generics - 在当前范围内找不到类型 ::Type 的名为 len 的方法

Question

我想通知 VecLengthDirector AbstractVecLengthBuider 的关联类型总是 [i32; n] (n ∈ N)。我写了下面的代码:

struct VecLengthDirector<T> {
    builder: T,
}

impl<T> VecLengthDirector<T>
where
    T: AbstractVecLengthBuider,
{
    fn construct(&self) -> f64 {
        let s = self.builder.get_start_point();
        let e = self.builder.get_end_point();

        let mut sum: i32 = 0;
        for i in 0..s.len() {
            sum += (s[i] - e[i]).pow(2);
        }

        (sum as f64).sqrt()
    }
}

trait AbstractVecLengthBuider {
    type PointType;
    fn add_start_point(&mut self, point: Self::PointType);
    fn get_start_point(&self) -> Self::PointType;
    fn add_end_point(&mut self, point: Self::PointType);
    fn get_end_point(&self) -> Self::PointType;
}

并报告错误。

error[E0599]: no method named `len` found for type `<T as AbstractVecLengthBuider>::PointType` in the current scope
  --> src/main.rs:14:23
   |
14 |         for i in 0..s.len() {
   |                       ^^^

error[E0608]: cannot index into a value of type `<T as AbstractVecLengthBuider>::PointType`
  --> src/main.rs:15:21
   |
15 |             sum += (s[i] - e[i]).pow(2);
   |                     ^^^^

error[E0608]: cannot index into a value of type `<T as AbstractVecLengthBuider>::PointType`
  --> src/main.rs:15:28
   |
15 |             sum += (s[i] - e[i]).pow(2);
   |                            ^^^^

Answer 1

快速修复

您需要指定 PointType 的种类.例如T: AbstractVecLengthBuider<PointType = [i32]>> .但是，[i32] 的大小在编译时未知，因此您可以将其替换为 Vec<i32> : T: AbstractVecLengthBuider<PointType = Vec<i32>> .

如果还是想泛型，可以约束PointType可借为[i32] :

impl<T, P> VecLengthDirector<T>
where
    T: AbstractVecLengthBuider<PointType = P>,
    P: ::std::borrow::Borrow<[i32]>,
{
    fn construct(&self) -> f64 {
        let s = self.builder.get_start_point().borrow();
        let e = self.builder.get_end_point().borrow();
        // ...
    }
}

这不是惯用的 Rust。

惯用的 Rust 方式

您的循环可以重写为更惯用的 Rust:

s.iter()
    .zip(e.iter())
    .map(|(se, ee)| (se - ee).pow(2) as f64)
    .sum()

那么你只需要约束PointType可迭代 i32 :

impl<T, P> VecLengthDirector<T>
where
    T: AbstractVecLengthBuider<PointType = P>,
    P: ::std::iter::IntoIterator<Item = i32>,
{
    fn construct(&self) -> f64 {
        let s = self.builder.get_start_point();
        let e = self.builder.get_end_point();

        s.into_iter()
            .zip(e.into_iter())
            .map(|(se, ee)| (se - ee).pow(2) as f64)
            .sum::<f64>()
            .sqrt()
    }
}