Method Syntax - The Rust Programming Language

# Method Syntax

Functions are great, but if you want to call a bunch of them on some data, it can be awkward. Consider this code:

    baz(bar(foo));

We would read this left-to-right, and so we see ‘baz bar foo’. But this isn’t the order that the functions would get called in, that’s inside-out: ‘foo bar baz’. Wouldn’t it be nice if we could do this instead?

    foo.bar().baz();

Luckily, as you may have guessed with the leading question, you can! Rust provides the ability to use this ‘method call syntax’ via the `impl` keyword.

# Method calls

Here’s how it works:

    struct Circle {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl Circle {
        fn area(&self) -> f64 {
            std::f64::consts::PI * (self.radius * self.radius)
        }
    }

    fn main() {
        let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
        println!("{}", c.area());
    }

This will print `12.566371`.

We’ve made a `struct` that represents a circle. We then write an `impl` block, and inside it, define a method, `area`.

Methods take a special first parameter, of which there are three variants: `self`, `&self`, and `&mut self`. You can think of this first parameter as being the `foo` in `foo.bar()`. The three variants correspond to the three kinds of things `foo` could be: `self` if it’s a value on the stack, `&self` if it’s a reference, and `&mut self` if it’s a mutable reference. Because we took the `&self` parameter to `area`, we can use it like any other parameter. Because we know it’s a `Circle`, we can access the `radius` like we would with any other `struct`.

We should default to using `&self`, as you should prefer borrowing over taking ownership, as well as taking immutable references over mutable ones. Here’s an example of all three variants:

    # #![allow(unused_variables)]
    #fn main() {
    struct Circle {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl Circle {
        fn reference(&self) {
           println!("taking self by reference!");
        }

        fn mutable_reference(&mut self) {
           println!("taking self by mutable reference!");
        }

        fn takes_ownership(self) {
           println!("taking ownership of self!");
        }
    }
    #}

You can use as many `impl` blocks as you’d like. The previous example could have also been written like this:

    # #![allow(unused_variables)]
    #fn main() {
    struct Circle {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl Circle {
        fn reference(&self) {
           println!("taking self by reference!");
        }
    }

    impl Circle {
        fn mutable_reference(&mut self) {
           println!("taking self by mutable reference!");
        }
    }

    impl Circle {
        fn takes_ownership(self) {
           println!("taking ownership of self!");
        }
    }
    #}

# Chaining method calls

So, now we know how to call a method, such as `foo.bar()`. But what about our original example, `foo.bar().baz()`? This is called ‘method chaining’. Let’s look at an example:

    struct Circle {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl Circle {
        fn area(&self) -> f64 {
            std::f64::consts::PI * (self.radius * self.radius)
        }

        fn grow(&self, increment: f64) -> Circle {
            Circle { x: self.x, y: self.y, radius: self.radius + increment }
        }
    }

    fn main() {
        let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
        println!("{}", c.area());

        let d = c.grow(2.0).area();
        println!("{}", d);
    }

Check the return type:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Circle;
    # impl Circle {
    fn grow(&self, increment: f64) -> Circle {
    # Circle } }
    #}

We say we’re returning a `Circle`. With this method, we can grow a new `Circle` to any arbitrary size.

# Associated functions

You can also define associated functions that do not take a `self` parameter. Here’s a pattern that’s very common in Rust code:

    struct Circle {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl Circle {
        fn new(x: f64, y: f64, radius: f64) -> Circle {
            Circle {
                x: x,
                y: y,
                radius: radius,
            }
        }
    }

    fn main() {
        let c = Circle::new(0.0, 0.0, 2.0);
    }

This ‘associated function’ builds a new `Circle` for us. Note that associated functions are called with the `Struct::function()` syntax, rather than the `ref.method()` syntax. Some other languages call associated functions ‘static methods’.

# Builder Pattern

Let’s say that we want our users to be able to create `Circle`s, but we will allow them to only set the properties they care about. Otherwise, the `x` and `y` attributes will be `0.0`, and the `radius` will be `1.0`. Rust doesn’t have method overloading, named arguments, or variable arguments. We employ the builder pattern instead. It looks like this:

    struct Circle {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl Circle {
        fn area(&self) -> f64 {
            std::f64::consts::PI * (self.radius * self.radius)
        }
    }

    struct CircleBuilder {
        x: f64,
        y: f64,
        radius: f64,
    }

    impl CircleBuilder {
        fn new() -> CircleBuilder {
            CircleBuilder { x: 0.0, y: 0.0, radius: 1.0, }
        }

        fn x(&mut self, coordinate: f64) -> &mut CircleBuilder {
            self.x = coordinate;
            self
        }

        fn y(&mut self, coordinate: f64) -> &mut CircleBuilder {
            self.y = coordinate;
            self
        }

        fn radius(&mut self, radius: f64) -> &mut CircleBuilder {
            self.radius = radius;
            self
        }

        fn finalize(&self) -> Circle {
            Circle { x: self.x, y: self.y, radius: self.radius }
        }
    }

    fn main() {
        let c = CircleBuilder::new()
                    .x(1.0)
                    .y(2.0)
                    .radius(2.0)
                    .finalize();

        println!("area: {}", c.area());
        println!("x: {}", c.x);
        println!("y: {}", c.y);
    }

What we’ve done here is make another `struct`, `CircleBuilder`. We’ve defined our builder methods on it. We’ve also defined our `area()` method on `Circle`. We also made one more method on `CircleBuilder`: `finalize()`. This method creates our final `Circle` from the builder. Now, we’ve used the type system to enforce our concerns: we can use the methods on `CircleBuilder` to constrain making `Circle`s in any way we choose.