创建特征对象的向量

Question

我正在尝试创建一个特征对象的向量，但我得到了类型不匹配。我还尝试使用. map（）代替for循环，但遇到了同样的问题。这是我真实代码的最小版本，我意识到这个例子不需要使用特征对象，但我的真实代码需要，因为列表中除了Link之外还有其他类型，它们也实现了Speak。如何使用正确的类型创建Vec以便将其传递给trait_objects？

use std::fs;

fn main() {
    // let links = fs::read_dir("my_dir").unwrap();
    // let links: Vec<Box<dyn Speak>> = links
    //     .map(|file| {
    //         let myfile = file.unwrap();
    //         let href = myfile.path().to_str().unwrap();
    //         Box::new(Link { attr: href })
    //     })
    //     .collect();

    let links = Vec::new();
    for entry in fs::read_dir("my_dir").unwrap() {
        let myfile = entry.unwrap();
        let href = myfile.path().to_str().unwrap();
        links.push(Box::new(Link { attr: href }));
    }

    let link_objects = ListOfTraitObjects {
        trait_objects: links,
    };
    for link in link_objects.trait_objects.iter() {
        println!("{}", link.speak());
    }
}
trait Speak {
    fn speak(&self) -> String;
}

struct ListOfTraitObjects {
    trait_objects: Vec<Box<dyn Speak>>,
}

struct Link<'a> {
    attr: &'a str,
}
impl Speak for Link<'_> {
    fn speak(&self) -> String {
        self.attr.to_string()
    }
}

Answer 1

您的代码存在几个问题。首先，您正在创建一个 Vec

links.push(Box::new(Link { attr: href }) as Box::<dyn Speak>);

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然而，这会产生两个新的错误：

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:7:20
  |
7 |         let href = myfile.path().to_str().unwrap();
  |                    ^^^^^^^^^^^^^                  - temporary value is freed at the end of this statement
  |                    |
  |                    creates a temporary which is freed while still in use
8 |         links.push(Box::new(Link { attr: href }) as Box::<dyn Speak>);
  |                    ----------------------------- cast requires that borrow lasts for `'static`

error[E0596]: cannot borrow `links` as mutable, as it is not declared as mutable
 --> src/lib.rs:8:9
  |
4 |     let links = Vec::new();
  |         ----- help: consider changing this to be mutable: `mut links`
...
8 |         links.push(Box::new(Link { attr: href }) as Box::<dyn Speak>);
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

第一个错误是因为 Link 结构从条目中借用了一个字符串，但条目只在当前循环迭代时存在。即使错误指出强制转换需要“静态生存期”，如果您只是尝试构建Vec，也会收到类似的错误

struct Link {
    attr: String,
}
impl Speak for Link {
    fn speak(&self) -> String {
        self.attr.clone()
    }
}

第二个错误更容易，只需按照编译器的建议将 let links = ... 替换为 let mut links = ...即可修复。

完整工作示例:

use std::fs;

fn main() {
    let mut links = Vec::new();
    for entry in fs::read_dir("my_dir").unwrap() {
        let myfile = entry.unwrap();
        let href = myfile.path().to_str().unwrap().to_string();
        links.push(Box::new(Link { attr: href }) as Box::<dyn Speak>);
    }

    let link_objects = ListOfTraitObjects {
        trait_objects: links,
    };
    for link in link_objects.trait_objects.iter() {
        println!("{}", link.speak());
    }
}
trait Speak {
    fn speak(&self) -> String;
}

struct ListOfTraitObjects {
    trait_objects: Vec<Box<dyn Speak>>,
}

struct Link {
    attr: String,
}
impl Speak for Link {
    fn speak(&self) -> String {
        self.attr.clone()
    }
}

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同样的修复也可以应用于基于迭代器的解决方案：

let links = fs::read_dir("my_dir").unwrap();
let links: Vec<_> = links
    .map(|file| {
        let myfile = file.unwrap();
        let href = myfile.path().to_str().unwrap().to_string();
        Box::new(Link { attr: href }) as Box<dyn Speak>
    })
    .collect();

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