% The Guide to Rust Strings

Strings are an important concept to master in any programming language. If you
come from a managed language background, you may be surprised at the complexity
of string handling in a systems programming language. Efficient access and
allocation of memory for a dynamically sized structure involves a lot of
details. Luckily, Rust has lots of tools to help us here.

A **string** is a sequence of unicode scalar values encoded as a stream of
UTF-8 bytes. All strings are guaranteed to be validly-encoded UTF-8 sequences.
Additionally, strings are not null-terminated and can contain null bytes.

Rust has two main types of strings: `&str` and `String`.

# &str

The first kind is a `&str`. This is pronounced a 'string slice.' String literals
are of the type `&str`:

```{rust}
let string = "Hello there.";
```

Like any Rust type, string slices have an associated lifetime. A string literal
is a `&'static str`.  A string slice can be written without an explicit
lifetime in many cases, such as in function arguments. In these cases the
lifetime will be inferred:

```{rust}
fn takes_slice(slice: &str) {
    println!("Got: {}", slice);
}
```

Like vector slices, string slices are simply a pointer plus a length. This
means that they're a 'view' into an already-allocated string, such as a
`&'static str` or a `String`.

# String

A `String` is a heap-allocated string. This string is growable, and is also
guaranteed to be UTF-8.

```{rust}
let mut s = "Hello".to_string();
println!("{}", s);

s.push_str(", world.");
println!("{}", s);
```

You can coerce a `String` into a `&str` with the `as_slice()` method:

```{rust}
fn takes_slice(slice: &str) {
    println!("Got: {}", slice);
}

fn main() {
    let s = "Hello".to_string();
    takes_slice(s.as_slice());
}
```

You can also get a `&str` from a stack-allocated array of bytes:

```{rust}
use std::str;

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

# Best Practices

## `String` vs. `&str`

In general, you should prefer `String` when you need ownership, and `&str` when
you just need to borrow a string. This is very similar to using `Vec<T>` vs. `&[T]`,
and `T` vs `&T` in general.

This means starting off with this:

```{rust,ignore}
fn foo(s: &str) {
```

and only moving to this:

```{rust,ignore}
fn foo(s: String) {
```

If you have good reason. It's not polite to hold on to ownership you don't
need, and it can make your lifetimes more complex. Furthermore, you can pass
either kind of string into `foo` by using `.as_slice()` on any `String` you
need to pass in, so the `&str` version is more flexible.

## Comparisons

To compare a String to a constant string, prefer `as_slice()`...

```{rust}
fn compare(string: String) {
    if string.as_slice() == "Hello" {
        println!("yes");
    }
}
```

... over `to_string()`:

```{rust}
fn compare(string: String) {
    if string == "Hello".to_string() {
        println!("yes");
    }
}
```

Converting a `String` to a `&str` is cheap, but converting the `&str` to a
`String` involves an allocation.

## Indexing strings

You may be tempted to try to access a certain character of a `String`, like
this:

```{rust,ignore}
let s = "hello".to_string();

println!("{}", s[0]);
```

This does not compile. This is on purpose. In the world of UTF-8, direct
indexing is basically never what you want to do. The reason is that each
charater can be a variable number of bytes. This means that you have to iterate
through the characters anyway, which is a O(n) operation. 

To iterate over a string, use the `graphemes()` method on `&str`:

```{rust}
let s = "αἰθήρ";

for l in s.graphemes(true) {
    println!("{}", l);
}
```

This will print out each character in turn, as you'd expect: first "α", then
"ἰ", etc. You can see that this is different than just the individual bytes.
Here's a version that prints out each byte:

```{rust}
let s = "αἰθήρ";

for l in s.as_bytes().iter() {
    println!("{}", l);
}
```

This will print:

```{notrust,ignore}
206
177
225
188
176
206
184
206
174
207
129
```

Many more bytes than graphemes!

# Other Documentation

* [the `&str` API documentation](/std/str/index.html)
* [the `String` API documentation](std/string/index.html)