Strings

We can now understand the two string types in Rust:

  • &str is a slice of UTF-8 encoded bytes, similar to &[u8].
  • String is an owned, heap-allocated buffer of UTF-8 bytes.
fn main() {
    let s1: &str = "World";
    println!("s1: {s1}");

    let mut s2: String = String::from("Hello ");
    println!("s2: {s2}");
    s2.push_str(s1);
    println!("s2: {s2}");

    let s3: &str = &s2[s2.len() - s1.len()..];
    println!("s3: {s3}");
}
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]);
}

  • Raw strings allow you to create a &str value with escapes disabled: r"\n" == "\\n". You can embed double-quotes by using an equal amount of # on either side of the quotes:

    fn main() {
    println!(r#"<a href="link.html">link</a>"#);
    println!("<a href=\"link.html\">link</a>");
}