Rust5.1 Error Handling

Rust学习笔记

Rust编程语言入门教程课程笔记

参考教材: The Rust Programming Language (by Steve Klabnik and Carol Nichols, with contributions from the Rust Community)

Lecture 9: Error Handling

use std::error::Error;
use std::io::ErrorKind;
use std::fs::File;
use std::io::Read;
use std::net::IpAddr;
use std::io;


fn main() -> Result<(), Box<dyn Error>> {//Box means the function will return a type that implements the Error trait

    // Rust groups errors into two major categories: recoverable and unrecoverable errors.
    // For a recoverable error, such as a file not found error, it’s reasonable to report
    // the problem to the user and retry the operation. Unrecoverable errors are always
    // symptoms of bugs, like trying to access a location beyond the end of an array.

    // Recoverable errors are instances of the Result enum, which has variants
    // Ok(T), representing success and containing a value, and Err(E), representing error
    // and containing an error value. The Result enum is defined as part of the
    // standard library.

    // The panic! macro can be used to generate a panic and start unwinding its stack.
    // While unwinding, the runtime will take care of freeing all the resources owned by
    // the thread by calling the destructors of all its objects.

    let f = File::open("hello.txt");
    match f {
        Ok(file) => file,
        Err(error) => panic!("Problem opening the file: {:?}", error),
    };

    let _f = File::open("hello.txt").unwrap();//unwrap returns the Ok value inside the Ok variant
    //unwrap cannot define the type of the error so it will panic if it is Err

    let _f = File::open("hello.txt").expect("Failed to open hello.txt");//expect is similar to unwrap but it allows us to choose the panic message

    //matching on different errors
    let f = File::open("hello.txt");
    let _f = match f {
        Ok(file) => file,
        Err(error) => match error.kind() {
            ErrorKind::NotFound => match File::create("hello.txt") {
                Ok(fc) => fc,
                Err(error) => panic!("Problem creating the file: {:?}", error),
            },
            other_error => panic!("Problem opening the file: {:?}", other_error),
        },
    };

    let _f = File::open("hello.txt").unwrap_or_else(|error| {//unwrap_or_else takes a closure
        if error.kind() == ErrorKind::NotFound {
            File::create("hello.txt").unwrap_or_else(|error| {
                panic!("Problem creating the file: {:?}", error);
            })
        } else {
            panic!("Problem opening the file: {:?}", error);
        }
    });

    //propagating errors
    let _ = read_username_from_file();

    //shortcut for propagating errors: the ? operator
    let _ = _read_username_from_file();
    //the ? operator can only be used in functions that return Result

    //translating errors from one type into another using From trait
    //when implementing the From trait, the standard library provides a generic
    //implementation of From for any type T that implements the Into trait
    //this is useful when we want to return a specific error type but the function
    //we are calling returns a generic error type

    //panic! vs Result

    let _home: IpAddr = "127.0.0.1"
        .parse()
        .expect("Hardcoded IP address should be valid");

    //create a custom type for the error
    let guess = Guess::new(99);//the constructor will check if the value is valid
    println!("Guess value: {}", guess.value());

    //use ? in main
    let _greeting_file = File::open("hello.txt")?;
    Ok(()) //fn main() -> Result<(), Box> 
}

fn read_username_from_file() -> Result<String, io::Error> {
    let f = File::open("hello.txt");
    let mut f = match f {
        Ok(file) => file,
        Err(e) => return Err(e),//early return
    };

    let mut s = String::new();

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

pub struct Guess {
    value: i32,
}

impl Guess {
    pub fn new(value: i32) -> Guess {
        if value < 1 || value > 100 {
            panic!("Guess value must be between 1 and 100, got {}.", value);
        }

        Guess { value }
    }

    pub fn value(&self) -> i32 {//we need to return a reference to the value
        self.value
    }
}

fn _read_username_from_file() -> Result<String, io::Error> {
    let mut f = File::open("hello.txt")?;//? can only be used in functions that return Result
    let mut s = String::new();
    f.read_to_string(&mut s)?;//? can only be used in functions that return Result
    Ok(s)
}   //? can be chained to simplify the code