[jarrettc]

I'm hoping the next post will talk about this use case: Suppose I have an enum called "Canine" and I want each of its variants to implement a different "bark" method. Currently, as far as I know, I have to write a match statement dispatching "bark" to each "Canine" variant. If I have "bark" and "growl," I have to write two match statements, and so on for each method that needs to be dispatched to different variants.

So it's a lot of boilerplate. I think it can be slimmed down with a macro, but still.

You might think traits rather than enums are the way to go here. Sometimes that may be true. But often, an enum is far preferable because in Rust, a trait is not a type, but an enum is. That means you cannot, for example, have a "Vec<CanineTrait>," but you can have a "Vec<CanineEnum>."

[dcb18]

A trait is a type, but the type does not have a size, which is the reason it is not possible to store a trait instance directly in a Vec (or any other place needing a sized type). It is possible to store a pointer to a trait object in a Vec, which would look like Vec<Box<CanineTrait>>.

[jarrettc]

Ah, my mistake. So would I be correct in saying that a trait is a type, but it does not necessarily implement the Sized trait, and if it does not, then it cannot be allocated directly on the stack?

Also, I've been wondering something about dereferencing a Box where the inner type is just a trait. If I dereference a Box<CanineTrait> and call "bark," how is the correct implementation found at runtime? Is there a vtable or something analogous?

[Rusky]

Trait object pointers are "fat"- they consist of both a pointer to the object and a vtable pointer (much like Go interfaces if you've used those). Other DSTs are similar- pointers to slices consist of both a pointer to the elements and a count.

[losvedir]

> If I dereference a Box<CanineTrait> and call "bark," how is the correct implementation found at runtime? Is there a vtable or something analogous?

I think a vtable. It's my understanding that "trait objects" (boxed traits) are how you implement dynamic dispatch in rust. If instead you were talking about a `fn foo<T: CanineTrait>(x: T) { ... }` trait bound (the more common case), there will be monomorphization and static dispatch.

Source: http://doc.rust-lang.org/1.0.0-beta/book/static-and-dynamic-...

[steveklabnik]

After the beta landed, I re-did the TOC for the book, and so this information will be at http://doc.rust-lang.org/nightly/book/trait-objects.html in the future. (where nightly = 1.0.0, of course)