Rust : ? 操作符（待续）

Rust从1.13版对语言进行了一些扩展，包括期待已久的?操作符。今天重点来谈谈？操作符。

最初，加入了一个新的操作符?，是为了简化Result

fn read_username_from_file() -> Resultio::Error> {
    let f = File::open("username.txt");

    let mut f = match f {
        Ok(file) => file,
        Err(e) => return Err(e),
    };

    let mut s = String::new();

    match f.read_to_string(&mut s) {
        Ok(_) => Ok(s),
        Err(e) => Err(e),
    }
}

有了？操作符：

fn read_username_from_file() -> Result<String, io::Error> {
    //对象本身返回类型就是Result; 且在Result类函数体内
    let mut f = File::open("username.txt")?; 
    let mut s = String::new();
    f.read_to_string(&mut s)?;
    Ok(s)
}

use std::fs::File;
use std::io::prelude::*;
use std::io;

struct Info {
    name: String,
    age: i32,
    rating: i32,
}

fn write_info(info: &Info) -> io::Result<()> {
    // Early return on error
    let mut file = match File::create("my_best_friends.txt") {
           Err(e) => return Err(e),
           Ok(f) => f,
    };
    if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
        return Err(e)
    }
    if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
        return Err(e)
    }
    if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
        return Err(e)
    }
    Ok(())
}

有了？操作符，可以改成下面：

use std::fs::File;
use std::io::prelude::*;
use std::io;

struct Info {
    name: String,
    age: i32,
    rating: i32,
}

fn write_info(info: &Info) -> io::Result<()> {
    let mut file = File::create("my_best_friends.txt")?;
    // Early return on error
    file.write_all(format!("name: {}\n", info.name).as_bytes())?;
    file.write_all(format!("age: {}\n", info.age).as_bytes())?;
    file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
    Ok(())
}

细查一下write_all:

use std::io;

trait Write {
    fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
}

还有：File::create

pub fn create<P: AsRef>(path: P) -> Result

感觉幸福了不少。

注意：test_io函数体中的内容不能直接放到main函数中。因为不符合我们前面提的几个条件，main函数的返回为（），不是Result或Option类型，或实现了Try trait。

use std::fs::File;
use std::io::prelude::*;
fn main() {
    let st = SystemTime::now();
    println!("i will go asleep!");
    test_io("C:\\Users\\wowotuo\\Desktop\\test.txt");
    let et =SystemTime::now();    
    thread::sleep_ms(500000);

}

fn test_io(s:&str)->io::Result<()>{
    let mut f = File::create(s)?;
    f.write_all(b"Hello, world!")?;
    f.sync_data()?;
    Ok(())
}

二、发展至Option

从1.22版本开始，为了简化Option类型的处理，最新版本的Rust允许在Option类型上使用？操作符了。