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

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
– Jmb, Commented Oct 16, 2019 at 6:17

Shepmaster · Accepted Answer · 2019-10-16 14:47:22Z

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.

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