rust - 从迭代器返回切片时无法推断出合适的生命周期

Question

我有一个 Vec<Point>用一个简单的 struct Point {x: f32, y: f32, z: f32} .我的向量在 3D 中代表数十万条线(实际上可能是 Vec<Vec<Point>>)，因此我跟踪所有线的开始/结束。

pub struct Streamlines {
    lengths: Vec<usize>,
    offsets: Vec<usize>,  // cumulative sum of lengths
    data: Vec<Point>,
}

我想为它创建一个非消耗迭代器，像这样使用:

for streamline in &streamlines {
    for point in &streamline {
        println!("{} {} {}", point.x, point.y, point.z);
    }
    println!("")
}

我找到了 How to implement Iterator and IntoIterator for a simple struct?并开始 copyi-err，改编 :)

impl IntoIterator for Streamlines {
    type Item = &[Point];
    type IntoIter = StreamlinesIterator;

    fn into_iter(self) -> Self::IntoIter {
        StreamlinesIterator {
            streamlines: self,
            it_idx: 0
        }
    }
}

struct StreamlinesIterator {
    streamlines: &Streamlines,
    it_idx: usize
}

impl Iterator for StreamlinesIterator {
    type Item = &[Point];

    fn next(&mut self) -> Option<&[Point]> {
        if self.it_idx < self.streamlines.lengths.len() {
            let start = self.streamlines.offsets[self.it_idx];
            self.it_idx += 1;
            let end = self.streamlines.offsets[self.it_idx];

            Some(self.streamlines.data[start..end])
        }
        else {
            None
        }
    }
}

我使用切片是因为我只想返回向量的一部分，然后我添加了生命周期因为它是必需的，但现在我有这个错误 cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements

事实上，我并不知道我在用该死的 <'a> 做什么.

Answer 1

cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements

那是因为你没有正确实现 Iterator并有这样的东西:

impl<'a> Iterator for StreamlinesIterator<'a> {
    type Item = &'a [Point];

    fn next(&mut self) -> Option<&[Point]> { /* ... */ }

    // ...
}

由于生命周期推断，这等同于:

impl<'a> Iterator for StreamlinesIterator<'a> {
    type Item = &'a [Point];

    fn next<'b>(&'b mut self) -> Option<&'b [Point]> { /* ... */ }

    // ...
}

这试图返回一个与迭代器一样长的引用，which you cannot do .

如果您正确实现 Iterator , 它有效:

impl<'a> Iterator for StreamlinesIterator<'a> {
    type Item = &'a [Point];

    fn next(&mut self) -> Option<&'a [Point]> { /* ... */ }

    // Even better:   
    fn next(&mut self) -> Option<Self::Item> { /* ... */ }

    // ...
}

I don't actually know what I'm doing with the damn <'a>.

你应该回去重新阅读The Rust Programming Language, second edition .当您有具体问题时，Stack Overflow、IRC、用户论坛都将等待。