[英]How to get mutable struct from HashMap?
I have a hashmap for all my states, which is a HashMap<String, Rc<State>>
, and I want to call the current state's member fn init(&mut self)
. 我有一个针对我所有状态的
HashMap<String, Rc<State>>
,它是HashMap<String, Rc<State>>
,我想调用当前状态的成员fn init(&mut self)
。 But I'm getting an error with the following code: 但是我在以下代码中遇到了错误:
...
if let Some(state) = self.states.get_mut(state_id) {
(*state).init();
}
...
Here's the error: 这是错误:
src/main.rs:70:25: 70:33 error: cannot borrow immutable borrowed content as mutable
src/main.rs:70 (*state).shutdown();`
afaict from the documentation, the problem is that get_mut
returns a mutable reference to the state, not a reference to a mutable state. 从文档来看,问题在于
get_mut
返回对该状态的可变引用,而不是对可变状态的引用。 So how would I get a reference to a mutable state? 那么我如何获得对可变状态的引用?
A fundamental idea in Rust is: either Aliasing or Mutability, but not both. Rust中的一个基本思想是: 别名或可变性,但不能同时存在。
Aliasing means having multiple active pointers to the same value. 混叠意味着具有指向相同值的多个活动指针。
What is Rc<T>
? 什么是
Rc<T>
? It's sharing ownership, aliasing a value. 它共享所有权,给值起别名。 Thus
Rc<T>
does not allow mutating the value inside. 因此,
Rc<T>
不允许对内部的值进行突变。
There is a way around this with Rc
, to use interior mutability with types like either Cell<U>
or RefCell<U>
. 使用
Rc
可以解决此问题,可以将内部可变性与Cell<U>
或RefCell<U>
类的类型一起使用。
(If you write a multithreaded program, you'd use Arc
for thread safe shared ownership / aliasing, and you could use Mutex<U>
for thread safe interior mutability instead.) (如果编写多线程程序,则将
Arc
用于线程安全共享所有权/别名,而可以将Mutex<U>
用于线程安全内部可变性。)
Rc<Cell<U>>
allows mutating U
by only allowing write-in and read-out, but no pointers to the inner U
value. Rc<Cell<U>>
允许突变U
通过只允许写入和读出,但没有指针到内U
值。 No pointers, no aliasing! 没有指针,没有锯齿!
Rc<RefCell<U>>
allows mutating by the method .borrow_mut()
that will keep a borrow count at runtime and dynamically make sure that any mutable borrow is exclusive. Rc<RefCell<U>>
允许通过方法.borrow_mut()
进行.borrow_mut()
,该方法将在运行时保留借用计数,并动态确保任何可变借用都是排他的。 No aliasing, you have mutability! 没有别名,您具有可变性!
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