5 ms·

It fundamentally doesn't make sense to have a Vec<Trait>, because Vec expects each of its elements to be the same size (otherwise you couldn't index into it in

by comex 9y ago

It fundamentally doesn't make sense to have a Vec<Trait>, because Vec expects each of its elements to be the same size (otherwise you couldn't index into it in O(1)), but different types that implement Trait can have different sizes. Other languages allow this because they automatically box everything, but Vec<Box<Trait>> should accomplish the same in Rust; I'm curious what the "other reasons" you referred to are, that make that unsuitable. Alternately, if any given instance of the Vec is only expected to have values of a single type (but the type can be different for different Vecs), you may be able to accomplish what you want with generics instead. That said, trait objects in Rust are pretty broken in general, so - this is just speculation, but - I think your problem might actually arise from that, and the Vec<Trait> issue is a red herring.

SCdF 9y ago
I didn't really want to get into the nitty gritty because it detracts from the point (I considered not having the rant paragraph at all for fear that it would just be deconstructed and the exact examples becoming the focus of what I said and not what I was actually trying to say). But anyway!: https://doc.rust-lang.org/error-index.html#E0038 https://doc.rust-lang.org/error-index.html#E0038 <-- these are restrictions on Box<Trait>. The one that screws me is the first one, requiring Sized.
- ehsanu1 9y ago
  I can agree with the initial frustration coming from a GC-ed language until one figures out the right way to design code and structure data in a language with manual memory management. But I got over it eventually. I'm also curious about this nitty gritty bit. I see this: Generally, Self : Sized is used to indicate that the trait should not be used as a trait object. If the trait comes from your own crate, consider removing this restriction. I guess you needed that sized restriction for some reason, assuming this was a trait of your own?
  - SCdF 9y ago
    Yeah. So I'm following a ray tracing book that uses C++ as example code, and am using Traits as a form of interface: there is a trait of Material that has a function on it, and then there are different "implementations" of Material with different implementations of that fn, and can have arbitrary data stored against them. I need to have them sized because I need to copy them at a point where I only understand they are a Material and not what the actual struct are, and I need to do this because I can't get a reference to work in this instance due to the fun of lifetimes. AFAICT the way to do this without Traits in Rust would be to have an enum, and then have an external function that takes the enum, matches against it and runs different code depending. To me the second one is kind of gross, but I may just go with it, or just give up and do something else.
    - tangent128 9y ago
      Would it work to add a boxed_clone() function to your Material trait that returns a Box<Material>? (or create a BoxedClone trait if you need that pattern more generally) Implementations of boxed_clone() could still use &self.clone() to limit boilerplate. Alternatively, if the Materials are not going to be modified and you just need multiple references to the same object, you could replace the Box<> with an Rc<> or Arc<>.
      - comex 9y ago
        Yep, those are the best approaches. For the former, you might consider the 'objekt' crate, which avoids the boilerplate: https://docs.rs/objekt/0.1.0/objekt/ https://docs.rs/objekt/0.1.0/objekt/
    - physguy1123 9y ago
      > there is a trait of Material that has a function on it, and then there are different "implementations" of Material with different implementations of that fn, and can have arbitrary data stored against them. Do you mean like a class hierarchy+virtual functions, or just having allocated data of arbitrary length past the end of a struct? If it's the former, C++ fares no better in this regard but it's pretty easy to do in Rust. You just have a cloner in the interface. If it's the latter, that's pretty nonstandard and Rust doesn't make that easy for good reason. Unless this raytracer is doing something wild the code should hopefully be refactorable into a more normal struct+trait layout.

by comex 9y ago

SCdF 9y ago

I didn't really want to get into the nitty gritty because it detracts from the point (I considered not having the rant paragraph at all for fear that it would just be deconstructed and the exact examples becoming the focus of what I said and not what I was actually trying to say). But anyway!: https://doc.rust-lang.org/error-index.html#E0038 https://doc.rust-lang.org/error-index.html#E0038 <-- these are restrictions on Box<Trait>. The one that screws me is the first one, requiring Sized.

ehsanu1 9y ago
I can agree with the initial frustration coming from a GC-ed language until one figures out the right way to design code and structure data in a language with manual memory management. But I got over it eventually. I'm also curious about this nitty gritty bit. I see this: Generally, Self : Sized is used to indicate that the trait should not be used as a trait object. If the trait comes from your own crate, consider removing this restriction. I guess you needed that sized restriction for some reason, assuming this was a trait of your own?
- SCdF 9y ago
  Yeah. So I'm following a ray tracing book that uses C++ as example code, and am using Traits as a form of interface: there is a trait of Material that has a function on it, and then there are different "implementations" of Material with different implementations of that fn, and can have arbitrary data stored against them. I need to have them sized because I need to copy them at a point where I only understand they are a Material and not what the actual struct are, and I need to do this because I can't get a reference to work in this instance due to the fun of lifetimes. AFAICT the way to do this without Traits in Rust would be to have an enum, and then have an external function that takes the enum, matches against it and runs different code depending. To me the second one is kind of gross, but I may just go with it, or just give up and do something else.
  - tangent128 9y ago
    Would it work to add a boxed_clone() function to your Material trait that returns a Box<Material>? (or create a BoxedClone trait if you need that pattern more generally) Implementations of boxed_clone() could still use &self.clone() to limit boilerplate. Alternatively, if the Materials are not going to be modified and you just need multiple references to the same object, you could replace the Box<> with an Rc<> or Arc<>.
    - comex 9y ago
      Yep, those are the best approaches. For the former, you might consider the 'objekt' crate, which avoids the boilerplate: https://docs.rs/objekt/0.1.0/objekt/ https://docs.rs/objekt/0.1.0/objekt/
  - physguy1123 9y ago
    > there is a trait of Material that has a function on it, and then there are different "implementations" of Material with different implementations of that fn, and can have arbitrary data stored against them. Do you mean like a class hierarchy+virtual functions, or just having allocated data of arbitrary length past the end of a struct? If it's the former, C++ fares no better in this regard but it's pretty easy to do in Rust. You just have a cloner in the interface. If it's the latter, that's pretty nonstandard and Rust doesn't make that easy for good reason. Unless this raytracer is doing something wild the code should hopefully be refactorable into a more normal struct+trait layout.