collection algorithm. Retain/release would be manual memory management by reference counting, which is not being discussed.
    - cmrdporcupine 6y ago
      Even without ARC it's still garbage collection. The reference counts are kept for the purpose of managing garbage collection. That the programmer has to do some manual retain/release steps is beside the point, even if it does make the experience somewhat painful.

Reference counting is still not garbage collection. ARC causes the compiler to implicitly insert calls to retain and release, that’s it. Everything is still 100% deterministic at runtime.

jolux 6y ago

It’s not tracing GC, but it’s well-known as a garbage collection algorithm: https://en.wikipedia.org/wiki/Reference_counting#Garbage_collection https://en.wikipedia.org/wiki/Reference_counting#Garbage_col...

PaulDavisThe1st 6y ago

It's not deterministic because you don't know the timing relationship between threads which may drop references. The only sense in which it is deterministic is that "when the reference count reaches zero, the object will be destroyed". That's a long way from the determinism of manual/explicit memory management. It gets even worse if there are per-thread pools in use, though to be fair, this is mostly an application design issue rather than a language one.

pcwalton 6y ago

Reference counting is a form of garbage collection [1]. Any automatic memory management is garbage collection. There is in fact a continuum of memory management strategies between full reference counting on one end and full stop-the-world mark/sweep on the other. Malloc heuristics also mean that it is not true that everything is 100% deterministic at runtime. It's easy to forget that malloc is simply an abstraction like any other. When do free memory blocks get reshuffled between threads in a thread-caching malloc? When does unused memory get returned to the OS? These are all things that cannot be predicted. Even retain and release are not 100% deterministic, as they can and will be optimized away by the compiler. [1]: https://www.memorymanagement.org/glossary/g.html#term-garbage-collection https://www.memorymanagement.org/glossary/g.html#term-garbag...

cat199 6y ago
reference counting is reference counting. a reference counting gc is a form of garbage collection. not sure what applies here, as am not versed in objc, but, the terms are distinct - in theory one can have an object system that refcounts objects so that they are not mis-deleted if some reference still exists and the object is accidentally deleted by some code, but still allow for mostly-manual memory management.
- jolux 6y ago
  We're discussing ARC here, i.e. Automatic Reference Counting, which is a garbage collection algorithm. Retain/release would be manual memory management by reference counting, which is not being discussed.
  - cmrdporcupine 6y ago
    Even without ARC it's still garbage collection. The reference counts are kept for the purpose of managing garbage collection. That the programmer has to do some manual retain/release steps is beside the point, even if it does make the experience somewhat painful.