从struct获取关联类型的任何方法？

Question

这段代码:

use std::collections::HashMap;

struct MyNode;
struct MyEdge;

struct Graph<N, E> {
    h: HashMap<N, Vec<E>>,
}

type MyGraph = Graph<MyNode, MyEdge>;

fn main() {

    let x: MyGraph::N;//XXX

    println!("Results:")

}

无法编译错误:

error[E0223]: ambiguous associated type
  --> /home/xxx/.emacs.d/rust-playground/at-2017-07-26-164119/snippet.rs:21:12
   |
21 |     let x: MyGraph::N;
   |            ^^^^^^^^^^ ambiguous associated type
   |
   = note: specify the type using the syntax `<Graph<MyNode, MyEdge> as Trait>::N`

有没有办法从中获取N类型Graph<MyNode, MyEdge>？

我创建了一个type =不复制节点类型定义的别名(),所以在XXX我可以写的标记点上它会很棒let x: MyNode但是let x: expression with MyGraph as argument.

Answer 1

代码中没有关联的类型参数.关联类型仅适用于特征,允许您编写以下内容:

trait Graph {
    type Node;
    type Edge;
}

特别是,你在struct(N和E)中有普通的类型参数.如果没有共同特征,则必须手动解析类型.无论如何,这里做起来并不复杂.

struct GraphImpl<N, E> {
    h: HashMap<N, Vec<E>>,
}

type MyGraph = GraphImpl<MyNode, MyEdge>;

let x: MyNode;

但是,如果你这样做实现这一Graph特质对你的结构:

impl<N, E> Graph for GraphImpl<N, E> {
    type Node = N;
    type Edge = E;
}

然后,您可以检索此问题中显示的关联类型:

let x: <MyGraph as Graph>::Node;

操场