英文:
Define variable of type TimeZone
问题
以下是您要求的代码部分的翻译：
// 原始代码use chrono_tz::Tz;use chrono::{TimeZone, NaiveDate};use arrow2::temporal_conversions::parse_offset;fn my_func(tz: &str) -> () {let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();match parse_offset(&tz) {Ok(time_zone) => {println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));},Err(_) => match tz.parse::<Tz>() {Ok(time_zone) => {println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));}Err(error) => panic!("Problem opening the file: {:?}", error)},};}fn main() {let time_zone = "Asia/Seoul";my_func(&time_zone);}// 重构后的代码use chrono_tz::Tz;use chrono::{TimeZone, NaiveDate};use arrow2::temporal_conversions::parse_offset;fn my_func(tz: &str) -> () {let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();let parsed_time_zone: Tz = match parse_offset(&tz) {Ok(time_zone) => {time_zone},Err(_) => match tz.parse::<Tz>() {Ok(time_zone) => {time_zone}Err(error) => panic!("Problem opening the file: {:?}", error)},};println!("converted: {:?}", parsed_time_zone.from_utc_datetime(&ndt));}fn main() {let time_zone = "Asia/Seoul";my_func(&time_zone);}
希望这对您有所帮助。如果您有其他问题，请随时提问。
英文:
The following compiles and runs fine:
use chrono_tz::Tz;use chrono::{TimeZone, NaiveDate};use arrow2::temporal_conversions::parse_offset;fn my_func(tz: &str) -> (){let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();match parse_offset(&tz) {Ok(time_zone) => {println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));},Err(_) => match tz.parse::<Tz>() {Ok(time_zone) => {println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));}Err(error) => panic!("Problem opening the file: {:?}", error)},};}fn main() {let time_zone = "Asia/Seoul";my_func(&time_zone);}
Noticed though that I'm doing
println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));
twice.
I've tried refactoring to
use chrono_tz::Tz;use chrono::{TimeZone, NaiveDate};use arrow2::temporal_conversions::parse_offset;fn my_func(tz: &str) -> (){let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();let parsed_time_zone: TimeZone = match parse_offset(&tz) {Ok(time_zone) => {time_zone},Err(_) => match tz.parse::<Tz>() {Ok(time_zone) => {time_zone}Err(error) => panic!("Problem opening the file: {:?}", error)},};println!("converted: {:?}", parsed_time_zone.from_utc_datetime(&ndt));}fn main() {let time_zone = "Asia/Seoul";my_func(&time_zone);}
and get a long error:
error[E0782]: trait objects must include the `dyn` keyword--> src/main.rs:7:27|7 | let parsed_time_zone: TimeZone = match parse_offset(&tz) {| ^^^^^^^^|help: add `dyn` keyword before this trait|7 | let parsed_time_zone: dyn TimeZone = match parse_offset(&tz) {| +++error[E0191]: the value of the associated type `Offset` (from trait `TimeZone`) must be specified--> src/main.rs:7:27|7 | let parsed_time_zone: TimeZone = match parse_offset(&tz) {| ^^^^^^^^ help: specify the associated type: `TimeZone<Offset = Type>`error[E0038]: the trait `TimeZone` cannot be made into an object--> src/main.rs:7:27|7 | let parsed_time_zone: TimeZone = match parse_offset(&tz) {| ^^^^^^^^ `TimeZone` cannot be made into an object|= note: the trait cannot be made into an object because it requires `Self: Sized`= note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>error[E0308]: mismatched types--> src/main.rs:9:13|9 | time_zone| ^^^^^^^^^ expected trait object `dyn TimeZone`, found struct `FixedOffset`|= note: expected trait object `dyn TimeZone`found struct `FixedOffset`error[E0308]: mismatched types--> src/main.rs:13:17|13 | time_zone| ^^^^^^^^^ expected trait object `dyn TimeZone`, found enum `Tz`|= note: expected trait object `dyn TimeZone`found enum `Tz`error: the `from_utc_datetime` method cannot be invoked on a trait object--> src/main.rs:18:50|18 | println!("converted: {:?}", parsed_time_zone.from_utc_datetime(&ndt));| ^^^^^^^^^^^^^^^^^|::: /home/marcogorelli/.cargo/registry/src/github.com-1ecc6299db9ec823/chrono-0.4.23/src/offset/mod.rs:205:21|205 | pub trait TimeZone: Sized + Clone {| ----- this has a `Sized` requirementSome errors have detailed explanations: E0038, E0191, E0308, E0782.For more information about an error, try `rustc --explain E0038`.error: could not compile `tmp` due to 6 previous errors
I've tried the suggestion to do
let parsed_time_zone: dyn TimeZone = match parse_offset(&tz) {
but then it still errors with
error[E0191]: the value of the associated type `Offset` (from trait `TimeZone`) must be specified--> src/main.rs:7:31|7 | let parsed_time_zone: dyn TimeZone = match parse_offset(&tz) {| ^^^^^^^^ help: specify the associated type: `TimeZone<Offset = Type>`
How can I just parse tz as TimeZone, without specified whether it's Tz or FixedOffset, and then use it in the rest of the function?
答案1
得分: 1
你无法轻松重构这部分。Rust 是强类型语言，而你的两个 time_zone 变量不是相同的类型。一个是 chrono::FixedOffset，另一个是 chrono_tz::Tz。
如果你确实想将它们都存储在同一个变量中，你有两种可能性：
- 特质对象引用 (
&dyn TimeZone) - 包装的特质对象 (
Box<dyn TimeZone>).
dyn Timezone 本身不能直接成为变量的类型。它没有已知的大小，因为它可以是任何实际大小的类型。因此，你需要使用上述其中一种间接方法。
然而，对于你的情况来说，这两种方法都不太理想：
&dyn TimeZone不起作用，因为必须有某个拥有实际对象的人。你不能返回对内部范围变量的引用，因为它比引用更早被丢弃。Box<dyn TimeZone>可以工作，没有问题，但它会引入开销：它会在堆上分配实际对象的空间。正如前面所描述的，dyn对象没有已知的大小。因此，使用Box，实际对象被移到堆上，在堆栈上的是Box对象，它有已知的大小。
所以我建议的做法是：不要担心。你在这里的代码重复非常小。我不认为值得麻烦。
我在回答这个问题后注意到的进一步细节：
TimeZone是: Sized，这意味着它不是对象安全，并且完全不允许使用dyn TimeZone。因此，甚至Box<dyn TimeZone>也不会起作用。
英文:
You can't refactor this easily. Rust is strongly typed, and your two time_zone variables aren't the same type. One is chrono::FixedOffset, the other is chrono_tz::Tz.
If you do want to store both of them in the same variable, you have two possibilities:
- trait object references (
&dyn TimeZone) - Boxed trait objects (
Box<dyn TimeZone).
dyn Timezone itself cannot directly be the type of a variable. It doesn't have a known size, because it could be any actual type of any size. Therefore you need one of the indirections above.
Both are non-ideal for your case though:
&dyn TimeZonewon't work because someone has to own the actual object. You can't return a reference to a variable of an inner scope, as it gets dropped earlier than the reference does.Box<dyn TimeZone>would work, no problem, but it introduces overhead: It will cause a heap allocation where the actual object will be put. The reason, as described earlier, is that adynobject doesn't have a known size. Therefore, withBox, the actual object gets moved to the heap, and on the stack is theBoxobject, which has a known size.
So what I would do is: don't worry. The code duplication you have here is really minimal. I don't think it's worth the trouble.
Further detail I noticed after I wrote this answer:
- TimeZone is
: Sized, which means it isn't object safe anddyn TimeZoneis disallowed entirely. So not evenBox<dyn TimeZone>would work.
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