rhai/doc/src/rust/indexers.md

Custom Type Indexers

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A custom type can also expose an indexer by registering an indexer function.

A custom type with an indexer function defined can use the bracket notation to get a property value:

object [ index ]

Like getters and setters, indexers take a &mut reference to the first parameter.

Indexers are disabled when the [no_index] feature is used.

For efficiency reasons, indexers cannot be used to overload (i.e. override) built-in indexing operations for [arrays] and [object maps].

Engine API	Description	Return Value of Function
register_indexer_get	Register an index getter	Any
register_indexer_set	Register an index setter	None
register_indexer_get_set	Short-hand to register both an index getter and a setter	None
register_indexer_get_result	Register an index getter	Result<Dynamic, Box<EvalAltResult>>
register_indexer_set_result	Register an index setter	Result<(), Box<EvalAltResult>>

Examples

#[derive(Clone)]
struct TestStruct {
    fields: Vec<i64>
}

impl TestStruct {
    fn get_field(&mut self, index: i64) -> i64 {
        self.fields[index as usize]
    }
    fn set_field(&mut self, index: i64, value: i64) {
        self.fields[index as usize] = value
    }

    fn new() -> Self {
        TestStruct { fields: vec![1, 2, 3, 4, 5] }
    }
}

let mut engine = Engine::new();

engine
    .register_type::<TestStruct>()
    .register_fn("new_ts", TestStruct::new)
    // Short-hand: .register_indexer_get_set(TestStruct::get_field, TestStruct::set_field);
    .register_indexer_get(TestStruct::get_field)
    .register_indexer_set(TestStruct::set_field);

let result = engine.eval::<i64>("let a = new_ts(); a[2] = 42; a[2]")?;

println!("Answer: {}", result);                     // prints 42