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4 years ago
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<h1><a class="header" href="#use-borrowed-types-for-arguments" id="use-borrowed-types-for-arguments">Use borrowed types for arguments</a></h1>
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<h2><a class="header" href="#description" id="description">Description</a></h2>
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<p>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.
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In this way, the function will accept more input types.</p>
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<p>This is not limited to slice-able or fat pointer types. In fact you should always prefer using the <strong>borrowed type</strong> over <strong>borrowing the owned type</strong>. E.g., <code>&str</code> over <code>&String</code>, <code>&[T]</code> over <code>&Vec<T></code>, or <code>&T</code> over <code>&Box<T></code>.</p>
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<p>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 <code>String</code> has a layer of indirection, so a <code>&String</code> will have two layers of indrection.
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We can avoid this by using <code>&str</code> instead, and letting <code>&String</code> coerce to a <code>&str</code> whenever the function is invoked. </p>
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<h2><a class="header" href="#example" id="example">Example</a></h2>
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<p>For this example, we will illustrate some differences for using <code>&String</code> as a function argument versus using a <code>&str</code>, but the ideas apply as well to using <code>&Vec<T></code> versus using a <code>&[T]</code> or using a <code>&T</code> versus a <code>&Box<T></code>.</p>
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<p>Consider an example where we wish to determine if a word contains three consecutive vowels.
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We don't need to own the string to determine this, so we will take a reference.</p>
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<p>The code might look something like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2018">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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</code></pre></pre>
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<p>This works fine because we are passing a <code>&String</code> type as a parameter.
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If we comment in the last two lines this example fails because a <code>&str</code> type will not coerce to a <code>&String</code> type.
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We can fix this by simply modifying the type for our argument. </p>
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<p>For instance, if we change our function declaration to:</p>
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<pre><code class="language-rust ignore">fn three_vowels(word: &str) -> bool {
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</code></pre>
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<p>then both versions will compile and print the same output.</p>
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<pre><code class="language-bash">Ferris: false
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Curious: true
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</code></pre>
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<p>But wait, that's not all! There is more to this story.
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It's likely that you may say to yourself: that doesn't matter, I will never be using a <code>&'static str</code> as an input anways (as we did when we used <code>"Ferris"</code>).
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Even ignoring this special example, you may still find that using <code>&str</code> will give you more flexibility than using a <code>&String</code>.</p>
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<p>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 has a word that contains three consecutive vowels.
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We probably should make use of the function we have already defined and simply feed in each word from the sentence.</p>
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<p>An example of this could look like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2018">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!("{} has three consecutive vowels!", word);
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}
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}
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}
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</code></pre></pre>
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<p>Running this example using our function declared with an argument type <code>&str</code> will yield</p>
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<pre><code class="language-bash">curious has three consecutive vowels!
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</code></pre>
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<p>However, this example will not run when our function is declared with an argument type <code>&String</code>.
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This is because string slices are a <code>&str</code> and not a <code>&String</code> which would require an allocation to be converted to <code>&String</code> which is not implicit, whereas converting from <code>String</code> to <code>&str</code> is cheap and implicit.</p>
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<h2><a class="header" href="#see-also" id="see-also">See also</a></h2>
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<ul>
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<li><a href="https://doc.rust-lang.org/reference/type-coercions.html">Rust Language Reference on Type Coercions</a></li>
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<li>For more discussion on how to handle <code>String</code> and <code>&str</code> see <a href="https://web.archive.org/web/20201112023149/https://hermanradtke.com/2015/05/03/string-vs-str-in-rust-functions.html">this blog series (2015)</a> by Herman J. Radtke III.</li>
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