rust/src/doc/guide-strings.md

% The Strings Guide

# 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.

## Other Documentation

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