[英]How to send generic T to another thread?
如何发送通用T
?
我尝试将通用T
发送到另一个线程,但我得到:
error[E0308]: mismatched types
--> src/main.rs:23:22
|
23 | t1.merge(Element(vec![3]));
| ^^^^^^^^^^^^^^^^ expected associated type, found struct `Element`
|
= note: expected associated type `<T as Join>::Item`
found struct `Element`
= help: consider constraining the associated type `<T as Join>::Item` to `Element`
完整代码:
trait Join {
type Item;
fn merge(&mut self, other: Self::Item);
}
#[derive(Debug, Default)]
struct Element(Vec<u8>);
impl Join for Element {
type Item = Element;
fn merge(&mut self, mut other: Self::Item) {
self.0.append(&mut other.0);
}
}
fn work<T>()
where
T: Default + Join + Send + Sync + 'static,
{
let (sender, receiver) = std::sync::mpsc::channel::<(T)>();
std::thread::spawn(move || {
while let (mut t1) = receiver.recv().unwrap() {
t1.merge(Element(vec![3]));
}
});
loop {
let mut t1 = T::default();
sender.send(t1);
std::thread::sleep(std::time::Duration::from_secs(5));
}
}
fn main() {
// works!
let mut e = Element(vec![1]);
e.merge(Element(vec![2]));
// bad!
work::<Element>();
}
当您使用 generics 时,您让调用者决定您的通用 function 必须使用哪些类型。
您的示例中的这一行t1.merge(Element(vec;[3]));
是无效的,因为它假定T = Element
但调用者可以从无限多种可能的T
类型中进行选择,其中T != Element
这就是编译器抱怨的原因。
要使您的 function 完全通用,您必须执行一些操作,例如在 function 签名中添加Default
绑定到<T as Join>::Item
,然后将违规行更改为t1.merge(<T as Join>::Item::default());
.
更新的工作注释示例:
use std::fmt::Debug;
trait Join {
type Item;
fn merge(&mut self, other: Self::Item);
}
#[derive(Debug)]
struct Element(Vec<u8>);
// updated Default impl so we can observe merges
impl Default for Element {
fn default() -> Self {
Element(vec![1])
}
}
impl Join for Element {
type Item = Element;
fn merge(&mut self, mut other: Self::Item) {
self.0.append(&mut other.0);
}
}
fn work<T>() -> Result<(), Box<dyn std::error::Error>>
where
T: Default + Join + Send + Sync + Debug + 'static,
<T as Join>::Item: Default, // added Default bound here
{
let (sender, receiver) = std::sync::mpsc::channel::<T>();
std::thread::spawn(move || {
while let Ok(mut t1) = receiver.recv() {
// changed this to use Default impl
t1.merge(<T as Join>::Item::default());
// prints "Element([1, 1])" three times
println!("{:?}", t1);
}
});
let mut iterations = 3;
loop {
let t1 = T::default();
sender.send(t1)?;
std::thread::sleep(std::time::Duration::from_millis(100));
iterations -= 1;
if iterations == 0 {
break;
}
}
Ok(())
}
fn main() -> Result<(), Box<dyn std::error::Error>> {
// works!
let mut e = Element(vec![1]);
e.merge(Element(vec![2]));
// now also works!
work::<Element>()?;
Ok(())
}
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