跨线程共享具有特征对象作为属性的结构

问题描述

我有下面的代码.随着注释掉的部分，它的工作.当我取消注释部分时，它不再编译.

I have the code below. With the commented out parts, it's working. When I uncomment the parts it does not compile anymore.

如何调整注释部分使其工作，即我想让线程同时访问表达式树.

How can I adjust the commented parts to make them work, i.e., I want to make threads access the expression tree simultaneously.

当我尝试时，编译器从线程安全性错误开始.

When I try it, the compiler starts with errors about thread safeness.

我读过 Rust 的书，知道 C/C++，但还没有完全理解 Rust 的类型系统和语义.

I read the Rust book and know C/C++, but didn't understood everything about Rust type system and semantics yet.

use std::thread;
use std::sync::Arc;

pub trait Expr {
    fn run(&self) -> i32;
}

pub struct ConstantExpr {
    n: i32,
}

impl ConstantExpr {
    pub fn new(n: i32) -> Self {
        Self { n }
    }
}

impl Expr for ConstantExpr {
    fn run(&self) -> i32 {
        self.n
    }
}

pub struct AddExpr {
    expr1: Box<Expr>,
    expr2: Box<Expr>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr>, expr2: Box<Expr>) -> Self {
        Self { expr1, expr2 }
    }
}

impl Expr for AddExpr {
    fn run(&self) -> i32 {
        self.expr1.run() + self.expr2.run()
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr>,
}

impl Container {
    fn new() -> Self {
        Self {
            x: 0,
            cached_expr: Arc::new(AddExpr::new(
                Box::new(ConstantExpr::new(10)),
                Box::new(ConstantExpr::new(1)),
            )),
        }
    }
}

fn main() {
    let container = Arc::new(Container::new());

    let container1 = Arc::clone(&container);

    /*
    let thread1 = thread::spawn(move || {
        println!("thread1: {}", container1.x);
        println!("thread1: {}", container1.cached_expr.run());
    });
    */

    println!("main: {}", container.x);
    println!("main: {}", container.cached_expr.run());

    //thread1.join().unwrap();
}

错误:

error[E0277]: the trait bound `Expr + 'static: std::marker::Send` is not satisfied
  --> src/main.rs:64:19
   |
64 |     let thread1 = thread::spawn(move || {
   |                   ^^^^^^^^^^^^^ `Expr + 'static` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `Expr + 'static`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Expr + 'static>`
   = note: required because it appears within the type `Container`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Container>`
   = note: required because it appears within the type `[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]`
   = note: required by `std::thread::spawn`

error[E0277]: the trait bound `Expr + 'static: std::marker::Sync` is not satisfied
  --> src/main.rs:64:19
   |
64 |     let thread1 = thread::spawn(move || {
   |                   ^^^^^^^^^^^^^ `Expr + 'static` cannot be shared between threads safely
   |
   = help: the trait `std::marker::Sync` is not implemented for `Expr + 'static`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Expr + 'static>`
   = note: required because it appears within the type `Container`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Container>`
   = note: required because it appears within the type `[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]`
   = note: required by `std::thread::spawn`

推荐答案

我发现错误信息非常简单:

I find the error message pretty straightforward:

• 特征 std::marker::Send 没有为 Expr + 'static
实现
• 需要，因为 std::sync::Arc
的 std::marker::Send 的实现要求>
• 必需，因为它出现在 Container
类型中
• 需要，因为std::sync::Arc
的std::marker::Send的实现要求
• 必需，因为它出现在类型 [closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc]
• std::thread::spawn 需要

• the trait std::marker::Send is not implemented for Expr + 'static
• required because of the requirements on the impl of std::marker::Send for std::sync::Arc<Expr + 'static>
• required because it appears within the type Container
• required because of the requirements on the impl of std::marker::Send for std::sync::Arc<Container>
• required because it appears within the type [closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]
• required by std::thread::spawn

您正在尝试将 Arc 移动到另一个线程，但它包含一个 Arc，不能保证安全跨线程发送(Send)或共享(Sync).

You are trying to move your Arc<Container> to another thread, but it contains an Arc<Expr + 'static>, which cannot be guaranteed to be safely sent (Send) or shared (Sync) across threads.

将 Send 和 Sync 作为超特征添加到 Expr:

Either add Send and Sync as supertraits to Expr:

pub trait Expr: Send + Sync { /* ... */ }

或者将它们作为 trait bound 添​​加到你的 trait 对象中:

Or add them as trait bounds to your trait objects:

pub struct AddExpr {
    expr1: Box<Expr + Send + Sync>,
    expr2: Box<Expr + Send + Sync>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr + Send + Sync>, expr2: Box<Expr + Send + Sync>) -> Self {
        Self { expr1, expr2 }
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr + Send + Sync>,
}

另见:

• 如何跨线程共享引用?
• 多线程应用程序无法通过错误链编译
• 有没有办法为 ZipFile 实现 Send 特性?
• 如何使用 Arc>> 在线程之间共享通用结构?

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