What are the differences between Rust's `String` and `str`?

Question

Why does Rust have String and str? What are the differences between String and str? When does one use String

Answer 1

std::String is simply a vector of u8. You can find its definition in source code . It's heap-allocated and growable.

#[derive(PartialOrd, Eq, Ord)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct String {
    vec: Vec,
}

str is a primitive type, also called string slice. A string slice has fixed size. A literal string like let test = "hello world" has &'static str type. test is a reference to this statically allocated string. &str cannot be modified, for example,

let mut word = "hello world";
word[0] = 's';
word.push('\n');

str does have mutable slice &mut str, for example: pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str)

let mut s = "Per Martin-Löf".to_string();
{
    let (first, last) = s.split_at_mut(3);
    first.make_ascii_uppercase();
    assert_eq!("PER", first);
    assert_eq!(" Martin-Löf", last);
}
assert_eq!("PER Martin-Löf", s);

But a small change to UTF-8 can change its byte length, and a slice cannot reallocate its referent.