How to get the value of a Struct without named field?

help
1

Hi,

I am using Legion and it has a struct defined like below,
pub struct Entity(NonZeroU64);

When I visualize the entity in the debug window, it shows a value. However, I don't see a straight forward method to access it. Can you help in accessing the value?

I am expecting something like below,
let val: u64 = entity;

2

You can access in the same way you access something inside tuple - by using the my_entity.0 syntax.

See thee chapter on Tuple Index Expressions in the Rust Reference for more details.

1 Like
3

Thanks Michael. However, it is throwing an error field 0 of struct legion::Entity is private

4

Hi, there doesn't seem to be a method to access the inner value directly, probably by design:

An opaque identifier for an entity.

But it is repr(transparent) so you could transmute to a NonZeroU64 and get a u64 from there.
I wouldn't say it's a pretty solution :sweat_smile:

Someone might be able to help more if you explain what you want to do with the inner value.

5

Thanks @leudz. I have a function which is called for each entity. At the end of the function, I am inserting processed data into Sled. Since I want a unique key, I thought I will use the Entity.0 as the unique key so in other parts of the code, I can use this Entity.0 to fetch the processed data from Sled.

I directly tried sled.insert(entity, my_data) which didn't work as Sled is expecting &[u8]

6

If it is private, the library authors have deliberately chosen to not let users access that inner field.

Often this is done because the number is an internal implementation detail, and letting users read/modify it could cause them to write code that violates assumptions made by Legion.

I don't think we should be recommending to use transmute() to hack around privacy.

Legion actually has a CustomEntitySerializer implementation which will transform an Entity into a UUID you can pass to sled.

1 Like
7

Thanks @Michael-F-Bryan. Does this means I have to add below lines in the function?

let c = Canon::default();
let a = c.get_name(*entity).expect(&err);

Will calling Canon::default() inside function, changes the UUID?

8

You would need to check the source code or documentation to see whether the UUIDs are deterministic. I think it should be fine as long as you use the same Canon instance for serializing all your entities.

9

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