Strings - Rust By Example
Description: A description

# Strings

There are two types of strings in Rust: `String` and `&str`.

A `String` is stored as a vector of bytes (`Vec<u8>`), but guaranteed to always be a valid UTF-8 sequence. `String` is heap allocated, growable and not null terminated.

`&str` is a slice (`&[u8]`) that always points to a valid UTF-8 sequence, and can be used to view into a `String`, just like `&[T]` is a view into `Vec<T>`.

    fn main() {
        // (all the type annotations are superfluous)
        // A reference to a string allocated in read only memory
        let pangram: &'static str = "the quick brown fox jumps over the lazy dog";
        println!("Pangram: {}", pangram);

        // Iterate over words in reverse, no new string is allocated
        println!("Words in reverse");
        for word in pangram.split_whitespace().rev() {
            println!("> {}", word);
        }

        // Copy chars into a vector, sort and remove duplicates
        let mut chars: Vec<char> = pangram.chars().collect();
        chars.sort();
        chars.dedup();

        // Create an empty and growable `String`
        let mut string = String::new();
        for c in chars {
            // Insert a char at the end of string
            string.push(c);
            // Insert a string at the end of string
            string.push_str(", ");
        }

        // The trimmed string is a slice to the original string, hence no new
        // allocation is performed
        let chars_to_trim: &[char] = &[' ', ','];
        let trimmed_str: &str = string.trim_matches(chars_to_trim);
        println!("Used characters: {}", trimmed_str);

        // Heap allocate a string
        let alice = String::from("I like dogs");
        // Allocate new memory and store the modified string there
        let bob: String = alice.replace("dog", "cat");

        println!("Alice says: {}", alice);
        println!("Bob says: {}", bob);
    }

More `str`/`String` methods can be found under the std::str and std::string modules

## Literals and escapes

There are multiple ways to write string literals with special characters in them. All result in a similar `&str` so it's best to use the form that is the most convenient to write. Similarly there are multiple ways to write byte string literals, which all result in `&[u8; N]`.

Generally special characters are escaped with a backslash character: `\`. This way you can add any character to your string, even unprintable ones and ones that you don't know how to type. If you want a literal backslash, escape it with another one: `\\`

String or character literal delimiters occuring within a literal must be escaped: `"\""`, `'\''`.

    fn main() {
        // You can use escapes to write bytes by their hexadecimal values...
        let byte_escape = "I'm writing \x52\x75\x73\x74!";
        println!("What are you doing\x3F (\\x3F means ?) {}", byte_escape);

        // ...or Unicode code points.
        let unicode_codepoint = "\u{211D}";
        let character_name = "\"DOUBLE-STRUCK CAPITAL R\"";

        println!("Unicode character {} (U+211D) is called {}",
                    unicode_codepoint, character_name );

        let long_string = "String literals
                            can span multiple lines.
                            The linebreak and indentation here ->\
                            <- can be escaped too!";
        println!("{}", long_string);
    }

Sometimes there are just too many characters that need to be escaped or it's just much more convenient to write a string out as-is. This is where raw string literals come into play.

    fn main() {
        let raw_str = r"Escapes don't work here: \x3F \u{211D}";
        println!("{}", raw_str);

        // If you need quotes in a raw string, add a pair of #s
        let quotes = r#"And then I said: "There is no escape!""#;
        println!("{}", quotes);

        // If you need "# in your string, just use more #s in the delimiter.
        // There is no limit for the number of #s you can use.
        let longer_delimiter = r###"A string with "# in it. And even "##!"###;
        println!("{}", longer_delimiter);
    }

Want a string that's not UTF-8? (Remember, `str` and `String` must be valid UTF-8) Or maybe you want an array of bytes that's mostly text? Byte strings to the rescue!

    use std::str;

    fn main() {
        // Note that this is not actually a &str
        let bytestring: &[u8; 20] = b"this is a bytestring";

        // Byte arrays don't have Display so printing them is a bit limited
        println!("A bytestring: {:?}", bytestring);

        // Bytestrings can have byte escapes...
        let escaped = b"\x52\x75\x73\x74 as bytes";
        // ...but no unicode escapes
        // let escaped = b"\u{211D} is not allowed";
        println!("Some escaped bytes: {:?}", escaped);

        // Raw bytestrings work just like raw strings
        let raw_bytestring = br"\u{211D} is not escaped here";
        println!("{:?}", raw_bytestring);

        // Converting a byte array to str can fail
        if let Ok(my_str) = str::from_utf8(raw_bytestring) {
            println!("And the same as text: '{}'", my_str);
        }

        let quotes = br#"You can also use "fancier" formatting, \
                        like with normal raw strings"#;

        // Bytestrings don't have to be UTF-8
        let shift_jis = b"\x82\xe6\x82\xa8\x82\xb1\x82"; // "ようこそ" in SHIFT-JIS

        // But then they can't always be converted to str
        match str::from_utf8(shift_jis) {
            Ok(my_str) => println!("Conversion successful: '{}'", my_str),
            Err(e) => println!("Conversion failed: {:?}", e),
        };
    }

For conversions between character encodings check out the encoding crate.

A more detailed listing of the ways to write string literals and escape characters is given in the 'Tokens' chapter of the Rust Reference.