String and &str conversion

Learn conversion methods between String and &str.

Converting between String and &str

String is owned, growable text; &str is a borrowed view of text. Conversions:

let a: String = String::from("rust");
let b: String = "rust".to_string();
let c: String = "rust".to_owned();
let d: &str = &a;          // deref coercion
let e: &str = a.as_str();
let f: String = format!("{a}-{}", 2024);

Going from &str to String allocates new memory; the reverse is free.

Concatenation

The + operator takes ownership of the String on the left and needs a &str on the right:

let a = String::from("Merhaba");
let b = String::from("dünya");
let c = a + " " + &b;   // a was moved, b was lent
// println!("{a}");     // error: a was moved
println!("{c} {b}");

When joining several pieces, format! is more readable and moves no value. To append to an existing String, use push_str.

Exercise Two calls

Write fn selamla(ad: &str) -> String returning format!("Merhaba {ad}"). Call it with a String (&s) and with a literal ("Can"); print the results separated by |.

Show example solution

fn selamla(ad: &str) -> String {
    format!("Merhaba {ad}")
}

fn main() {
    let s = String::from("Ada");
    println!("{}|{}", selamla(&s), selamla("Can"));
}

Test yourself

If a function only reads text, what should the parameter type be?
- &str
- String
- &String
- &mut String
Show answer

&str — &str accepts both Strings (via &s) and literals; &String accepts only Strings.

Take a borrowed slice

let a = String::from("rust");
let e: &str = a.____();

Show answer

let a = String::from("rust");
let e: &str = a.as_str();

Expression and result type

"a".to_string()

String
s.as_str()

&str
format!("{x}")

String (new)
a + &b

String (a is moved)

Next lesson: chars and bytes

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies