Why does Rust have
str? What are the differences between
str? When does one use
String instead of
str and vice versa? Is one of them getting deprecated?
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
&'static str. The data is hardcoded into the executable and loaded into memory when the program runs.
Inside a heap allocated
Stringdereferences to a
&strview of the
On the stack: e.g. the following creates a stack-allocated byte array, and then gets a view of that data as a
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.
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
ä (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
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.
Answered By – huon
Answer Checked By – Candace Johnson (BugsFixing Volunteer)