无法推断生命周期参数克隆特征对象的适当生命周期

Question

这个问题的重复似乎并没有为我解决问题。 以下代码给出了错误：

use std::collections::HashMap;
use std::thread;


pub trait Spider : Sync + Send {
    fn add_request_headers(&self, headers: &mut Vec<String>);
}

pub struct Google {}

impl Spider for Google {
    fn add_request_headers(&self, headers: &mut Vec<String>) {
        headers.push("Hello".to_string())
    }
}

fn parallel_get(spiders: &HashMap<String, Box<Spider>>) -> std::thread::JoinHandle<()> {
    let thread_spiders = spiders.clone();
    thread::spawn(move || {
        let headers = &mut vec![];
        let spider = thread_spiders.get("Google").unwrap();
        spider.add_request_headers(headers);
    })
}

fn main() {
    let spiders = HashMap::new();
    let spider = Box::new(Google{});
    spiders.insert("Google", spider);
}

在这里的操场上跑步。

我明白了：

rustc 1.14.0 (e8a012324 2016-12-16)
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> <anon>:18:34
   |
18 |     let thread_spiders = spiders.clone();
   |                                  ^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the block at 17:87...
  --> <anon>:17:88
   |
17 | fn parallel_get(spiders: &HashMap<String, Box<Spider>>) -> std::thread::JoinHandle<()> {
   |                                                                                        ^
note: ...so that types are compatible (expected &&std::collections::HashMap<std::string::String, Box<Spider>>, found &&std::collections::HashMap<std::string::String, Box<Spider + 'static>>)
  --> <anon>:18:34
   |
18 |     let thread_spiders = spiders.clone();
   |                                  ^^^^^
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that the type `[closure@<anon>:19:19: 23:6 thread_spiders:&std::collections::HashMap<std::string::String, Box<Spider>>]` will meet its required lifetime bounds
  --> <anon>:19:5
   |
19 |     thread::spawn(move || {
   |     ^^^^^^^^^^^^^

我认为它告诉我它不能自动推断thread_spiders的生命周期，因为它需要'static才能为线程保持足够长的时间，但它不能超过'a这是输入参数的生命周期。

问题是，我可以在parallel_get克隆其他对象，并且它们可以毫无问题地移动到新线程中。 但由于某种原因， thread_spiders似乎绊倒了编译器。 它应该具有'a的生命周期，如果我是正确的，然后进入线程闭包。

我已经尝试将明确的生命周期参数添加到parallel_get但是无法获得任何工作。 如何编译此代码？

Answer 1

在这种情况下错误消息非常混乱，但请注意这里的双符号:( (expected &&std::collections::HashMap<std::string::String, Box<Spider>>, found &&std::collections::HashMap<std::string::String, Box<Spider + 'static>>) 。

看起来它试图克隆引用。 我假设您想要克隆整个HashMap 。 将clone显式调用为Clone::clone(spiders)提供更清晰的错误消息：

error[E0277]: the trait bound `Spider: std::clone::Clone` is not satisfied
error[E0277]: the trait bound `Spider: std::marker::Sized` is not satisfied
  --> error_orig.rs:19:26
   |
19 |     let thread_spiders = Clone::clone(spiders);
   |                          ^^^^^^^^^^^^ the trait `std::marker::Sized` is not implemented for `Spider`
   |
   = note: `Spider` does not have a constant size known at compile-time
   = note: required because of the requirements on the impl of `std::clone::Clone` for `Box<Spider>`
   = note: required because of the requirements on the impl of `std::clone::Clone` for `std::collections::HashMap<std::string::String, Box<Spider>>`
   = note: required by `std::clone::Clone::clone`

你在Box<Spider>上调用Clone::clone ，这是一个拥有的特征对象。 如何克隆包含盒装特征对象的HashMap？ 说明它可以通过向您的特征引入克隆方法来实现，如下所示：

pub trait Spider: Sync + Send {
    fn add_request_headers(&self, headers: &mut Vec<String>);
    fn clone_into_box(&self) -> Box<Spider>;
}

impl Clone for Box<Spider> {
    fn clone(&self) -> Self {
        self.clone_into_box()
    }
}

#[derive(Clone)]
pub struct Google {}

impl Spider for Google {
    fn add_request_headers(&self, headers: &mut Vec<String>) {
        headers.push("Hello".to_string())
    }

    fn clone_into_box(&self) -> Box<Spider> {
        Box::new(self.clone())
    }
}

如何克隆存储特征对象的结构？ 建议为这种多态克隆方法创建一个单独的特征。