&mut self

Define methods that mutate the struct with &mut self.

&mut self

Methods that change the instance's fields take &mut self. Such a method can only be called on a mut variable:

struct Sayac { deger: u32, adim: u32 }

impl Sayac {
    fn artir(&mut self) {
        self.deger += self.adim;
    }
    fn sifirla(&mut self) {
        self.deger = 0;
    }
}

Writing let s = Sayac { .. }; s.artir(); is error E0596: "cannot borrow as mutable".

Chaining

If a &mut self method returns &mut Self, calls can be chained:

impl Sayac {
    fn ekle(&mut self, n: u32) -> &mut Self {
        self.deger += n;
        self
    }
}

let mut s = Sayac { deger: 0, adim: 1 };
s.ekle(2).ekle(3);
println!("{}", s.deger);   // 5

This pattern is common in configuration objects (builders).

Exercise Step counter

For Sayac { deger: u32, adim: u32 }, write fn artir(&mut self) (deger += adim) and fn sifirla(&mut self). Start with deger: 0, adim: 5, increment twice and record the value; then reset and record it again (print on one line, separated by a space).

Show example solution

struct Sayac {
    deger: u32,
    adim: u32,
}

impl Sayac {
    fn artir(&mut self) {
        self.deger += self.adim;
    }
    fn sifirla(&mut self) {
        self.deger = 0;
    }
}

fn main() {
    let mut s = Sayac { deger: 0, adim: 5 };
    s.artir();
    s.artir();
    let once = s.deger;
    s.sifirla();
    println!("{} {}", once, s.deger);
}

Test yourself

What happens if another <code>&s</code> reference is alive while calling a method that takes <code>&mut self</code>?
- Runtime panic
- The reference is copied automatically
- Compile error: a reader and a writer can't coexist
- No problem
Show answer

Compile error: a reader and a writer can't coexist — A method call is a borrow too; the borrowing rules apply just the same.

A modifying method

fn sifirla(____) {
    self.deger = 0;
}

Show answer

fn sifirla(&mut self) {
    self.deger = 0;
}

Code and result

let s; s.artir()

E0596
let mut s; s.artir()

Works
-> &mut Self

Chaining
self.deger = 0

Field assignment

Next lesson: Associated functions

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