我将集合转储到磁盘上。当请求时,应该检索这些集合(没问题)和 iterator
应该为它构建返回对检索到的值的引用。
iterator
之后被丢弃了,我不再需要收藏了。我也希望它被删除。
到目前为止我尝试了什么:
Iterator
拥有收藏。这对我来说最有意义,但这是不可能的;我不太清楚为什么。有人说Iterator
next
的特征方法签名是问题所在。 ( example )引用计数:
Retriever
返回Rc<Vec<usize>>
.我遇到了与拥有迭代器相同的问题。 ( example )让寻回犬拥有收藏并分发对它的引用。我试图实现具有内部可变性的检索器 (
RefCell<HashMap>
),但我无法将引用返回到HashMap
中具有足够长的生命周期。
我看到了两种基本的可能性。
猎犬转移所有权。然后
Iterator
需要拥有数据。内容如下:use std::slice::Iter; fn retrieve(id: usize) -> Vec<usize> { //Create Data out of the blue (or disk, or memory, or network. I dont care) //Move the data out. Transfer ownership let data = vec![0, 1, 2, 3]; data } fn consume_iterator<'a, TIterator: Iterator<Item=&'a usize>>(iterator: TIterator) { for i in iterator { println!("{}", i); } } fn handler<'a>(id: usize) -> Iter<'a, usize> { //handle_request now owns the vector. //I now want to build an owning iterator.. //This does of course not compile as vector will be dropped at the end of this method retrieve(id).iter() } fn main() { consume_iterator(handler(0)) }
寻回犬拥有收藏。但是随后出现了两个新问题:
- 如何在迭代器超出范围时删除数据?
- 我如何告诉借阅检查员我将拥有该馆藏足够长的时间?
use std::cell::{Ref, RefCell}; struct Retriever { //Own the data. But I want it to be dropped as soon as the references to it go out of scope. data: RefCell<Vec<usize>> } impl Retriever{ fn retrieve<'a>(&'a self, id: usize) -> Ref<'a, Vec<usize>> { //Create Data out of the blue (or disk, or memory, or network. I dont care) //Now data can be stored internally and a referece to it can be supplied. let mut data = self.data.borrow_mut(); *data = vec![0, 1, 2, 3]; self.data.borrow() } } fn consume_iterator<'a, TIterator: Iterator<Item=&'a usize>>(iterator: TIterator) { for i in iterator { println!("{}", i); } } fn handler<'a>(ret: &'a Retriever, id: usize) -> IterWrapper<'a> { //andle_request now has a reference to the collection //So just call iter()? Nope. Lifetime issues. ret.retrieve(id).iter() } fn main() { let retriever = Retriever{data: RefCell::new(Vec::new())}; consume_iterator(handler(&retriever, 0)) }
我觉得这里有点迷茫,我忽略了一些显而易见的事情。
最佳答案
The Iterator owns the collection. [or joint ownership via reference-counting]
ContainerIterator { data: data, iter: data.iter(), }
不,你cannot have a value and a reference to that value in the same struct .
Letting the retriever own the collection and handing out a reference to it.
不,你cannot return references to items owned by the iterator .
正如评论者所说,使用 IntoIter
将项目的所有权转移到迭代器,然后将它们作为迭代值分发出去:
use std::vec::IntoIter;
struct ContainerIterator {
iter: IntoIter<usize>,
}
impl Iterator for ContainerIterator {
type Item = usize;
fn next(&mut self) -> Option<Self::Item> {
self.iter.next()
}
}
fn main() {
let data = vec![0, 1, 2, 3];
let cont = ContainerIterator { iter: data.into_iter() };
for x in cont {
println!("Hi {}", x)
}
}
如果您必须返回引用...那么您需要在所有引用可能存在的整个时间内保持拥有它们的东西。
How can I drop the data when the iterator is out of range?
不再使用该值:
fn main() {
{
let loaded_from_disk = vec![0, 1, 2, 3];
for i in &loaded_from_disk {
println!("{}", i)
}
// loaded_from_disk goes out of scope and is dropped. Nothing to *do*, per se.
}
}
How do I tell the borrow-checker that I will own the collection long enough?
拥有收藏足够长的时间。没有 Rust Illuminati 与借用检查器使用的 secret 握手。代码只需要结构化,使得借用的东西在借用未完成时不会变得无效。你不能移动它(改变内存地址)或放下它(改变内存地址)。
关于iterator - 遍历集合。 Iterator 被删除后立即删除它,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/38585599/