Strings

Introduction#

Unlike many other languages, Rust has two main string types: String (a heap-allocated string type) and &str (a borrowed string, which does not use extra memory). Knowing the difference and when to use each is vital to understand how Rust works.

Basic String manipulation

fn main() {
    // Statically allocated string slice
    let hello = "Hello world";

    // This is equivalent to the previous one
    let hello_again: &'static str = "Hello world";

    // An empty String
    let mut string = String::new();

    // An empty String with a pre-allocated initial buffer
    let mut capacity = String::with_capacity(10);

    // Add a string slice to a String
    string.push_str("foo");

    // From a string slice to a String
    // Note: Prior to Rust 1.9.0 the to_owned method was faster
    // than to_string. Nowadays, they are equivalent.      
    let bar = "foo".to_owned();
    let qux = "foo".to_string();

    // The String::from method is another way to convert a
    // string slice to an owned String.
    let baz = String::from("foo");

    // Coerce a String into &str with &
    let baz: &str = &bar;
}

Note: Both the String::new and the String::with_capacity methods will create empty strings. However, the latter allocates an initial buffer, making it initially slower, but helping reduce subsequent allocations. If the final size of the String is known, String::with_capacity should be preferred.

String slicing

fn main() {
    let english = "Hello, World!";

    println!("{}", &english[0..5]); // Prints "Hello"
    println!("{}", &english[7..]);  // Prints "World!"
}

Note that we need to use the & operator here. It takes a reference and thus gives the compiler information about the size of the slice type, which it needs to print it. Without the reference, the two println! calls would be a compile-time error.

Warning: Slicing works by byte offset, not character offset, and will panic when bounds are not on a character boundary:

fn main() {
    let icelandic = "Halló, heimur!"; // note that “ó” is two-byte long in UTF-8

    println!("{}", &icelandic[0..6]); // Prints "Halló", “ó” lies on two bytes 5 and 6
    println!("{}", &icelandic[8..]);  // Prints "heimur!", the “h” is the 8th byte, but the 7th char
    println!("{}", &icelandic[0..5]); // Panics!
}

This is also the reason why strings don’t support simple indexing (eg. icelandic[5]).

Split a string

let strings = "bananas,apples,pear".split(",");

split returns an iterator.

for s in strings {
  println!("{}", s)
}

And can be “collected” in a Vec with the Iterator::collect method.

let strings: Vec<&str> = "bananas,apples,pear".split(",").collect(); // ["bananas", "apples", "pear"]

From borrowed to owned

// all variables `s` have the type `String`
let s = "hi".to_string();  // Generic way to convert into `String`. This works
                           // for all types that implement `Display`.

let s = "hi".to_owned();   // Clearly states the intend of obtaining an owned object

let s: String = "hi".into();       // Generic conversion, type annotation required
let s: String = From::from("hi");  // in both cases!

let s = String::from("hi");  // Calling the `from` impl explicitly -- the `From` 
                             // trait has to be in scope!

let s = format!("hi");       // Using the formatting functionality (this has some
                             // overhead)

Apart from format!(), all of the methods above are equally fast.

Breaking long string literals

Break regular string literals with the \ character

let a = "foobar";
let b = "foo\
         bar";

// `a` and `b` are equal.
assert_eq!(a,b);

Break raw-string literals to separate strings, and join them with the concat! macro

let c = r"foo\bar";
let d = concat!(r"foo\", r"bar");

// `c` and `d` are equal.
assert_eq!(c, d);