Methods and Associated Functions

Methods

Methods are similar to functions in that we declare them with the fn keyword and they have parameters and return a value. However, unlike functions, Methods are defined within the context of a struct (or enum), and either refers to that type or mutates it. The first parameter of a method is always self, which represents the instance of the struct (or enum) the method is being called on.

Associated Functions

Associated functions are very similar to methods, in that they are also defined in the context of a struct or enum, but they do not actually use any of the data in the struct and as a result do not take self as a parameter. Associated functions could be standalone functions, but they are included in a specific type for organizational or semantic reasons.

Constructors

Constructors are associated functions that construct, or in other words instantiate, new instances of a type. Their return type is always the type itself. E.g., public structs that have private fields must provide a public constructor, or otherwise they cannot be instantiated outside of the module in which they are declared.

Declaring Methods and Associated Functions

To declare methods and associated functions for a struct or enum, use an impl block. Here, impl is short for implementation.

script;

struct Foo {
    bar: u64,
    baz: bool,
}

impl Foo {
    // this is a _method_, as it takes `self` as a parameter.
    fn is_baz_true(self) -> bool {
        self.baz
    }

    // this is an _associated function_, since it does not take `self` as a parameter.
    // it is at the same time a _constructor_ because it instantiates and returns
    // a new instance of `Foo`.
    fn new_foo(number: u64, boolean: bool) -> Foo {
        Foo {
            bar: number,
            baz: boolean,
        }
    }
}

fn main() {
    let foo = Foo::new_foo(42, true);
    assert(foo.is_baz_true());
}

To call a method, simply use dot syntax: foo.iz_baz_true().

Similarly to free functions, methods and associated functions may accept ref mut parameters.

For example:

struct Coordinates {
    x: u64,
    y: u64,
}

impl Coordinates {
    fn move_right(ref mut self, distance: u64) {
        self.x += distance;
    }
}

and when called:

    let mut point = Coordinates { x: 1, y: 1 };
    point.move_right(5);
    assert(point.x == 6);
    assert(point.y == 1);