rust错误处理

从read_username_from_file从hello.txt文件读取值并打印该值 kaisery.github.io/trpl-zh-cn/…

• 使用match,if let打印

#![allow(unused)]
fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let username_file_result = File::open("hello.txt");

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

        let mut username = String::new();

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

    let result = read_username_from_file();
    if let Ok(x) = result {
        println!("value: {x}")
    } else if let Err(e) = result {
        println!("Error reading file: {}", e);
    }

}

• 使用match,match打印

#![allow(unused)]
fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let username_file_result = File::open("hello.txt");

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

        let mut username = String::new();

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

    let result = read_username_from_file();

    match result {
        Ok(content) => println!("File content: {}", content),
        Err(e) => println!("Error reading file: {}", e),
    }
    
    
}

• 使用?运算符

#![allow(unused)]
fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let mut username_file = File::open("hello.txt")?;
        let mut username = String::new();
        username_file.read_to_string(&mut username)?;
        Ok(username)

    }

    let result = read_username_from_file();

    match result {
        Ok(content) => println!("File content: {}", content),
        Err(e) => println!("Error reading file: {}", e),
    }
    
    
}

• 使用?运算符的链式方法调用

#![allow(unused)]
fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let mut username = String::new();
        File::open("hello.txt")?.read_to_string(&mut username)?;
        Ok(username)

    }

    let result = read_username_from_file();

    match result {
        Ok(content) => println!("File content: {}", content),
        Err(e) => println!("Error reading file: {}", e),
    }
    
    
}

• 使用 fs::read_to_string 的更为简短的写法

#![allow(unused)]
fn main() {
use std::fs;
use std::io;

fn read_username_from_file() -> Result<String, io::Error> {
    fs::read_to_string("hello.txt")
}
}

• 修改 main 返回 Result<(), E> 允许对 Result 值使用 ? 运算符

use std::error::Error;
use std::fs::File;

fn main() -> Result<(), Box<dyn Error>> {
    let greeting_file = File::open("hello.txt")?;

    Ok(())
}