如何使用自定义结构实现错误

Question

我一直在关注here。

它包含一个看似相关的简化示例，但我的实现并不适合。这里的错误是：returns a reference to data owned by the current function。该示例建议使用结构的特定属性，即String。这很好，因为编译器可以知道该结构将持续多长时间。但是如果我们需要在运行时构建字符串呢？我该如何完成这项工作？

#[derive(Clone, Debug)]
pub enum ReadFailure {
    MissingFile(String),
    BadData(String),
    // ...
}

impl fmt::Display for ReadFailure {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            ReadFailure::MissingFile(path) => write!(f, "WARNING - File not read (file does not exist): {}", path),
            ReadFailure::BadData(path) => write!(f, "WARNING - File cannot be read (file has bad data): {}", path),
            // ...
        }
    }
}

/// Implements an error for failure to read with a message
#[derive(Clone, Debug)]
pub struct ReadError {
    /// Failure condition
    error: ReadFailure,
}

impl fmt::Display for ReadError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.error)
    }
}

impl Error for ReadError {
    fn description(&self) -> &str {
        let s = format!("{}", self.error);
        &s
    }
}

Answer 1

但是如果我们需要在运行时构建字符串呢？我该如何完成这项工作？

只需在创建ReadError 时在运行时构建它，这是一个示例：

use std::fmt;
use std::error::Error;

#[derive(Clone, Debug)]
pub enum ReadFailure {
    MissingFile(String),
    BadData(String),
}

impl fmt::Display for ReadFailure {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            ReadFailure::MissingFile(path) => write!(f, "WARNING - File not read (file does not exist): {}", path),
            ReadFailure::BadData(path) => write!(f, "WARNING - File cannot be read (file has bad data): {}", path),
        }
    }
}

#[derive(Clone, Debug)]
pub struct ReadError {
    error: ReadFailure,
    description: String, // NEW
}

// NEW
impl ReadError {
    fn new(error: ReadFailure) -> Self {
        ReadError {
            description: error.to_string(),
            error,
        }
    }
}

impl fmt::Display for ReadError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.error)
    }
}

impl Error for ReadError {
    fn description(&self) -> &str {
        &self.description
    }
}

fn main() {
    // example
    let error = ReadError::new(ReadFailure::MissingFile("./whatever.txt".to_string()));
    println!("{}", error.description());
}

playground

从description 的函数签名中可以清楚地看出，它只是假设是一个只读方法，它返回对错误中某个字段的引用。

无论如何，正如@justinas 在 cmets 中指出的那样，该方法已被弃用，文档建议您实施 Display 特征，而不是您已经在做的。