動態錯誤型別

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.

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read_count 函式可以傳回 std::io::Error (透過檔案作業) 或 std::num::ParseIntError (透過 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.

定義自訂錯誤型別時,請務必實作 std::error::Error 特徵,這樣才能裝箱。不過,如果您需要支援 no_std 屬性,請留意 std::error::Error 特徵目前僅與每夜版中的 no_std 相容。