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0 pointsPauan2y ago0 comments

I am very aware of the definition of zero-cost.

We're talking about the comparison between using an abstraction vs not using an abstraction.

When I said "doesn't have a runtime cost", I meant "the abstraction doesn't have a runtime cost compared to not using the abstraction".

If you want your computer to do anything useful, then you have to write code, and that code has a runtime cost.

That runtime cost is unavoidable, it is a simple necessity of the computer doing useful work, regardless of whether you use an abstraction or not.

Whenever you create or use an abstraction, you do a cost-benefit analysis in your head: "does this abstraction provide enough value to justify the EXTRA cost of the abstraction?"

But if there is no extra cost, then the abstraction is free, it is truly zero cost, because the code needed to be written no matter what, and the abstraction is the same speed as not using the abstraction. So there is no cost-benefit analysis, because the abstraction is always worth it.

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awused2y ago

The way you used it in your parent comment didn't make it clear that you were using it properly, hence my clarification. I'm honestly still not sure you've got it right, because Rust abstractions, in general, are not zero-cost. Rust has some zero-cost abstractions in the standard library and Rust has made choices, like monomorphization for generics, that make writing zero-cost abstractions easier and more common in the ecosystem. But there's nothing in the language or compiler that forces all abstractions written in Rust to be free of extra runtime costs.

PauanOP2y ago

I never said that ALL abstractions in Rust are zero-cost, though the vast majority of them are, and you actually have to explicitly go out of your way to use non-zero-cost abstractions.

awused2y ago

Are you sure about that?

>Rust embraces abstractions because Rust abstractions are zero-cost. So you can liberally create them and use them without paying a runtime cost.

>you never need to do a cost-benefit analysis in your head, abstractions are just always a good idea in Rust

Again though, and ignoring that, "zero-cost abstraction" can be very narrow and context specific, so you really don't need to go out of your way to find "costly" abstractions in Rust. As an example, if you have any uses of Rc that don't use weak references, then Rc is not zero-cost for those uses. This is rarely something to bother about, but rarely is not never, and it's going to be more common the more abstractions you roll yourself.

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awused2y ago

PauanOP2y ago

I never said that ALL abstractions in Rust are zero-cost, though the vast majority of them are, and you actually have to explicitly go out of your way to use non-zero-cost abstractions.

awused2y ago

Are you sure about that?

>Rust embraces abstractions because Rust abstractions are zero-cost. So you can liberally create them and use them without paying a runtime cost.

>you never need to do a cost-benefit analysis in your head, abstractions are just always a good idea in Rust

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