如何从向量中获取可变引用？

Question

run和send方法都需要一个self对象，send函数也需要一个可变的Packet引用：

struct Package {
    id: u32,
}

impl Package {
    fn new(id: u32) -> Package {
        Package { id }
    }
}

struct Manager {
    packages: Vec<Package>,
}

impl Manager {
    fn new() -> Manager {
        Manager {
            packages: vec![
                Package::new(1),
                Package::new(2),
                Package::new(3),
                Package::new(4),
            ],
        }
    }
    fn run(&mut self) {
        for package in self.packages.iter_mut() {
            if package.id == 1 {
                self.send(package);
            }
            println!("{}", package.id);
        }
    }
    fn send(&self, package: &mut Package) {
        package.id = 23;
    }
}

fn main() {
    let manager = Manager::new();
    manager.run();
}

error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
  --> src/main.rs:29:17
   |
27 |         for package in self.packages.iter_mut() {
   |                        ------------------------
   |                        |
   |                        mutable borrow occurs here
   |                        mutable borrow later used here
28 |             if package.id == 1 {
29 |                 self.send(package);
   |                 ^^^^ immutable borrow occurs here

error[E0596]: cannot borrow `manager` as mutable, as it is not declared as mutable
  --> src/main.rs:41:5
   |
40 |     let manager = Manager::new();
   |         ------- help: consider changing this to be mutable: `mut manager`
41 |     manager.run();
   |     ^^^^^^^ cannot borrow as mutable

如何重构这两个函数以使其正常工作？

我不想复制数据包，因为那样会给我的原始代码带来很多麻烦。

Answer 1

选项 1：不接受 send() 中的 &self。让它从 self 中获取它需要的东西，例如 &mut Package 和可能的其他字段：

// call it with Self::send(package)
fn send(package: &mut Package) {
    package.id = 23;
}

选项 2：接受包索引而不是 &mut Package:

fn run(&mut self) {
    for package_idx in 0..self.packages.len() {
        if self.packages[package_idx].id == 1 {
            self.send(package_idx);
        }
        println!("{}", self.packages[package_idx].id);
    }
}

fn send(&mut self, package_idx: usize) {
    self.packages[package_idx].id = 23;
}

选项 3：使用内部可变性。

struct Manager {
    packages: Vec<RefCell<Package>>,
}

impl Manager {
    fn new() -> Manager { ... }

    fn run(&mut self) {
        for package in &self.packages {
            if package.borrow().id == 1 {
                self.send(package);
            }
            println!("{}", package.borrow().id);
        }
    }
    fn send(&self, package: &RefCell<Package>) {
        let package = package.borrow_mut();
        package.id = 23;
    }
}

哪个选项适合您取决于您的用例。在其他条件相同的情况下，我更喜欢选项 1，然后是选项 2，然后是选项 3。

如何从向量中获取可变引用？

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