rust - 从对象池中借用时解决省略的静态生命周期

Question

这是我目前面临的问题的简化版本。

trait SuperObject {
    fn object_name(&self) -> String;
}

trait Inspect {
    fn inspect(&self);
}

impl Inspect for SuperObject {
    fn inspect(&self) {
        println!("I am a Superobject.");
    }
}

struct Object {
    name: String
}

impl SuperObject for Box<Object> {
    fn object_name(&self) -> String {
        format!("I am {}.", self.name.clone())
    }
}

struct ObjectPool {
    object1: Box<Object>,
    object2: Box<Object>,
    object3: Box<Object>
}

impl ObjectPool {
    pub fn new() -> ObjectPool {
        ObjectPool {
            object1: Box::new(Object { name: String::from("Object 1") }),
            object2: Box::new(Object { name: String::from("Object 2") }),
            object3: Box::new(Object { name: String::from("Object 3") })
        }
    }
    fn all_objects(&self) -> Vec<&SuperObject> {
        let mut ret: Vec<&SuperObject> = Vec::new();
        ret.push(&self.object1);
        ret.push(&self.object2);
        ret.push(&self.object3);
        ret
    }
}

fn main() {
    let objectpool: ObjectPool = ObjectPool::new();
    let allobjects: Vec<&SuperObject> = objectpool.all_objects();
    for i in &allobjects {
        println!("{}", i.object_name());
        // Comment the following line in order to drop error E0597
        i.inspect(); // FIXME: borrowed value must be valid for the static lifetime
    }
}

尝试编译这段代码时的错误如下:

error[E0597]: `objectpool` does not live long enough
  --> src/main.rs:50:41
   |
50 |     let allobjects: Vec<&SuperObject> = objectpool.all_objects();
   |                                         ^^^^^^^^^^ does not live long enough
...
56 | }
   | - borrowed value only lives until here
   |
   = note: borrowed value must be valid for the static lifetime...

error: aborting due to previous error

经过多次搜索，据我了解，被实例化的对象具有默认的静态生命周期，如 https://doc.rust-lang.org/book/second-edition/ch19-02-advanced-lifetimes.html 中所述

我相信 ObjectPool 的 all_objects 方法的输出被编译器忽略为静态的，正如我尝试调试代码片段时引发的错误之一所证明的那样:

error[E0308]: mismatched types
  --> src/main.rs:42:18
   |
42 |         ret.push(&self.object2);
   |                  ^^^^^^^^^^^^^ expected struct `std::boxed::Box`, found reference
   |
   = note: expected type `std::boxed::Box<SuperObject>`
              found type `&std::boxed::Box<SuperObject + 'static>`

不涉及完全废弃对象池的最佳行动方案是什么？或者是否有适合 Rust 实现的更优雅的抽象？

Answer 1

问题是您的 impl Inspect for SuperObject。为另一个特征实现一个特征不会达到您期望的效果。基本上规则是:永远不要这样做。从本质上讲，这意味着只有当您拥有 &(SuperObject + 'static) 时，您才能将其视为 Inspect。你想要的是

impl<T: SuperObject + ?Sized> Inspect for T {
    fn inspect(&self) {
        println!("I am a Superobject.");
    }
}