Methods

A method is a function that belongs to a type. You call it with a dot: rect.area().

impl blocks

Methods live in an impl block for the type. Inside it, Self means the type itself.

1struct Rect { w: u32, h: u32 }23impl Rect {4    fn area(&self) -> u32 {5        self.w * self.h6    }7}

Inside a method, read fields through self, like self.w. A plain w would not work.

&self

&self borrows the value to read it. The caller keeps ownership, so the value can be used again after the call. Most methods that only look at a value use &self.

&mut self

&mut self borrows the value so the method can change it. The variable must be declared with mut, or the call fails with E0596.

1impl Rect {2    fn grow(&mut self) { self.w += 1; }3}45let mut r = Rect { w: 2, h: 3 };6r.grow(); // r.w is now 3

self

A plain self takes ownership of the value. After the call, the old variable can't be used (E0382). Use it when a method turns the value into something else.

Associated functions

A function in an impl block with no self is an associated function. Call it on the type with ::, not on a value with a dot. new is the usual name for one that makes a value.

1impl Rect {2    fn square(n: u32) -> Self {3        Self { w: n, h: n }4    }5}67let s = Rect::square(4);

Remember

• &self reads, &mut self changes, self takes.
• A &mut self method needs a mut variable.
• No self means you call it as Type::name().