The Rust Programming Language

The Slice Type

Slices let you reference a contiguous sequence of elements in a collection rather than the whole collection. A slice is a kind of reference, so it is a non-owning pointer.

To motivate why slices are useful, let’s work through a small programming problem: write a function that takes a string of words separated by spaces and returns the first word it finds in that string. If the function doesn’t find a space in the string, the whole string must be one word, so the entire string should be returned. Without slices, we might write the signature of the function like this:

fn first_word(s: &String) -> ?

The first_word function has a &String as a parameter. We don’t want ownership of the string, so this is fine. But what should we return? We don’t really have a way to talk about part of a string. However, we could return the index of the end of the word, indicated by a space. Let’s try that, as shown in Listing 4-7.

Filename: src/main.rs

fn first_word(s: &String) -> usize {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return i;
        }
    }

    s.len()
}

fn main() {}

Listing 4-7: The first_word function that returns a byte index value into the String parameter

Because we need to go through the String element by element and check whether a value is a space, we’ll convert our String to an array of bytes using the as_bytes method:

fn first_word(s: &String) -> usize {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return i;
        }
    }

    s.len()
}

fn main() {}

Next, we create an iterator over the array of bytes using the iter method:

fn first_word(s: &String) -> usize {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return i;
        }
    }

    s.len()
}

fn main() {}

We’ll discuss iterators in more detail in Chapter 13. For now, know that iter is a method that returns each element in a collection and that enumerate wraps the result of iter and returns each element as part of a tuple instead. The first element of the tuple returned from enumerate is the index, and the second element is a reference to the element. This is a bit more convenient than calculating the index ourselves.

Because the enumerate method returns a tuple, we can use patterns to destructure that tuple. We’ll be discussing patterns more in Chapter 6. In the for loop, we specify a pattern that has i for the index in the tuple and &item for the single byte in the tuple. Because we get a reference to the element from .iter().enumerate(), we use & in the pattern.

Inside the for loop, we search for the byte that represents the space by using the byte literal syntax. If we find a space, we return the position. Otherwise, we return the length of the string by using s.len():

fn first_word(s: &String) -> usize {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return i;
        }
    }

    s.len()
}

fn main() {}

We now have a way to find out the index of the end of the first word in the string, but there’s a problem. We’re returning a usize on its own, but it’s only a meaningful number in the context of the &String. In other words, because it’s a separate value from the String, there’s no guarantee that it will still be valid in the future. Consider the program in Listing 4-8 that uses the first_word function from Listing 4-7.

Filename: src/main.rs

Listing 4-8: Storing the result from calling the first_word function and then changing the String contents

This program compiles without any errors, as s retains write permissions after calling first_word. Because word isn’t connected to the state of s at all, word still contains the value 5. We could use that value 5 with the variable s to try to extract the first word out, but this would be a bug because the contents of s have changed since we saved 5 in word.

Having to worry about the index in word getting out of sync with the data in s is tedious and error prone! Managing these indices is even more brittle if we write a second_word function. Its signature would have to look like this:

fn second_word(s: &String) -> (usize, usize) {

Now we’re tracking a starting and an ending index, and we have even more values that were calculated from data in a particular state but aren’t tied to that state at all. We have three unrelated variables floating around that need to be kept in sync.

Luckily, Rust has a solution to this problem: string slices.

String Slices

A string slice is a reference to part of a String, and it looks like this:

Rather than a reference to the entire String (like s2), hello is a reference to a portion of the String, specified in the extra [0..5] bit. We create slices using a range within brackets by specifying [starting_index..ending_index], where starting_index is the first position in the slice and ending_index is one more than the last position in the slice.

Slices are special kinds of references because they are “fat” pointers, or pointers with metadata. Here, the metadata is the length of the slice. We can see this metadata by changing our visualization to peek into the internals of Rust’s data structures:

Observe that the variables hello and world have both a ptr and a len field, which together define the underlined regions of the string on the heap. You can also see here what a String actually looks like: a string is a vector of bytes (Vec<u8>), which contains a length len and a buffer buf that has a pointer ptr and a capacity cap.

Because slices are references, they also change the permissions on referenced data. For example, observe below that when hello is created as a slice of s, then s loses write and own permissions:

Range syntax

With Rust’s .. range syntax, if you want to start at index zero, you can drop the value before the two periods. In other words, these are equal:

#![allow(unused)]
fn main() {
let s = String::from("hello");

let slice = &s[0..2];
let slice = &s[..2];
}

By the same token, if your slice includes the last byte of the String, you can drop the trailing number. That means these are equal:

#![allow(unused)]
fn main() {
let s = String::from("hello");

let len = s.len();

let slice = &s[3..len];
let slice = &s[3..];
}

You can also drop both values to take a slice of the entire string. So these are equal:

#![allow(unused)]
fn main() {
let s = String::from("hello");

let len = s.len();

let slice = &s[0..len];
let slice = &s[..];
}

Rewriting first_word with string slices

With all this information in mind, let’s rewrite first_word to return a slice. The type that signifies “string slice” is written as &str:

Filename: src/main.rs

fn first_word(s: &String) -> &str {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }

    &s[..]
}

fn main() {}

We get the index for the end of the word in the same way as we did in Listing 4-7, by looking for the first occurrence of a space. When we find a space, we return a string slice using the start of the string and the index of the space as the starting and ending indices.

Now when we call first_word, we get back a single value that is tied to the underlying data. The value is made up of a reference to the starting point of the slice and the number of elements in the slice.

Returning a slice would also work for a second_word function:

fn second_word(s: &String) -> &str {

We now have a straightforward API that’s much harder to mess up, because the compiler will ensure the references into the String remain valid. Remember the bug in the program in Listing 4-8, when we got the index to the end of the first word but then cleared the string so our index was invalid? That code was logically incorrect but didn’t show any immediate errors. The problems would show up later if we kept trying to use the first word index with an emptied string. Slices make this bug impossible and let us know we have a problem with our code much sooner. For example:

Filename: src/main.rs

You can see that calling first_word now removes the write permission from s, which prevents us from calling s.clear(). Here’s the compiler error:

$ cargo run
   Compiling ownership v0.1.0 (file:///projects/ownership)
error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable
  --> src/main.rs:18:5
   |
16 |     let word = first_word(&s);
   |                           -- immutable borrow occurs here
17 |
18 |     s.clear(); // error!
   |     ^^^^^^^^^ mutable borrow occurs here
19 |
20 |     println!("the first word is: {word}");
   |                                  ------ immutable borrow later used here

For more information about this error, try `rustc --explain E0502`.
error: could not compile `ownership` (bin "ownership") due to 1 previous error

Recall from the borrowing rules that if we have an immutable reference to something, we cannot also take a mutable reference. Because clear needs to truncate the String, it needs to get a mutable reference. The println! after the call to clear uses the reference in word, so the immutable reference must still be active at that point. Rust disallows the mutable reference in clear and the immutable reference in word from existing at the same time, and compilation fails. Not only has Rust made our API easier to use, but it has also eliminated an entire class of errors at compile time!

String Literals Are Slices

Recall that we talked about string literals being stored inside the binary. Now that we know about slices, we can properly understand string literals:

#![allow(unused)]
fn main() {
let s = "Hello, world!";
}

The type of s here is &str: it’s a slice pointing to that specific point of the binary. This is also why string literals are immutable; &str is an immutable reference.

String Slices as Parameters

Knowing that you can take slices of literals and String values leads us to one more improvement on first_word, and that’s its signature:

fn first_word(s: &String) -> &str {

A more experienced Rustacean would write the signature shown in Listing 4-9 instead because it allows us to use the same function on both &String values and &str values.

fn first_word(s: &str) -> &str {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }

    &s[..]
}

fn main() {
    let my_string = String::from("hello world");

    // `first_word` works on slices of `String`s, whether partial or whole
    let word = first_word(&my_string[0..6]);
    let word = first_word(&my_string[..]);
    // `first_word` also works on references to `String`s, which are equivalent
    // to whole slices of `String`s
    let word = first_word(&my_string);

    let my_string_literal = "hello world";

    // `first_word` works on slices of string literals, whether partial or whole
    let word = first_word(&my_string_literal[0..6]);
    let word = first_word(&my_string_literal[..]);

    // Because string literals *are* string slices already,
    // this works too, without the slice syntax!
    let word = first_word(my_string_literal);
}

Listing 4-9: Improving the first_word function by using a string slice for the type of the s parameter

If we have a string slice, we can pass that directly. If we have a String, we can pass a slice of the String or a reference to the String. This flexibility takes advantage of deref coercions, a feature we will cover in the “Implicit Deref Coercions with Functions and Methods” section of Chapter 15.

Defining a function to take a string slice instead of a reference to a String makes our API more general and useful without losing any functionality:

Filename: src/main.rs

fn first_word(s: &str) -> &str {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }

    &s[..]
}

fn main() {
    let my_string = String::from("hello world");

    // `first_word` works on slices of `String`s, whether partial or whole
    let word = first_word(&my_string[0..6]);
    let word = first_word(&my_string[..]);
    // `first_word` also works on references to `String`s, which are equivalent
    // to whole slices of `String`s
    let word = first_word(&my_string);

    let my_string_literal = "hello world";

    // `first_word` works on slices of string literals, whether partial or whole
    let word = first_word(&my_string_literal[0..6]);
    let word = first_word(&my_string_literal[..]);

    // Because string literals *are* string slices already,
    // this works too, without the slice syntax!
    let word = first_word(my_string_literal);
}

Other Slices

String slices, as you might imagine, are specific to strings. But there’s a more general slice type, too. Consider this array:

#![allow(unused)]
fn main() {
let a = [1, 2, 3, 4, 5];
}

Just as we might want to refer to a part of a string, we might want to refer to part of an array. We’d do so like this:

#![allow(unused)]
fn main() {
let a = [1, 2, 3, 4, 5];

let slice = &a[1..3];

assert_eq!(slice, &[2, 3]);
}

This slice has the type &[i32]. It works the same way as string slices do, by storing a reference to the first element and a length. You’ll use this kind of slice for all sorts of other collections. We’ll discuss these collections in detail when we talk about vectors in Chapter 8.

Summary

Slices are a special kind of reference that refer to sub-ranges of a sequence, like a string or a vector. At runtime, a slice is represented as a “fat pointer” which contains a pointer to the beginning of the range and a length of the range. One advantage of slices over index-based ranges is that the slice cannot be invalidated while it’s being used.