尝试转换

? 的有效展开比前面介绍的内容略微复杂一些：

  1. expression?

效果等同于

  1. match expression {
  2.     Ok(value) => value,
  3.     Err(err)  => return Err(From::from(err)),
  4. }

The From::from call here means we attempt to convert the error type to the type returned by the function. This makes it easy to encapsulate errors into higher-level errors.

示例

  1. use std::error::Error;
  2. use std::fmt::{self, Display, Formatter};
  3. use std::fs::File;
  4. use std::io::{self, Read};
  6. #[derive(Debug)]
  7. enum ReadUsernameError {
  8.     IoError(io::Error),
  9.     EmptyUsername(String),
  10. }
  12. impl Error for ReadUsernameError {}
  14. impl Display for ReadUsernameError {
  15.     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
  16.         match self {
  17.             Self::IoError(e) => write!(f, "IO error: {e}"),
  18.             Self::EmptyUsername(path) => write!(f, "Found no username in {path}"),
  19.         }
  20.     }
  21. }
  23. impl From<io::Error> for ReadUsernameError {
  24.     fn from(err: io::Error) -> Self {
  25.         Self::IoError(err)
  26.     }
  27. }
  29. fn read_username(path: &str) -> Result<String, ReadUsernameError> {
  30.     let mut username = String::with_capacity(100);
  31.     File::open(path)?.read_to_string(&mut username)?;
  32.     if username.is_empty() {
  33.         return Err(ReadUsernameError::EmptyUsername(String::from(path)));
  34.     }
  35.     Ok(username)
  36. }
  38. fn main() {
  39.     //fs::write("config.dat", "").unwrap();
  40.     let username = read_username("config.dat");
  41.     println!("username or error: {username:?}");
  42. }

This slide should take about 5 minutes.

The ? operator must return a value compatible with the return type of the function. For Result, it means that the error types have to be compatible. A function that returns Result<T, ErrorOuter> can only use ? on a value of type Result<U, ErrorInner> if ErrorOuter and ErrorInner are the same type or if ErrorOuter implements From<ErrorInner>.

From 实现的常见替代方案是 Result::map_err，尤其是只在一个位置进行转换时。

There is no compatibility requirement for Option. A function returning Option<T> can use the ? operator on Option<U> for arbitrary T and U types.

A function that returns Result cannot use ? on Option and vice versa. However, Option::ok_or converts Option to Result whereas Result::ok turns Result into Option.