casting - 转换为未调整大小的类型 : `std::io::Stdout` as `std::io::Write` error

Question

我正在尝试将 Stdout 转换为 Write:

use std::io::{self, Write};

pub struct A<Output: Write> {
    output: Output,
}

impl<Output: Write> A<Output> {
    pub fn new() -> A<Output> {
        A {
            output: io::stdout() as Write,
        }
    }
}

编译器报错:

error[E0620]: cast to unsized type: `std::io::Stdout` as `std::io::Write`
  --> src/main.rs:10:21
   |
10 |             output: io::stdout() as Write,
   |                     ^^^^^^^^^^^^^^^^^^^^^
   |
help: consider using a box or reference as appropriate
  --> src/main.rs:10:21
   |
10 |             output: io::stdout() as Write,
   |                     ^^^^^^^^^^^^

我想转换它并尝试按照编译器的建议进行操作，但它说 as 关键字只能用于基元，我应该实现 From特征。

如何将 Stdout 转换为 Write 特征？

Answer 1

cast Stdout to Write

这没有意义，因为 Write不是您将其转换为 的类型。 Write是一种特质。有 trait objects 是类型，例如 Box<Write>或 &Write .重读 trait objects chapter of The Rust Programming Language刷新你对这个话题的内存。 Rust 1.27 将改进此处的语法，使其更明显，如 Box<dyn Write>/&dyn Write .

你可以使用 Box::new(io::stdout()) as Box<Write> ，但您很快就会遇到 "Expected type parameter" error in the constructor of a generic struct .

impl A<Box<Write>> {
    pub fn new() -> Self {
        A {
            output: Box::new(io::stdout()) as Box<Write>,
        }
    }
}

另见:

• What makes something a "trait object"?
• Why is `let ref a: Trait = Struct` forbidden
• "Expected type parameter" error in the constructor of a generic struct