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Patricia Aas

@pati_gallardo

May 13, 2021

I don’t know Rust, so I was wondering how this would look in C++, does anyone have an example?

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Patrick Walton

@pcwalton

May 13, 2021

Conceptually, the thing that makes Rust different from modern C++ (and other languages) is the "alias xor mutable" guideline, enforced by the borrow check. You could write C++ that way, but in practice nobody does. It's much stricter than just using smart pointers.

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You can have any number of immutable views (references) of data but only one mutable reference at a time. The compiler also enforces that the references can't be used beyond the lifetime of the object and the lifetime scopes are part of the types of references (as subtypes).

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Mutex<T> and RwLock<T> naturally provide a dynamic implementation of those checks as part of implementing their interior mutability. Those give out objects representing locks with the destructor releasing the lock, similar to a modern C++ idiom, but safe.

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RefCell<T> is similar within a single thread with dynamic failure on conflicts rather than waiting for the lock to be released. It's used in cases where statically checked borrow checking is too strict and can only support immutability due to shared ownership.

9:00 AM · May 13, 2021Twitter Web App

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Arc<T> is comparable to std::shared_ptr<T>. Rc<T> is a thread-specific variant of it without the atomic reference counting. Due to the shared ownership, those only provide immutable references. Arc<Mutex<T>> and Rc<RefCell<T>> provide a way to have shared ownership + mutability.

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The only way you can have overlapping references with mutability is via very restricted internal mutability via &Cell<T>. RefCell<T> is more flexible (allows references) but has to enforce the usual unique mutable reference rule dynamically. doc.rust-lang.org/std/cell/

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