String References

We can now understand the two string types in Rust: &str is almost like &[char], but with its data stored in a variable-length encoding (UTF-8).

fn main() {
    let s1: &str = "World";
    println!("s1: {s1}");

    let mut s2: String = String::from("Hello ");
    println!("s2: {s2}");
    println!("s2: {s2}");

    let s3: &str = &s2[6..];
    println!("s3: {s3}");

Rust terminology:

  • &str an immutable reference to a string slice.
  • String a mutable string buffer.
This slide should take about 10 minutes.
  • &str introduces a string slice, which is an immutable reference to UTF-8 encoded string data stored in a block of memory. String literals (”Hello”), are stored in the program’s binary.

  • Rust’s String type is a wrapper around a vector of bytes. As with a Vec<T>, it is owned.

  • As with many other types String::from() creates a string from a string literal; String::new() creates a new empty string, to which string data can be added using the push() and push_str() methods.

  • The format!() macro is a convenient way to generate an owned string from dynamic values. It accepts the same format specification as println!().

  • You can borrow &str slices from String via & and optionally range selection. If you select a byte range that is not aligned to character boundaries, the expression will panic. The chars iterator iterates over characters and is preferred over trying to get character boundaries right.

  • For C++ programmers: think of &str as std::string_view from C++, but the one that always points to a valid string in memory. Rust String is a rough equivalent of std::string from C++ (main difference: it can only contain UTF-8 encoded bytes and will never use a small-string optimization).

  • Byte strings literals allow you to create a &[u8] value directly:

    fn main() {
        println!("{:?}", b"abc");
        println!("{:?}", &[97, 98, 99]);