HashMap inside struct error: cannot move out of xxx which is behind a shared reference

Question

I have the following Rust structure which has a HashMap to children structures.

use std::collections::HashMap;

#[derive(Debug)]
struct Node {
    children: HashMap<i32, Node>,
}

impl Node {
    fn no_children(&self) -> usize {
        if self.children.is_empty() {
            1
        } else {
            1 + self
                .children
                .into_iter()
                .map(|(_, child)| child.no_children())
                .sum::<usize>()
        }
    }
}

I implemented no_children(&self) to find the total number of nodes. However, under self.children, Rust highlights an error because:

error[E0507]: cannot move out of `self.children` which is behind a shared reference
  --> src/lib.rs:13:17
   |
13 |               1 + self
   |  _________________^
14 | |                 .children
   | |_________________________^ move occurs because `self.children` has type `std::collections::HashMap<i32, Node>`, which does not implement the `Copy` trait

I am not sure what is missing. I have tried adding &self.children... but still got the same error.

As a side note, your `no_immediate_children` counts **all** the children recursively, not just the immediate ones… — Jmb, Oct 16 '19 at 06:17

score 2 · Accepted Answer · edited Oct 16 '19 at 14:47

The issue is that .into_iter(self) needs to take ownership of the HashMap, but in no_immediate_children(&self) the HashMap is behind a reference -> i.e. &self instead of self;

You can work around that in two ways, depending on what you want to achieve:

If you want to consume the elements of the hash map and leave it empty after the method invocation:
- Change the receiver to &mut self
- Use .drain() instead of .into_iter():
```
self.children.drain().map(|(_, mut v)| v.no_immediate_children()).sum::<usize>() + 1
```
If you just want to get the sum, but do not want to modify the HashMap:
- Use .iter() instead of .into_iter():
```
self.children.iter().map(|(_k, v)| v.no_immediate_children()).sum::<usize>() + 1
```
You want to consume the whole Node chain:
- Change the method signature to fn no_immediate_children(self) and use .into_iter() as it is.

HashMap inside struct error: cannot move out of xxx which is behind a shared reference

1 Answers1