Creating a String

Learn String::new, String::from and to_string constructors.

Creating Strings

There are several equivalent ways to create a String:

let a = String::new();              // empty
let b = String::from("Rust");
let c = "Rust".to_string();
let d: String = "Rust".into();
let e = 42.to_string();             // number to text

to_string works on every type that implements Display. If the size is known, String::with_capacity(n) avoids repeated allocations.

Going between String and &str

Getting a &str from a String is cheap (no copy): &s, s.as_str() or &s[..]. Making a String from a &str allocates new memory (to_string, to_owned). That's why working with &str is more efficient unless you need to store the value.

Exercise Build text

Create let mut s = String::with_capacity(16);. Add s.push_str("Yıl: "); and s.push_str(&2026.to_string());; print s and s.len().

Show example solution

fn main() {
    let mut s = String::with_capacity(16);
    s.push_str("Yıl: ");
    s.push_str(&2026.to_string());
    println!("{} {}", s, s.len());
}

Test yourself

What is the difference between <code>String::from("a")</code> and <code>"a".to_string()</code>?
- from is immutable
- The result is the same; both create a new String
- to_string returns a &str
- from is slower
Show answer

The result is the same; both create a new String — Both produce an owned String on the heap; which to use is mostly a matter of taste.

Number to text

let s = 2026.____();
println!("{}", s.len());  // 4

Show answer

let s = 2026.to_string();
println!("{}", s.len());  // 4

Expression and result

String::new()

An empty String
"a".to_string()

A new String
s.as_str()

&str (no copy)
42.to_string()

"42"

Next lesson: Concatenating strings

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