在生成的线程中读取不可变值 [英] Reading immutable value inside spawned thread
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问题描述
我正在努力让这个例子发挥作用
I'm trying to make this example work
use std::sync::mpsc::{Sender, Receiver};
use std::sync::mpsc;
use std::thread;
use std::sync::Arc;
struct User {
reference: String,
email: String
}
struct UserIndex {
reference: usize,
email: usize
}
fn main() {
let rows = vec![
vec!["abcd", "test@test.com"],
vec!["efgh", "test1@test.com"],
vec!["wfee", "test2@test.com"],
vec!["rrgr", "test3@test.com"]
];
let user_index = Arc::new(
UserIndex {
reference: 0,
email: 1
}
);
let chunk_len = (rows.len() / 2) as usize;
let mut chunks = Vec::new();
for chunk in rows.chunks(chunk_len) {
chunks.push(chunk.to_owned());
}
let (tx, rx): (Sender<Vec<User>>, Receiver<Vec<User>>) = mpsc::channel();
for chunk in chunks {
let thread_tx = tx.clone();
thread::spawn(move || {
let user_index_cloned = user_index.clone();
let result = chunk.iter().map( |row|
User {
reference: row[user_index_cloned.reference].to_string(),
email: row[user_index_cloned.email].to_string()
}
).collect::<Vec<User>>();
thread_tx.send(result).unwrap();
});
}
let mut users = Vec::new();
for _ in 0..chunk_len {
users.push(rx.recv());
}
}
但它抛出一个错误
src/main.rs:43:28: 43:38 error: capture of moved value: `user_index` [E0382]
src/main.rs:43 let user_index_cloned = user_index.clone();
^~~~~~~~~~
note: in expansion of closure expansion
src/main.rs:42:17: 51:4 note: expansion site
note: in expansion of for loop expansion
src/main.rs:40:2: 52:3 note: expansion site
src/main.rs:42:17: 51:4 note: `user_index` moved into closure environment here because it has type `[closure(())]`, which is non-copyable
src/main.rs:42 thread::spawn(move || {
src/main.rs:43 let user_index_cloned = user_index.clone();
src/main.rs:44 let result = chunk.iter().map( |row|
src/main.rs:45 User {
src/main.rs:46 reference: row[user_index_cloned.reference].to_string(),
src/main.rs:47 email: row[user_index_cloned.email].to_string()
...
note: in expansion of closure expansion
src/main.rs:42:17: 51:4 note: expansion site
note: in expansion of for loop expansion
src/main.rs:40:2: 52:3 note: expansion site
src/main.rs:42:17: 51:4 help: perhaps you meant to use `clone()`?
error: aborting due to previous error
现在根据 这个讨论它应该有效,但是它没有.我在这里错过了什么?
now according to this discussion it should have worked, but it doesn't. What am I missing here?
推荐答案
大功告成.只是Arc
必须在衍生线程的外部被克隆:
You're almost there. It's just that Arc
has to be cloned outside of the spawned thread:
for chunk in chunks {
let thread_tx = tx.clone();
let user_index_cloned = user_index.clone();
thread::spawn(move || {
let result = chunk.iter().map( |row|
User {
reference: row[user_index_cloned.reference].to_string(),
email: row[user_index_cloned.email].to_string()
}
).collect::<Vec<User>>();
thread_tx.send(result).unwrap();
});
}
必须这样做,否则 user_index
确实会被移动到线程中,并且在循环中这样做是违反所有权规则的.
This has to be done because otherwise user_index
indeed will be moved into the thread, and doing this in a loop is a violation of ownership rules.
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