返回结果<Box<dyn Trait> &gt; 在比赛中

Question

我有一组实现给定特征的类型，我想从字符串名称中获取一个具体的类型对象，奇怪的是当我的match返回Box<dyn Trait>时它可以工作，但当我将它包装在Result时它不会。

鉴于此特征和类型：

trait Shape {
    fn edges(&self) -> u8;
}

struct Triangle {}

impl Shape for Triangle {
    fn edges(&self) -> u8 {
        3
    }
}

struct Rectangle {}

impl Shape for Rectangle {
    fn edges(&self) -> u8 {
        4
    }
}

这有效：

fn get_shape_edges(name: &str) -> Result<u8, &str> {
    let shape: Box<dyn Shape> = match name {
        "triangle" => Box::new(Triangle {}),
        "rectangle" => Box::new(Rectangle {}),
        _ => panic!("Bad value"),
    };
    Ok(shape.edges())
}

但是，这不会：

fn get_shape_edges(name: &str) -> Result<u8, &str> {
    let shape: Box<dyn Shape> = match name {
        "triangle" => Ok(Box::new(Triangle {})),
        "rectanble" => Ok(Box::new(Rectangle {})),
        _ => Err("bad value")
    }?;
    Ok(shape.edges())
}

错误：

error[E0308]: `match` arms have incompatible types
  --> src/main.rs:24:24
   |
22 |       let shape: Box<dyn Shape> = match name {
   |  _________________________________-
23 | |         "triangle" => Ok(Box::new(Triangle {})),
   | |                       ------------------------- this is found to be of type `Result<Box<Triangle>, _>`
24 | |         "rectanble" => Ok(Box::new(Rectangle {})),
   | |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `Triangle`, found struct `Rectangle`
25 | |         _ => Err("bad value")
26 | |     }?;
   | |_____- `match` arms have incompatible types
   |
   = note: expected type `Result<Box<Triangle>, _>`
              found enum `Result<Box<Rectangle>, _>`

当然这只是一个例子，在我的代码中我想使用后一种情况来处理错误。

为什么后一种情况不起作用？

Answer 1

您需要将Box强制转换为Box<dyn Shape> ，否则编译器不够聪明（仍然）来忽略类型：

fn get_shape_edges(name: &str) -> Result<u8, &str> {
    let shape: Box<dyn Shape> = match name {
        "triangle" => Ok(Box::new(Triangle {}) as Box<dyn Shape>),
        "rectanble" => Ok(Box::new(Rectangle {}) as Box<dyn Shape>),
        _ => Err("bad value")
    }?;
    Ok(shape.edges())
}

操场

顺便说一句，由于您已经在构建结果，因此您可以将edges映射到原始构建的边缘：

fn get_shape_edges(name: &str) -> Result<u8, &str> {
    let shape = match name {
        "triangle" => Ok(Box::new(Triangle {}) as Box<dyn Shape>),
        "rectanble" => Ok(Box::new(Rectangle {}) as Box<dyn Shape>),
        _ => Err("bad value")
    };
    shape.map(|s| s.edges())
}