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138 lines
4.6 KiB
Markdown
138 lines
4.6 KiB
Markdown
# Use borrowed types for arguments
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## Description
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Using a target of a deref coercion can increase the flexibility of your code
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when you are deciding which argument type to use for a function argument. In
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this way, the function will accept more input types.
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This is not limited to slice-able or fat pointer types. In fact, you should
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always prefer using the **borrowed type** over **borrowing the owned type**.
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Such as `&str` over `&String`, `&[T]` over `&Vec<T>`, or `&T` over `&Box<T>`.
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Using borrowed types you can avoid layers of indirection for those instances
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where the owned type already provides a layer of indirection. For instance, a
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`String` has a layer of indirection, so a `&String` will have two layers of
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indirection. We can avoid this by using `&str` instead, and letting `&String`
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coerce to a `&str` whenever the function is invoked.
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## Example
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For this example, we will illustrate some differences for using `&String` as a
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function argument versus using a `&str`, but the ideas apply as well to using
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`&Vec<T>` versus using a `&[T]` or using a `&Box<T>` versus a `&T`.
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Consider an example where we wish to determine if a word contains three
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consecutive vowels. We don't need to own the string to determine this, so we
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will take a reference.
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The code might look something like this:
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```rust
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fn three_vowels(word: &String) -> bool {
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let mut vowel_count = 0;
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for c in word.chars() {
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match c {
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'a' | 'e' | 'i' | 'o' | 'u' => {
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vowel_count += 1;
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if vowel_count >= 3 {
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return true;
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}
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}
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_ => vowel_count = 0,
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}
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}
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false
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}
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fn main() {
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let ferris = "Ferris".to_string();
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let curious = "Curious".to_string();
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println!("{}: {}", ferris, three_vowels(&ferris));
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println!("{}: {}", curious, three_vowels(&curious));
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// This works fine, but the following two lines would fail:
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// println!("Ferris: {}", three_vowels("Ferris"));
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// println!("Curious: {}", three_vowels("Curious"));
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}
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```
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This works fine because we are passing a `&String` type as a parameter. If we
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remove the comments on the last two lines, the example will fail. This is
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because a `&str` type will not coerce to a `&String` type. We can fix this by
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simply modifying the type for our argument.
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For instance, if we change our function declaration to:
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```rust, ignore
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fn three_vowels(word: &str) -> bool {
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```
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then both versions will compile and print the same output.
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```bash
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Ferris: false
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Curious: true
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```
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But wait, that's not all! There is more to this story. It's likely that you may
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say to yourself: that doesn't matter, I will never be using a `&'static str` as
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an input anyways (as we did when we used `"Ferris"`). Even ignoring this special
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example, you may still find that using `&str` will give you more flexibility
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than using a `&String`.
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Let's now take an example where someone gives us a sentence, and we want to
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determine if any of the words in the sentence contain three consecutive vowels.
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We probably should make use of the function we have already defined and simply
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feed in each word from the sentence.
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An example of this could look like this:
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```rust
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fn three_vowels(word: &str) -> bool {
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let mut vowel_count = 0;
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for c in word.chars() {
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match c {
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'a' | 'e' | 'i' | 'o' | 'u' => {
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vowel_count += 1;
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if vowel_count >= 3 {
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return true;
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}
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}
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_ => vowel_count = 0,
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}
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}
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false
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}
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fn main() {
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let sentence_string =
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"Once upon a time, there was a friendly curious crab named Ferris".to_string();
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for word in sentence_string.split(' ') {
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if three_vowels(word) {
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println!("{word} has three consecutive vowels!");
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}
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}
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}
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```
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Running this example using our function declared with an argument type `&str`
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will yield
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```bash
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curious has three consecutive vowels!
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```
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However, this example will not run when our function is declared with an
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argument type `&String`. This is because string slices are a `&str` and not a
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`&String` which would require an allocation to be converted to `&String` which
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is not implicit, whereas converting from `String` to `&str` is cheap and
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implicit.
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## See also
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- [Rust Language Reference on Type Coercions](https://doc.rust-lang.org/reference/type-coercions.html)
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- For more discussion on how to handle `String` and `&str` see
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[this blog series (2015)](https://web.archive.org/web/20201112023149/https://hermanradtke.com/2015/05/03/string-vs-str-in-rust-functions.html)
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by Herman J. Radtke III
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