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 instead of str and vice versa? Is one of them getting deprecated?

Answer 1

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

str is an immutable¹ 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 commonly² 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, e.g.

• 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: e.g. the following creates a stack-allocated byte array, and then gets a view of that data as a &str:

  use std::str;
  
  let x: &[u8] = &[b'a', b'b', b'c'];
  let stack_str: &str = str::from_utf8(x).unwrap();
  

In summary, use String if you need owned string data (like passing strings to other threads, or building them at runtime), and use &str if you only need a view of a string.

This is identical to the relationship between a vector Vec<T> and a slice &[T], and is similar to the relationship between by-value T and by-reference &T for general types.

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¹ A str is fixed-length; you cannot write bytes beyond the end, or leave trailing invalid bytes. Since UTF-8 is a variable-width encoding, this effectively forces all strs to be immutable in many cases. In general, mutation requires writing more or fewer bytes than there were before (e.g. replacing an a (1 byte) with an ? (2+ bytes) would require making more room in the str). There are specific methods that can modify a &mut str in place, mostly those that handle only ASCII characters, like make_ascii_uppercase.

² Dynamically sized types allow things like Rc<str> for a sequence of reference counted UTF-8 bytes since Rust 1.2. Rust 1.21 allows easily creating these types.