closures - 为什么 `take_while` 的闭包通过引用来获取它的参数？

Question

这是来自 Rust by Example 的示例:

fn is_odd(n: u32) -> bool {
    n % 2 == 1
}

fn main() {
    println!("Find the sum of all the squared odd numbers under 1000");
    let upper = 1000;

    // Functional approach
    let sum_of_squared_odd_numbers: u32 =
        (0..).map(|n| n * n)             // All natural numbers squared
             .take_while(|&n| n < upper) // Below upper limit
             .filter(|n| is_odd(*n))     // That are odd
             .fold(0, |sum, i| sum + i); // Sum them
    println!("functional style: {}", sum_of_squared_odd_numbers);
}

为什么 take_while 的闭包通过引用获取其参数，而所有其他闭包通过值获取？

Answer 1

执行Iterator::take_while很有启发性:

fn next(&mut self) -> Option<I::Item> {
    if self.flag {
        None
    } else {
        self.iter.next().and_then(|x| {
            if (self.predicate)(&x) {
                Some(x)
            } else {
                self.flag = true;
                None
            }
        })
    }
}

如果从底层迭代器返回的值直接传递给谓词，那么该值的所有权也将转移。调用谓词后，如果谓词为真，则不再有值可从 TakeWhile 适配器返回!