Rust: str vs String

As a Rust newbie, I was confused by the different ways used to represent strings. The “References and Borrowing” chapter of the Rust book uses three different types of string variables in the examples: String, &String and &str.

Let’s start with the difference between str and String: String is a growable, heap-allocated data structure whereas str is an immutable fixed-length string somewhere in memory ¹.

String

If you’re a Java programmer, a Rust String is semantically equivalent to StringBuffer (this was probably a factor in my confusion, as I’m so used to equating String with immutable). As such, a String maintains a length and a capacity whereas a str only has a len() method. As an example:

let mut s = String::from("Hello, Rust!");
println!("{}", s.capacity()); // prints 12
s.push_str("Here I come!");
println!("{}", s.len()); // prints 24

let s = "Hello, Rust!";
println!("{}", s.capacity()); // compile error: no method named `capacity` found for type `&str`
println!("{}", s.len()); // prints 12

&str

You can only ever interact with str as a borrowed type aka &str. This is called a string slice, an immutable view of a string. This is the preferred way to pass strings around, as we shall see.

&String

This is a reference to a String, also called a borrowed type. This is nothing more than a pointer which you can pass around without giving up ownership. Turns out a &String can be coerced to a &str:

fn main() {
    let s = String::from("Hello, Rust!");
    foo(&s);
}

fn foo(s: &str) {
    println!("{}", s);
}

In the above example, foo() can take either string slices or borrowed Strings, which is super convenient. As such, you almost never need to deal with &Strings. The only real use case I can think of is if you want to pass a mutable reference to a function that needs to modify the string:

fn main() {
    let mut s = String::from("Hello, Rust!");
    foo(&mut s);
}

fn foo(s: &mut String) {
    s.push_str("appending foo..");
    println!("{}", s);
}

Summary

Prefer &str as a function parameter or if you want a read-only view of a string; String when you want to own and mutate a string.