我正在尝试实现一个二叉树。我希望节点数据是分开的,因为有很多不同的方法可以实现这一点,而树上的算法应该是通用的并且独立于数据的存储方式。
但我在借用检查器方面遇到了一个奇怪的问题。基本上,当我切换时impl<TValue> Display for dyn Tree<TValue>
with impl<TValue> Display for TreeNode<TValue>
,问题就消失了。但我不知道为什么。为什么该 Trait 会导致此问题?
我的代码是这样的:
use std::fmt::{Display, Formatter};
struct TreeNode<TValue> {
value: TValue,
left: Option<Box<TreeNode<TValue>>>,
right: Option<Box<TreeNode<TValue>>>,
}
trait Tree<TValue> {
fn value(&self) -> &TValue;
fn left(&self) -> Option<&dyn Tree<TValue>>;
fn right(&self) -> Option<&dyn Tree<TValue>>;
}
impl<TValue> Display for dyn Tree<TValue>
where
TValue: Display,
{
fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
f.write_str("(")?;
Display::fmt(self.value(), f)?;
f.write_str(", ")?;
match self.left() {
Some(ref x) => x.fmt(f)?,
None => f.write_str("None")?,
}
f.write_str(", ")?;
match self.right().as_ref() {
Some(x) => x.fmt(f)?,
None => f.write_str("None")?,
}
f.write_str(")")
}
}
impl<TValue> Tree<TValue> for TreeNode<TValue>
where
TValue: Display,
{
fn value(&self) -> &TValue {
&self.value
}
fn left(&self) -> Option<&dyn Tree<TValue>> {
self.left.as_ref().map(|x| &**x as &dyn Tree<TValue>)
}
fn right(&self) -> Option<&dyn Tree<TValue>> {
self.right.as_ref().map(|x| &**x as &dyn Tree<TValue>)
}
}
fn main() {
let tree = Box::new(TreeNode {
value: 1,
left: Some(Box::new(TreeNode {
value: 2,
left: None,
right: None,
})),
right: Some(Box::new(TreeNode {
value: 3,
left: None,
right: None,
})),
}) as Box<dyn Tree<i32>>;
println!("{}", tree);
}
编译器打印:
error[E0521]: borrowed data escapes outside of associated function
--> src\main.rs:24:15
|
19 | fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
| -----
| |
| `self` declared here, outside of the associated function body
| `self` is a reference that is only valid in the associated function body
| let's call the lifetime of this reference `'1`
...
24 | match self.left() {
| ^^^^^^^^^^^
| |
| `self` escapes the associated function body here
| argument requires that `'1` must outlive `'static`
对我来说,这毫无意义。函数体中没有任何内容捕获该值并尝试使其超出该函数的范围。这是借用检查器的限制吗?