Use borrowed types for arguments

Description

Using a target of a deref coercion can increase the flexibility of your code when you are deciding which argument type to use for a function argument. In this way, the function will accept more input types.

This is not limited to slice-able or fat pointer types. In fact, you should always prefer using the borrowed type over borrowing the owned type. Such as &str over &String, &[T] over &Vec<T>, or &T over &Box<T>.

Using borrowed types you can avoid layers of indirection for those instances where the owned type already provides a layer of indirection. For instance, a String has a layer of indirection, so a &String will have two layers of indirection. We can avoid this by using &str instead, and letting &String coerce to a &str whenever the function is invoked.

Example

For this example, we will illustrate some differences for using &String as a function argument versus using a &str, but the ideas apply as well to using &Vec<T> versus using a &[T] or using a &Box<T> versus a &T.

Consider an example where we wish to determine if a word contains three consecutive vowels. We don’t need to own the string to determine this, so we will take a reference.

The code might look something like this:

fn three_vowels(word: &String) -> bool {
    let mut vowel_count = 0;
    for c in word.chars() {
        match c {
            'a' | 'e' | 'i' | 'o' | 'u' => {
                vowel_count += 1;
                if vowel_count >= 3 {
                    return true;
                }
            }
            _ => vowel_count = 0,
        }
    }
    false
}

fn main() {
    let ferris = "Ferris".to_string();
    let curious = "Curious".to_string();
    println!("{}: {}", ferris, three_vowels(&ferris));
    println!("{}: {}", curious, three_vowels(&curious));

    // This works fine, but the following two lines would fail:
    // println!("Ferris: {}", three_vowels("Ferris"));
    // println!("Curious: {}", three_vowels("Curious"));
}

This works fine because we are passing a &String type as a parameter. If we remove the comments on the last two lines, the example will fail. This is because a &str type will not coerce to a &String type. We can fix this by simply modifying the type for our argument.

For instance, if we change our function declaration to:

fn three_vowels(word: &str) -> bool {

then both versions will compile and print the same output.

Ferris: false
Curious: true

But wait, that’s not all! There is more to this story. It’s likely that you may say to yourself: that doesn’t matter, I will never be using a &'static str as an input anyways (as we did when we used "Ferris"). Even ignoring this special example, you may still find that using &str will give you more flexibility than using a &String.

Let’s now take an example where someone gives us a sentence, and we want to determine if any of the words in the sentence contain three consecutive vowels. We probably should make use of the function we have already defined and simply feed in each word from the sentence.

An example of this could look like this:

fn three_vowels(word: &str) -> bool {
    let mut vowel_count = 0;
    for c in word.chars() {
        match c {
            'a' | 'e' | 'i' | 'o' | 'u' => {
                vowel_count += 1;
                if vowel_count >= 3 {
                    return true;
                }
            }
            _ => vowel_count = 0,
        }
    }
    false
}

fn main() {
    let sentence_string =
        "Once upon a time, there was a friendly curious crab named Ferris".to_string();
    for word in sentence_string.split(' ') {
        if three_vowels(word) {
            println!("{word} has three consecutive vowels!");
        }
    }
}

Running this example using our function declared with an argument type &str will yield

curious has three consecutive vowels!

However, this example will not run when our function is declared with an argument type &String. This is because string slices are a &str and not a &String which would require an allocation to be converted to &String which is not implicit, whereas converting from String to &str is cheap and implicit.

See also

Last change: 2024-06-11, commit: 3aa5c21