Day7:String and &str - 100DayOfRust

String and &str

String is Rust's String type, where strings are saved in heap.

&str is a string slice, pointed to the real strings.

String literals are &str, they are saved at the "static storage" area in a program, so technically they are type of 'static &str.

As parameter

If a function takes &str as parameter, you can pass String, &str and String literals to it. For example:

fn test(&str) {

}

let s1 = String::from("hello");
// to pass String to this function, we need change it to string slice
test(&s1[..]);

// directly pass a string slice
let s2 = &s1[1..3];
test(s2);

// b/c string literals are &str, we can directly pass string literals
let s3 = "Hello World";
test(s3);

Conversion

To change String to &str, we can use &str[..] like the above example.

To change string literals to String, we can use the to_string() function, for example "hello".to_string() or String::from("hello");

Example

You can infer if a string function returning String or &str by checking if the function changes the origin string letters.

For example, .trim() will remove the leading and trailing spaces, which means it can return a pointer to the original string starting with the first non-space letter and ending with the last ending space letter, it doesn't need to change the original string or create a new string.

While .replace() or the to_lowercase() method will need to create a new string to make the change, because just create a pointer pointing to the original string won't work.

fn string_slice(arg: &str) {
    println!("{}", arg);
}
fn string(arg: String) {
    println!("{}", arg);
}

fn main() {
    string_slice("blue");          // string literals are &str
    string("red".to_string());     // to_string return String
    string(String::from("hi"));    // String::from return String
    string("rust is fun!".to_owned());    // to_owned return String
    string("nice weather".into()); // Same as String::from
    string(format!("Interpolation {}", "Station"));  // Created a new String
    string_slice(&String::from("abc")[0..1]);  // Create a slice
    string_slice("  hello there ".trim());     // create a slice
    string("Happy Monday!".to_string().replace("Mon", "Tues"));  // Create a new String
    string("mY sHiFt KeY iS sTiCkY".to_lowercase());  // Create a new String
}