如何从“文件”推广到“阅读”？

Question

我有一些读取文件的工作代码，但我需要将其概括为从简单磁盘文件以外的其他来源提取数据。

为了替换File

• If，

1. 是正确的泛化，因此，如何在下面的示例代码中修复example2？实际上，它在注释行的编译错误dyn async_std::io::Read cannot be unpinned中失败。如果没有，我应该从example2

返回哪些类型，并且是否需要在get_read中进行相应的更改？

//! [dependencies]
//! tokio = { version = "1.0.1", features = ["full"] }
//! async-std = "1.8.0"
//! anyhow = "1.0.32"

use async_std::io::prelude::*;
use async_std::fs::File;
use anyhow::Result;

#[tokio::main]
async fn main() -> Result<()> {
    example1().await?;
    example2().await?;
    Ok(())
}

// Example of consuming `File` ... works great!
async fn example1() -> Result<()> {
    let mut file = get_file().await?;
    let mut contents = String::new();
    let _ = file.read_to_string(&mut contents).await?;
    println!("read {} characters", contents.len());
    Ok(())
}

// Example of consuming `Read` ... does not compile?
async fn example2() -> Result<()> {
    let mut read = get_read().await?;
    let mut contents = String::new();
    // ERROR:  `dyn async_std::io::Read` cannot be unpinned
    let _ = read.read_to_string(&mut contents).await?;
    println!("read {} characters", contents.len());
    Ok(())
}

async fn get_read() -> Result<Box<dyn Read>> {
    let file = get_file().await?;
    Ok(Box::new(file))
}

async fn get_file() -> Result<File> {
    let file = File::open("/etc/hosts").await?;
    Ok(file)
}

Answer 1

你需要钉：

async fn get_read() -> Result<Pin<Box<dyn Read>>> {
    let file = get_file().await?;
    Ok(Box::pin(file))
}

Box<File> (没有Pin)工作是因为文件实现了Unpin。Box<dyn Read + Unpin>也能工作。