代码之家 › 专栏 › 技术社区 › David

`match的手臂类型不兼容

rust-2021 rust

1

David · 技术社区 · 2 年前

我有一些代码需要创建和使用 quick_xml::Writer 使用任何一个 File 或 Cursor 取决于用户输入。正确的生锈方式(非继承/非向下投射)是什么 Writer

关于堆栈溢出的所有答案似乎都很古老,建议在带有Box的堆上进行分配,但这不再有效,而且就个人而言,它感觉无论如何都是错误的。

代码:

use std::fs::File;
use std::io::Cursor;
use quick_xml::Writer;

fn main() {
    let some_option = Some("some_file.txt");

    let writer = match &some_option {
        Some(file_name) => {
            Writer::new(File::create(file_name).unwrap())
        },
        _ => {
            Writer::new(Cursor::new(Vec::new()))
        },
    };
}

错误:

error[E0308]: `match` arms have incompatible types
  --> src\main.rs:13:13
   |
8  |       let writer = match &some_option {
   |  __________________-
9  | |         Some(file_name) => {
10 | |             Writer::new(File::create(file_name).unwrap())
   | |             --------------------------------------------- this is found to be of type `Writer<File>`
11 | |         },
12 | |         _ => {
13 | |             Writer::new(Cursor::new(Vec::new()))
   | |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `File`, found struct `std::io::Cursor`
14 | |         },
15 | |     };
   | |_____- `match` arms have incompatible types
   |
   = note: expected type `Writer<File>`
            found struct `Writer<std::io::Cursor<Vec<u8>>>`

For more information about this error, try `rustc --explain E0308`.

2 回复 | 直到 2 年前

1

Ry- 2 年前

这个 dyn Writer . 例如,对于装箱特征对象,

let destination: Box<dyn Write> = match &some_option {
    Some(file_name) => {
        Box::new(File::create(file_name).unwrap())
    }
    None => {
        Box::new(Cursor::new(Vec::new()))
    }
};

let writer = Writer::new(destination);

Write the necessary blanket implementations 对这件作品进行修改。

2

1

cdhowie 2 年前

二者都 File 和 Cursor 使生效 std::io::Write ,所以你可以通过装箱这些内在价值观来解决这个问题,给自己一个 Writer<Box<Write>> :

let writer: Writer<Box<dyn std::io::Write>> = Writer::new(match &some_option {
    Some(file_name) => {
        Box::new(File::create(file_name).unwrap())
    },
    _ => {
        Box::new(Cursor::new(Vec::new()))
   },
});

文件 光标 价值或者,您可以使用 either Either 相反:

let writer = Writer::new(match &some_option {
    Some(file_name) => {
        Either::Left(File::create(file_name).unwrap())
    },
    _ => {
        Either::Right(Cursor::new(Vec::new()))
    },
});

Either implements Write when both the Left and Right variants do