&mut self
- Rust course
- Beginner
- Lesson 73/115
- 4 min
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); // 5This pattern is common in configuration objects (builders).
Practice 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).
Starter code
struct Sayac {
deger: u32,
adim: u32,
}
fn main() {
// buraya yaz
}Sample 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);
}Expected output
10 0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- No problem
Explanation: A method call is a borrow too; the borrowing rules apply just the same.
2 A modifying method
fn sifirla(____) {
self.deger = 0;
}fn sifirla(&mut self) {
self.deger = 0;
}Explanation: Changing a field requires a mutable borrow.
3 Code and result
| Term | Meaning |
|---|---|
let s; s.artir() |
E0596 |
let mut s; s.artir() |
Works |
-> &mut Self |
Chaining |
self.deger = 0 |
Field assignment |