Rust- 错误处理

Rust approaches error handling with the aim of being explicit about possible error conditions. It separates the concerns of "normal" return values and "error" return values by using a specific set of types for each concern. Rust's primary tools for expressing these concepts are the Result and Option types, along with the unwrap method.

The Result type : This is an enum that represents either successful (Ok) or unsuccessful (Err) computation. Functions that can fail generally return a Result variant.

Here's a simple example:
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```
fn divide(numerator: f64, denominator: f64) -> Result<f64, &str> {
    if denominator == 0.0 {
        Err("Cannot divide by zero")
    } else {
        Ok(numerator / denominator)
    }
}
```
In this case, if you try to divide by zero, the function will return an error wrapped in the Err variant. Otherwise, it will return the division result wrapped in the Ok variant.
The Option type : This is another enum that represents a value that could be something (Some) or nothing (None). It's often used when you have a situation where a value could be absent or present.

For example:
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```
fn find_word_starting_with<'a, 'b>(sentence: &'a str, initial: &'b str) -> Option<&'a str> {
    for word in sentence.split_whitespace() {
        if word.starts_with(initial) {
            return Some(word);
        }
    }
    None
}
```
This function tries to find a word starting with a specific string in a sentence. If it finds such a word, it returns Some(word), otherwise, it returns None.
The unwrap method : Both Option and Result types have an unwrap method, which is used to get the value out. If the instance of Option is a Some variant, unwrap will return the value inside the Some. If the instance is a None, unwrap will panic.

Similarly, for Result, if the instance is an Ok, unwrap will return the value inside the Ok. If the instance is an Err, unwrap will panic.
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```
let maybe_number: Option<i32> = Some(42);
println!("{}", maybe_number.unwrap()); // Prints 42

let definitely_not_a_number: Option<i32> = None;
println!("{}", definitely_not_a_number.unwrap()); // This will panic
```
It's generally advised to avoid using unwrap in most situations, as it can lead to your program crashing. Instead, you should handle errors appropriately using pattern matching or methods like unwrap_or, unwrap_or_else, expect, etc., which allow you to provide alternative values or actions in case of an error.
The ? operator : This is a convenient way to propagate errors upwards in the call stack. When you use ? on a Result value, it does pretty much the same as a match expression would do: if the value is Ok, it unwraps it; if it's Err, it returns it from the current function. This allows for more concise error handling.

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fn main() {
    match foo() {
        Ok(_) => println!("Success"),
        Err(e) => println!("Error: {}", e),
    }
}

fn foo() -> Result<(), &'static str> {
    bar()?;
    Ok(())
}

fn bar() -> Result<(), &'static str> {
    Err("Some error")
}

In the foo function, bar()? will return the error from bar, thus ending the foo function early.

These are the basic building blocks of error handling in Rust. Rust also provides advanced features like creating your own error types, using the From trait for flexible error conversions, etc., but those are more advanced topics.

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use std::fs::File;

fn main() {
    panic!("出错了");
    println!("Hello Rust");

    // panic 程序立即退出，退出的时候调用者抛出退出原因。

    // 数组越界
    let v = vec!["Rust", "Programming", "Language"];
    println!("{}", v[5]); // thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 5'

    // 打开文件出错
    let f = File::open("abc.txt");
    println!("{:?}", f); // Err(Os { code: 2, kind: NotFound, message: "系统找不到指定的文件。" })

    // unwrap expect
    let result = is_even(6).unwrap();
    println!("结果{}", result);

    let result = is_even(11).unwrap();
    println!("结果{}", result);

    // unwrap()是Result<T, E>的方法，实例上调用此方法时，如果是Ok，就返回Ok中的对象
    // 如果是Err枚举，在运行时会panic

    // expect()接收msg::&str作为参数，可以自定义报错信息。
}

fn is_even(no: i32) -> Result<bool, String> {
    return if no % 2 == 0 {
        Ok(true)
    } else {
        Err("输入的值不是偶数".to_string())
    };
}