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What is the difference between String and &str?

Short answer: String owns the buffer its text sits in, and can grow it. &str is a read-only view of bytes that belong to something else — a String, a literal baked into the binary, or any other owner.

What each type is

Section titled “What each type is”

A String is a Vec<u8> with one promise attached: the bytes are valid UTF-8. The value itself is three machine words, 24 bytes on a 64-bit machine: a pointer, a length and a capacity. The text lives in a heap buffer those words point at, and owning that buffer is what lets it grow — push_str appends, and the buffer reallocates when it runs out of room.

str is the text itself, and it’s unsized, so a str never exists on its own. A &str is a borrow of one: a pointer and a length, two words, pointing at bytes owned elsewhere.

Literals have type &'static str, borrowed for the whole run of the program, because the text sits in the binary itself. Writing "hello" allocates nothing.

Which one to reach for

Section titled “Which one to reach for”

Build text with a String; read it through &str. A shared reference can’t change what it points at, so anything that appends or edits in place, like push_str and insert_str, needs the owned type.

Function parameters are where the split pays off. String implements Deref<Target = str>, so &some_string coerces to &str, and a parameter typed &str accepts both a literal and a borrow of a String with no clone at the call site. Taking String by value moves ownership in instead, and the caller is left without the text; why String isn’t Copy explains why there’s no free second copy. Copying a &str, by contrast, is harmless: a borrow owns nothing, and it’s two words.

A struct that stores text usually holds a String; one that views text someone else owns holds a &str. When a function you don’t control asks for a String, String::from("text") makes the owned copy; the next article in this chapter compares those spellings.

Section titled “Related”