Binding modes
- Rust course
- Advanced
- Lesson 76/81
- 4 min
Learn binding modes with ref and ref mut.
Binding modes
A pattern can bind a value in three ways: by moving (the default), by borrowing with ref, by mutable borrowing with ref mut. Today you mostly don't need to write these: when matching on a reference (match &x, match &mut x), the compiler picks the binding mode itself:
let mut cift = (String::from("a"), 1);
match &mut cift {
(s, n) => { // s: &mut String, n: &mut i32
s.push('!');
*n += 1;
}
}
println!("{:?}", cift); // ("a!", 2)When is ref needed?
When matching on the value itself and you want to borrow one part while moving another, ref is still useful:
let kayit = (String::from("ece"), vec![1, 2]);
let (ref ad, puanlar) = kayit; // ad borrowed, puanlar moved
println!("{ad} {:?}", puanlar);
// kayit.1 can no longer be used; kayit.0 canPartial move and partial borrow together; this fine control lets you split large structures without needless copies.
Practice Change in place
Declare let mut cift = (String::from("a"), 1);. With match &mut cift, append '!' to the text and increment the number by 1; then print cift with {:?}.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let mut cift = (String::from("a"), 1);
match &mut cift {
(s, n) => {
s.push('!');
*n += 1;
}
}
println!("{:?}", cift);
}Expected output
("a!", 2)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Can <code>opt</code> be used after <code>match opt { Some(s) => .., None => .. }</code> (<code>opt: Option<String></code>)?
- Yes
- Only if it's None
- It doesn't compile
- No; the String was moved into s. You need to match on &opt Correct answer
Explanation: With match &opt, s becomes a &String and opt keeps ownership; Some(ref s) is the older form.
2 Mutable matching
match ____ cift {
(s, n) => {
s.push('!');
*n += 1;
}
}match &mut cift {
(s, n) => {
s.push('!');
*n += 1;
}
}Explanation: When matching on &mut, the bindings become &mut.
3 Matching and binding type
| Term | Meaning |
|---|---|
match x { (s, _) } |
Move |
match &x { (s, _) } |
&String |
match &mut x { (s, _) } |
&mut String |
let (ref a, b) = x; |
a borrowed, b moved |