我有一个函数需要递归地传递一个闭包参数
use std::cell::RefCell;
use std::rc::Rc;
pub struct TreeNode {
val: i32,
left: Option<Rc<RefCell<TreeNode>>>,
right: Option<Rc<RefCell<TreeNode>>>,
}
pub fn pre_order<F>(root: Option<Rc<RefCell<TreeNode>>>, f: F)
where
F: FnOnce(i32) -> (),
{
helper(&root, f);
fn helper<F>(root: &Option<Rc<RefCell<TreeNode>>>, f: F)
where
F: FnOnce(i32),
{
match root {
Some(node) => {
f(node.borrow().val);
helper(&node.borrow().left, f);
helper(&node.borrow().right, f);
}
None => return,
}
}
}
这个不可行:
error[E0382]: use of moved value: `f`
--> src/lib.rs:23:45
|
22 | f(node.borrow().val);
| - value moved here
23 | helper(&node.borrow().left, f);
| ^ value used here after move
|
= note: move occurs because `f` has type `F`, which does not implement the `Copy` trait
error[E0382]: use of moved value: `f`
--> src/lib.rs:24:46
|
23 | helper(&node.borrow().left, f);
| - value moved here
24 | helper(&node.borrow().right, f);
| ^ value used here after move
|
= note: move occurs because `f` has type `F`, which does not implement the `Copy` trait
如果我尝试将
f: F
的类型更改为f: &F
,我会收到编译器错误。error[E0507]: cannot move out of borrowed content
--> src/lib.rs:22:17
|
22 | f(node.borrow().val);
| ^ cannot move out of borrowed content
我该怎样解决这个问题?
我是这样调用函数的:
let mut node = TreeNode::new(15);
node.left = Some(Rc::new(RefCell::new(TreeNode::new(9))));
let node_option = Some(Rc::new(RefCell::new(node)));
pre_order(node_option, |x| {
println!("{:?}", x);
});
F:FnOnce()
的类型为FnOnce()
,这是有原因的吗?如果将其更改为类型Fn()
并使helper()
接受&F
,它将起作用。F:FnOnce()
使闭包F
按值移动到调用中,因此只能使用一次。 - vikram2784