Call Method as Function

Method-Call Style vs. Function-Call Style

Any registered function with a first argument that is a &mut reference can be used as method because internally they are the same thing: methods on a custom type is implemented as a functions taking a &mut first argument.

This design is similar to Rust.

fn main() {
impl TestStruct {
    fn foo(&mut self) -> i64 {

engine.register_fn("foo", TestStruct::foo);

let result = engine.eval::<i64>(
    let x = new_ts();
    foo(x);                         // normal call to 'foo'                         // 'foo' can also be called like a method on 'x'

println!("result: {}", result);     // prints 1

Under no_object, however, the method-call style is no longer supported.

fn main() {
// Below is a syntax error under 'no_object'.
let result = engine.eval("let x = [1, 2, 3]; x.clear();")?;
                                           // ^ cannot call method-style

First &mut Parameter

The opposite direction also works – methods in a Rust custom type registered with the Engine can be called just like a regular function. In fact, like Rust, object methods are registered as regular functions in Rhai that take a first &mut parameter.

Unlike functions defined in script (for which all arguments are passed by value), native Rust functions may mutate the first &mut argument.

Sometimes, however, there are more subtle differences. Methods called in normal function-call style may end up not muting the object afterall – see the example below.

Custom types, properties, indexers and methods are disabled under the no_object feature.

fn main() {
let a = new_ts();   // constructor function
a.field = 500;      // property setter
a.update();         // method call, 'a' can be modified

update(a);          // <- this de-sugars to 'a.update()'
                    //    'a' can be modified and is not a copy

let array = [ a ];

update(array[0]);   // <- 'array[0]' is an expression returning a calculated value,
                    //    a transient (i.e. a copy), so this statement has no effect
                    //    except waste time cloning 'a'

array[0].update();  // <- call in method-call style will update 'a'

IMPORTANT: Rhai does NOT support normal references (i.e. &T) as parameters.

Number of Parameters in Methods

Native Rust methods registered with an Engine take one additional parameter more than an equivalent method coded in script, where the object is accessed via the this pointer instead.

The following table illustrates the differences:

Function typeNo. of parametersObject referenceFunction signature
Native RustN + 1first &mut T parameterFn(obj: &mut T, x: U, y: V)
Rhai scriptNthisFn(x: U, y: V)

&mut is Efficient, Except for &mut ImmutableString

Using a &mut first parameter is highly encouraged when using types that are expensive to clone, even when the intention is not to mutate that argument, because it avoids cloning that argument value.

Even when a function is never intended to be a method – for example an operator, it is still sometimes beneficial to make it method-like (i.e. with a first &mut parameter) if the first parameter is not modified.

For types that are expensive to clone (remember, all function calls are passed cloned copies of argument values), this may result in a significant performance boost.

For primary types that are cheap to clone (e.g. those that implement Copy), including ImmutableString, this is not necessary.

fn main() {
// This is a type that is very expensive to clone.
#[derive(Debug, Clone)]
struct VeryComplexType { ... }

// Calculate some value by adding 'VeryComplexType' with an integer number.
fn do_add(obj: &VeryComplexType, offset: i64) -> i64 {

      .register_fn("+", add_pure /* or  add_method*/);

// Very expensive to call, as the 'VeryComplexType' is cloned before each call.
fn add_pure(obj: VeryComplexType, offset: i64) -> i64 {
    do_add(obj, offset)

// Efficient to call, as only a reference to the 'VeryComplexType' is passed.
fn add_method(obj: &mut VeryComplexType, offset: i64) -> i64 {
    do_add(obj, offset)

Data Race Considerations

Because methods always take a mutable reference as the first argument, even it the value is never changed, care must be taken when using shared values with methods.

Usually data races are not possible in Rhai because, for each function call, there is ever only one value that is mutable – the first argument of a method. All other arguments are cloned.

It is possible, however, to create a data race with a shared value, when the same value is used both as the object of a method call (including the this pointer) and also as an argument.

fn main() {
// A method using the 'this' pointer and an argument
fn foo(x) {
    this + x

let value = 42;     // 'value' is not shared by default

let f = || value;   // this closure captures 'value'

// ... at this point, 'value' is shared

value.is_shared() == true;;   // <- error: data race detected!

The reason why it is a data race is because the variable value is shared, and cloning it merely clones a shared reference to it. Using it as the method call object (i.e. the this pointer) takes a mutable reference to the underlying value, which then cause a data race because a non-mutable reference is already outstanding due to the argument (which uses the same variable).

Shared values are typically created from closures which capture external variable, so data races are not possible in Rhai under the no_closure feature.