Borrow value from HashMap? - rust

Question

Still in my learning of rust while trying to make an ECS, I ecountered this issue :
I'm having a hashmap as a component table, where the keys are components ids and values are components arrays. Components array is a wrapper around another hashmap, where keys are entity ids and values are the components (so components arrays are generics with the type of component).
Here is the structure :
pub struct ComponentTable {
components: HashMap<u32, Box<dyn ComponentArrayOnly>>,
}
pub struct ComponentArray<C> {
comp_array: HashMap<u64, C>,
}
impl<C> ComponentArray<C> {
pub fn new() -> ComponentArray<C> {
return ComponentArray {
comp_array: HashMap::new(),
};
}
}
trait ComponentArrayOnly {
fn add_component(self, entity_id: u64);
}
impl<C: Component> ComponentArrayOnly for ComponentArray<C> {
fn add_component(mut self, entity_id: u64) {
if !self.comp_array.contains_key(&entity_id) {
self.comp_array.insert(entity_id, C::new());
}
}
}
(the trait component array only is to hide the generics in the component table)
Now, when I try to add components from the component table, which should check if the component array exists (if not, add it) then call the add_component method on the array, I'm getting an error :
impl ComponentTable {
pub fn new() -> ComponentTable {
return ComponentTable {
components: HashMap::new(),
};
}
pub fn add_component_to<C: Component + 'static>(mut self, entity: u64) {
if !self.components.contains_key(&C::id()) {
// add a new component array
self.components.insert(C::id(), Box::new(ComponentArray::<C>::new()));
}
// add the component at the entity id in the component array
match self.components.get(&C::id()) {
None => println!("Unable to add component : component array was not created !"),
Some(c_arr) => c_arr.add_component(entity),
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// error[E0507]: cannot move out of `**c_arr` which is behind a shared reference
// move occurs because `**c_arr` has type `dyn ComponentArrayOnly`, which does not implement the `Copy` trait
}
}
}
I don't really understand what is going on. I don't want c_arr to be a shared reference, I would like to borrow it to modify it with add_component then release it back in the hashmap.
I looked the HashMap documentation but couldn't find anything to achieve this.
Am I missing something? or is just poor design choices?

Answer 1

As mentionned in the comments, the issue was that I was getting the ComponentArray by value and not reference when adding components to it.
So I needed to slightly change the implementation of add_component on the component array :
trait ComponentArrayOnly {
fn add_component(&mut self, entity_id: u64);
}
impl<C: Component> ComponentArrayOnly for ComponentArray<C> {
fn add_component(&mut self, entity_id: u64) {
if !self.comp_array.contains_key(&entity_id) {
self.comp_array.insert(entity_id, C::new());
}
}
}
And borrow the component array as a mut from the hashmap :
match self.components.get_mut(&C::id()) {
None => println!("Unable to add component : component array was not created !"),
Some(c_arr) => c_arr.add_component(entity),
}