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0 pointsbrundolf5y ago0 comments

Taking a second look at this particular example, I guess it is a bit of a "trick" because of the ahead-declaration of both possible "holder" variables on the stack. It's just... wildly unintuitive that this should be possible. Even if you showed me this code without saying whether or not it should compile, I wouldn't be sure.

Here's the train of intuition:

1) dyn requires a pointer that may be to one of multiple types of structs

2) a group of multiple types of structs has an undefined memory layout, so the value must either live on the heap or be wrapped up in an enum

That feels like an airtight understanding. But then Rust lets you do this weird juggling maneuver based on control-flow that allows you to do it on the stack.

I'm not saying Rust shouldn't let you do this, and I'm not really sure how it could be made intuitive given the "normal" case. I'm just expressing that subjectively, this feels very weird and non-obvious, and it's far from the first example like this that I've encountered. Here's another example: https://news.ycombinator.com/item?id=25595120

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steveklabnik5y ago

I think where your intuition is leading you wrong is that in #2, you are assuming ownership. That is, you're saying "I need a place to put an arbitrary thing." But you don't! A &dyn T doesn't own T, just like a &T doesn't own T. Trait objects are a (pointer to data, pointer to vtable), (may be in the reverse order we don't guarantee layout) and so that pointer can point to anywhere, heap or stack.

Interestingly enough, this was special in Rust 1.0 to Rust 1.5. In 1.5, it finally became non-special. It's interesting because I totally get what you're saying, but at the same time, this is an example of Rust being orthogonal, not special cased.

(It is really hard to address the string thing without an example, to be honest.)

brundolfOP5y ago

I think you're right about my intuition, and that's interesting about the syntax change. It could just be that during my learning stage I learned (based on example bias maybe?) that dyn is for owned values, and not just any reference. I'm sure this was never stated explicitly, but somehow that idea got lodged in my brain

For the String thing (really, the Deref thing), further down others weigh in with a case where it doesn't work as expected, and then the workaround &*. The latter is something that feels like it should be a non-op, yet it's required in certain cases like this one to trigger something in the compiler. I'm sure there's some internal reason for this, but from the user's perspective it's, "What does dereferencing and then re-referencing this value have to do with performing what amounts to a cast?"

I want to emphasize that I'm not complaining just to complain, nor placing blame on any specific party. I'm just "reporting a bug" in my learning experience with the language, and trying to provide as much info as possible :)

steveklabnik5y ago

> I want to emphasize that I'm not complaining just to complain,

Oh yeah totally! It is very helpful.

> It could just be that during my learning stage I learned

I mean, I think this is very reasonable and intentional. Trait objects are a pretty niche feature of Rust already, and non-owned trait objects are even more niche than that. The book does guide you towards Box<dyn Trait> for this reason.

> or the String thing (really, the Deref thing), further down others weigh in with a case where it doesn't work as expected,

Yeah so the trick here is a balance between not wanting coercion willy-nilly, and also making some cases work well. You had cited method calls specifically, and those should work due to auto-ref/deref. The example given isn't about method calls, it's about match not doing Deref coercion. That being said it's really easy to assume that it always does it, because it does do it in the right places most of the time! There's interesting tradeoffs here...

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0 pointsbrundolf5y ago0 comments

Here's the train of intuition:

1) dyn requires a pointer that may be to one of multiple types of structs

2) a group of multiple types of structs has an undefined memory layout, so the value must either live on the heap or be wrapped up in an enum

That feels like an airtight understanding. But then Rust lets you do this weird juggling maneuver based on control-flow that allows you to do it on the stack.

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steveklabnik5y ago

(It is really hard to address the string thing without an example, to be honest.)

brundolfOP5y ago

steveklabnik5y ago

> I want to emphasize that I'm not complaining just to complain,

Oh yeah totally! It is very helpful.

> It could just be that during my learning stage I learned

> or the String thing (really, the Deref thing), further down others weigh in with a case where it doesn't work as expected,

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