Visualizing Rust's Vtables: How dyn Trait Works In Memory(sofiabelen.github.io)
126 pointsby torutofu11 hours ago3 comments
  • tialaramex3 hours ago
    This has a section on Object Safety, I checked and the article was written this week, but "Object Safety" is a confusing name for this idea, and so for a little while now Rust calls this idea "dyn compatibility" because the most important thing you're getting if a trait is "dyn compatible" is that you can use "dyn Trait" - https://doc.rust-lang.org/1.98.1/reference/items/traits.html...

    That link more comprehensively explains the rules too.

    [Edit: Realized the end of the article explains this, the author began writing it months ago, likely before they read about the improved "dyn compatibility" naming]

    • loeg2 hours ago
      The aside about C++ seems a little confused too?

      > C++ doesn’t have this problem at all, since virtual dispatch always goes through pointers and return types are always pointers too.

      Uh. C++ can return objects by value. Maybe it isn't idiomatic. And I don't know if there's a convenient spelling like `Self`. But it runs into the same problem, of course -- you would need to know the concrete value type returned to have storage for it.

      And Rust has the ~same solution as imagined for C++ here, I think? Have a `DynClone` trait that returns `Box<dyn Trait>` instead of `Self`.

  • returningfory23 hours ago
    Very nice. As a follow up would be interesting to also reverse engineer the structure of the vtable itself. I guess it’s a list of pointers to the method implementations?
  • ketzu4 hours ago
    > In Rust, that question is answered by the borrow-checker at compile time:

    (About zero sized objects being the same)

    But why does that mean the programmer never has to check? (or if they want that information from the borrow checker how would they get it?) It's not motivated as the intro above for c++ was just "In C++, we might do this to check if two pointers refer to the same object".

    So the borrow checker knows already, why does that stop the programmer from wanting to know or separate these cases?

    • steveklabnik4 hours ago
      > if they want that information from the borrow checker how would they get it?

      If you want to compare if two pointers point to the same place, you use https://doc.rust-lang.org/stable/std/ptr/fn.eq.html

      Rust just defaults to value equality over reference equality. This is true for everything, not just ZSTs.

      (I find the post's framing of "it's stored in the borrow checker" to be a bit odd, but I can't put my finger on exactly what it is. The borrow checker doesn't determine these sorts of semantics, it checks for liveliness and aliasing, so "do these pointers alias" isn't inherently not the borrow checker's job, it just strikes me as an odd way to put it. Maybe it's because you don't "ask the borrow checker for that information" really.)

      • ketzu4 hours ago
        As the article itself discusses, pointers to zero sized objects are not necessarily different (they write it is only the case in debug mode).

        > I find the post's framing of "it's stored in the borrow checker" to be a bit odd

        That's exactly what I wanted to say as well.

        I feel like it would have been better to just skip the borrow checker mention and just go "in rust this can not be done reliably ..(section about pointers being the same)"

      • 4 hours ago
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    • TazeTSchnitzel4 hours ago
      I'm not sure if this quite answers your question, but the difference in philosophy here is that C++ objects with pointers always have identity, whereas a Rust object only has identity if it has a non-zero size. It's an application of the zero-overhead principle, "you only pay for what you use".
      • ketzu4 hours ago
        "zero sized objects don't have identity" actually would answer my question and is a really interesting factoid, thanks!
        • tialaramex3 hours ago
          This is why C++ doesn't have ZSTs, it wants all objects to have identities, the obvious way to distinguish them is by where they are in memory, but ZSTs don't have distinct addresses in memory.
        • 4 hours ago
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    • wrs3 hours ago
      I've tried (admittedly for just a few minutes) to come up with a case where you'd need to compare two pointers to ZSTs in any real algorithm, and failed. Can you think of one?
    • loeg4 hours ago
      It's not obvious to me that the borrow checker actually knows, so much as it can prove there are no illegal conflicts between mut and non-mut references to the same object.

      If you somehow need this property in your programs, I think you can just add a 1-byte member and use pointer equality. (I'm not a Rust expert.)