Comment by zozbot234

6 years ago

> Currently it seems like the main limiting factor in Swift is ARC: it uses atomic operations to ensure thread-safe reference counts

This is exactly what Rust avoids by having both Arc and plain-vanilla Rc. Plus reference counts are only updated when the ownership situation changes, not for any reads/writes to the object.

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3PS  6 years ago

Rust also backs up this design with the Send and Sync traits, which statically prevent programmers from, say, accidentally sending an Rc<T> between threads when they really should have used an Arc<T> instead.

jb1991  6 years ago

Now I'm curious, what is the difference between automated ref counting and "vanilla" ref counting? And of these two, where does the C++ shared pointer fit?

  • skohan  6 years ago

    ARC as in "Atomic Reference Counting". ARC uses atomic operations to increment and decrement reference counts. That means these operations must be synchronized between threads. synchronization between threads/cores tends to be an expensive operation.

    This is required for reference counting objects between threads. Otherwise, you might have one thread try to release an object at the same time another thread is trying to increment the reference count. It's just overkill for objects which are only ever referenced from a single thread.