rust - What happens in Rust using "match" if nothing matches?

Question

I'm learning Rust and it looks very interesting. I'm not yet very familiar with "match", but it looks to be fairly integral. I was using the following code (below) to convert String to i64 which included the commented-out line "None" in place of the next line "_". I wondered what happened in the event of a non-match without underscore or whether "None" may be a catch-all. The calling-code requires a positive i64, so negative results in an invalid input (in this case). I'm not sure if it's possible to indicate an error in a return using this example other than perhaps using a struct.

1. Without underscore as a match item, will "None" catch all, and can it be used instead of underscore?

2. Is it possible to return an error in a function like this without using a struct as the return value?

3. In general, is it possible for a non-match using "match" and if so, what happens?

4. In the example below, is there any difference between using underscore and using "None"?

Example code :

fn fParseI64(sVal: &str) -> i64 {
    match from_str::<i64>(sVal) {
        Some(iVal) => iVal,
//        None => -1
     _ => -1    
    }
}

Answer 1

1. 没有下划线作为匹配项，“None”会全部捕获吗，可以用它来代替下划线吗？

在这种情况下，是的。Option<T>:Some<T>或.只有 2 种情况None。您已经匹配Some所有值的大小写，None是唯一剩下的大小写，因此 None行为与_.

 2. 这样的函数是否可以在不使用结构体作为返回值的情况下返回错误？

我建议您阅读这份详细指南以了解处理错误的所有可能方法 - http://static.rust-lang.org/doc/master/tutorial-conditions.html

使用选项和结果将要求您返回一个结构。

 3. 一般来说，是否可以使用“匹配”来进行不匹配，如果可以，会发生什么？

match如果不能保证处理所有可能的情况，Rust 将不会编译 a 。_如果您不匹配所有可能性，则需要有一个子句。

 4. 在下面的例子中，使用下划线和使用“None”有什么区别吗？

不。见答案#1。

以下是编写函数的多种方法中的 3 种：

// Returns -1 on wrong input.
fn alt_1(s: &str) -> i64 {
  match from_str::<i64>(s) {
    Some(i) => if i >= 0 {
      i
    } else {
      -1
    },
    None => -1
  }
}

// Returns None on invalid input.
fn alt_2(s: &str) -> Option<i64> {
  match from_str::<i64>(s) {
    Some(i) => if i >= 0 {
      Some(i)
    } else {
      None
    },
    None => None
  }
}

// Returns a nice message describing the problem with the input, if any.
fn alt_3(s: &str) -> Result<i64, ~str> {
  match from_str::<i64>(s) {
    Some(i) => if i >= 0 {
      Ok(i)
    } else {
      Err(~"It's less than 0!")
    },
    None => Err(~"It's not even a valid integer!")
  }
}

fn main() {
  println!("{:?}", alt_1("123"));
  println!("{:?}", alt_1("-123"));
  println!("{:?}", alt_1("abc"));

  println!("{:?}", alt_2("123"));
  println!("{:?}", alt_2("-123"));
  println!("{:?}", alt_2("abc"));

  println!("{:?}", alt_3("123"));
  println!("{:?}", alt_3("-123"));
  println!("{:?}", alt_3("abc"));
}

输出：

123i64
-1i64
-1i64
Some(123i64)
None
None
Ok(123i64)
Err(~"It's less than 0!")
Err(~"It's not even a valid integer!")