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

String is the dynamic heap string type, it is considered like mutable variable, like Vec: use it when you need to own or modify your string data.

str is an immutable sequence of UTF-8 bytes of dynamic length somewhere in memory. Since the size is unknown, one can only handle it behind a pointer. This means that str most commonly appears as &str: a reference to some UTF-8 data, normally called a "string slice" or just a "slice". A slice is just a view onto some data, and that data can be anywhere.

  • In static storage: a string literal "foo" is a &'static str. The data is hardcoded into the executable and loaded into memory when the program runs.
  • Inside a heap allocated String: String dereferences to a &str view of the String's data.
  • On the stack:

Example

use std::str;
fn main() {
    let x: &[u8] = &[b'a', b'b', b'c'];
    let stack_str: &str = str::from_utf8(x).unwrap();
    println!("x value = {:?}", x);
    println!("stack_str value = {:?}", stack_str);
}

Output

x value = [97, 98, 99]
stack_str value = "abc"


Conclusion

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