英文:
Extracting an attribute from a Box inside an enum
问题
#[derive(Clone, Debug)]
pub struct Node<T> {
value: T,
left: Addr<T>,
right: Addr<T>,
}
#[derive(Clone, Debug)]
pub enum Addr<T> {
Addr(Box<Node<T>>),
None,
}
impl<T> Addr<T> {
pub fn is_none(&self) -> bool {
matches!(&*self, Addr::None)
}
}
impl<T> Node<T> {
fn node_insert(mut n: Addr<T>, val: T) {
let nd = Box::new(Node {
value: val,
left: Addr::None,
right: Addr::None,
});
if n.is_none() {
n = Addr::Addr(nd);
} else {
// You should access the left attribute like this:
if let Addr::Addr(node) = n {
// Access the left attribute of the Node struct
node.left = node_insert(node.left, val);
}
}
}
}
在 node_insert 函数中，要访问 Node 结构的 left 属性，您应该像上面的示例代码中那样进行访问，使用 if let 来将 n 解构为 Addr::Addr(node)，然后您可以访问 node 的 left 属性并调用 node_insert 递归。
英文:
#[derive(Clone, Debug)]
pub struct Node<T> {
value: T,
left: Addr<T>,
right: Addr<T>,
}
#[derive(Clone, Debug)]
pub enum Addr<T> {
Addr(Box<Node<T>>),
None,
}
impl<T> Addr<T> {
pub fn is_none(&self) -> bool {
matches!(&*self, Addr::None)
}
}
impl<T> Node<T> {
fn node_insert(mut n: Addr<T>, val: T) {
let nd = Box::new(Node {
value: val,
left: Addr::None,
right: Addr::None,
});
if n.is_none() {
n = Addr::Addr(nd);
} else {
node_insert(n.left, val);
}
}
}
I'm trying to implement a simple binary tree, and in the node_insert function I can't figure out how to access the left attribute of the Node struct when calling node_insert recursively.
Nothing I've done worked and I couldn't find any answer to help with it, probably because its somehow specific.
答案1
得分: 0
使用模式匹配：
fn node_insert(mut n: Addr<T>, val: T) {
match n {
Addr::None => {
n = Addr::Addr(Box::new(Node {
value: val,
left: Addr::None,
right: Addr::None,
}))
}
Addr::Addr(n) => Self::node_insert(n.left, val),
}
}
然而，编译器正确地发出警告：
warning: value assigned to `n` is never read
--> src/lib.rs:23:13
|
23 | n = Addr::Addr(Box::new(Node {
| ^
|
= help: maybe it is overwritten before being read?
= note: `#[warn(unused_assignments)]` on by default
由于您拥有 n，您正在写入一个局部变量而不是树。相反，您应该获取 &mut Addr：
fn node_insert(n: &mut Addr<T>, val: T) {
match n {
Addr::None => {
*n = Addr::Addr(Box::new(Node {
value: val,
left: Addr::None,
right: Addr::None,
}))
}
Addr::Addr(n) => Self::node_insert(&mut n.left, val),
}
}
英文:
Use pattern matching:
fn node_insert(mut n: Addr<T>, val: T) {
match n {
Addr::None => {
n = Addr::Addr(Box::new(Node {
value: val,
left: Addr::None,
right: Addr::None,
}))
}
Addr::Addr(n) => Self::node_insert(n.left, val),
}
}
However, the compiler rightfully warns:
warning: value assigned to `n` is never read
--> src/lib.rs:23:13
|
23 | n = Addr::Addr(Box::new(Node {
| ^
|
= help: maybe it is overwritten before being read?
= note: `#[warn(unused_assignments)]` on by default
Since you took ownership of n, you're writing to a local variable and not to the tree. Instead, you should take &mut Addr:
fn node_insert(n: &mut Addr<T>, val: T) {
match n {
Addr::None => {
*n = Addr::Addr(Box::new(Node {
value: val,
left: Addr::None,
right: Addr::None,
}))
}
Addr::Addr(n) => Self::node_insert(&mut n.left, val),
}
}
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