rust - 打印树 — 尝试访问字段，但未找到具有该名称的字段

Question

我正在尝试编写我的第一个 Rust 程序。我想在屏幕上打印一棵简单的树，但我无法访问 value 属性，它说

Error 1 attempted access of field value on type Node, but no field with that name was found c:\users\zhukovskiy\documents\visual studio 2013\Projects\rust_application1\rust_application1\src\main.rs 21 20 rust_application1

use std::io;

enum Node {
    Branch { value: i32, next: *const Node },
    Leaf { value: i32 }
}

fn main() {
    let leaf = Node::Leaf { value: 15 };
    let branch = Node::Branch { value: 10, next: &leaf };
    let root = Node::Branch { value: 50, next: &branch };

    let current = root;
    loop {
        match current {
            Node::Branch => { println!("{}", current.value); current = current.next; },
            Node::Leaf => { println!("{}", current.value); break; }, 
        }
    }
}

Answer 1

仅仅因为 Node 的两个变体都有一个 value 字段，并不意味着您可以直接访问它。您可以通过匹配值来获取它(这些是等价的):

let value = match leaf {
    Node::Branch { value, .. } => value,
    Node::Leaf { value } => value,
};

let value = match leaf {
    Node::Branch { value, .. } | Node::Leaf { value } => value,
};

但是如果你要经常这样做，你可能想要添加一个方法:

impl Node {
    pub fn get_value(&self) -> i32 {
        match self {
            &Node::Branch { value, .. } => value,
            &Node::Leaf { value } => value,
        }
    }
}

...然后您可以像这样使用它:

let value = leaf.get_value();