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"Выпуск языка программирования Rust 1.54" Отправлено Аноним, 30-Июл-21 14:41
> Еще можно глянуть пример PhantomData как частный вариант ZST. > https://doc.rust-lang.org/stable/core/marker/struct.PhantomD... Я бы дал ссылку на https://doc.rust-lang.org/nomicon/phantom-data.html > PhantomData consumes no space, but simulates a field of the given type for the purpose of static analysis. This was deemed to be less error-prone than explicitly telling the type-system the kind of variance that you want, while also providing other useful things such as the information needed by drop check. там и пример для ZST более лаконичен и поняте не-рустовцам https://doc.rust-lang.org/nomicon/exotic-sizes.html > On their own, Zero Sized Types (ZSTs) are, for obvious reasons, pretty useless. ... > One of the most extreme examples of this is Sets and Maps. Given a Map<Key, Value>, it is common to implement a Set<Key> as just a thin wrapper around Map<Key, UselessJunk>. In many languages, this would necessitate allocating space for UselessJunk and doing work to store and load UselessJunk only to discard it. Proving this unnecessary would be a difficult analysis for the compiler. > However in Rust, we can just say that Set<Key> = Map<Key, ()>. Now Rust statically knows that every load and store is useless, and no allocation has any size. The result is that the monomorphized code is basically a custom implementation of a HashSet with none of the overhead that HashMap would have to support values.

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Сообщение*:	>> Еще можно глянуть пример PhantomData как частный вариант ZST. >> https://doc.rust-lang.org/stable/core/marker/struct.PhantomData.html > Я бы дал ссылку на > https://doc.rust-lang.org/nomicon/phantom-data.html >> PhantomData consumes no space, but simulates a field of the given type for the purpose of static analysis. This was deemed to be less error-prone than explicitly telling the type-system the kind of variance that you want, while also providing other useful things such as the information needed by drop check. > там и пример для ZST более лаконичен и поняте не-рустовцам > https://doc.rust-lang.org/nomicon/exotic-sizes.html >> On their own, Zero Sized Types (ZSTs) are, for obvious reasons, pretty useless. > ... >> One of the most extreme examples of this is Sets and Maps. Given a Map<Key, Value>, it is common to implement a Set<Key> as just a thin wrapper around Map<Key, UselessJunk>. In many languages, this would necessitate allocating space for UselessJunk and doing work to store and load UselessJunk only to discard it. Proving this unnecessary would be a difficult analysis for the compiler. >> However in Rust, we can just say that Set<Key> = Map<Key, ()>. Now Rust statically knows that every load and store is useless, and no allocation has any size. The result is that the monomorphized code is basically a custom implementation of a HashSet with none of the overhead that HashMap would have to support values.
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