如何在不移动或不需要复制的情况下在闭包中使用 FnMut 参数？

Question

我仍然对 Fn-FnMut-FnOnce 与其他特征的关系感到困惑。我想从以下遍历树结构的函数中消除 Copy 约束。

pub fn for_each<F>(&mut self, mut f: F)
where
    F: FnMut(&mut Tree<T>) + Copy,
{
    self.children.iter_mut().for_each(|c| c.for_each(f));
    f(self);
}

原因是我试图传递一个将外部变量修改为 for_each 的闭包，而 Copy 阻止了这种情况 (E0277)。但是，当我去掉 Copy 时，我收到以下错误消息：

error[E0507]: cannot move out of `f`, a captured variable in an `FnMut` closure
  --> src/tree.rs:34:58
   |
30 |     pub fn for_each<F>(&mut self, mut f: F)
   |                                   ----- captured outer variable
...
34 |         self.children.iter_mut().for_each(|c| c.for_each(f));
   |                                                          ^ move occurs because `f` has type `F`, which does not implement the `Copy` trait

error[E0382]: borrow of moved value: `f`
  --> src/tree.rs:35:9
   |
30 |     pub fn for_each<F>(&mut self, mut f: F)
   |                                   ----- move occurs because `f` has type `F`, which does not implement the `Copy` trait
...
34 |         self.children.iter_mut().for_each(|c| c.for_each(f));
   |                                           ---            - variable moved due to use in closure
   |                                           |
   |                                           value moved into closure here
35 |         f(self);
   |         ^ value borrowed here after move
   |
help: consider further restricting this bound
   |
32 |         F: FnMut(&mut Tree<T>) + Copy,
   |                                ^^^^^^

error: aborting due to 2 previous errors

我该如何解决这个问题？如果这样更容易，我也愿意将它变成一个迭代器。

Answer 1

原因是 for_each 取得了 f 的所有权。

这意味着，一旦您调用 c.for_each(f)，您将失去对 f 的所有权，因此您以后无法使用 f。为了解决这个问题，您可以将 for_each 函数更改为引用，这样您就可以像这样传递它

pub fn for_each<F>(&mut self, f: &mut F)
where
    F: FnMut(&mut Tree<T>),
{
    // Only a reference is passed into for_each here
    self.children.iter_mut().for_each(|c| c.for_each(f));
    // f can be used again, as reference is reborrowed implicitly
    f(self);
}

或者您也可以（如果可能的话）将呼叫移到 f 上一行

pub fn for_each<F>(&mut self, mut f: F)
where
    F: FnMut(&mut Tree<T>),
{
    // f is used, but ownership is kept
    f(self);
    // ownership of f can now be transferred, as there is no further use of it
    self.children.iter_mut().for_each(|c| c.for_each(f));
}

如何在不移动或不需要复制的情况下在闭包中使用 FnMut 参数？

How to use a FnMut argument in a closure without moving it or requiring Copy?

closures

rust