动态错误类型

Sometimes we want to allow any type of error to be returned without writing our own enum covering all the different possibilities. The std::error::Error trait makes it easy to create a trait object that can contain any error.

1. use std::error::Error;
2. use std::fs;
3. use std::io::Read;

5. fn read_count(path: &str) -> Result<i32, Box<dyn Error>> {
6. let mut count_str = String::new();
7. fs::File::open(path)?.read_to_string(&mut count_str)?;
8. let count: i32 = count_str.parse()?;
9. Ok(count)
10. }

12. fn main() {
13. fs::write("count.dat", "1i3").unwrap();
14. match read_count("count.dat") {
15. Ok(count) => println!("Count: {count}"),
16. Err(err) => println!("Error: {err}"),
17. }
18. }

This slide should take about 5 minutes.

read_count 函数可以返回 std::io::Error（通过文件操作）或 std::num::ParseIntError（通过 String::parse）。

Boxing errors saves on code, but gives up the ability to cleanly handle different error cases differently in the program. As such it’s generally not a good idea to use Box<dyn Error> in the public API of a library, but it can be a good option in a program where you just want to display the error message somewhere.

Make sure to implement the std::error::Error trait when defining a custom error type so it can be boxed. But if you need to support the no_std attribute, keep in mind that the std::error::Error trait is currently compatible with no_std in nightly only.