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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Update</a></li><li class="chapter-item expanded "><a href="Chapter_1.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="Chapter_2.html"><strong aria-hidden="true">3.</strong> Who am I?</a></li><li class="chapter-item expanded "><a href="Chapter_3.html"><strong aria-hidden="true">4.</strong> Writing Rust in Easy English</a></li><li class="chapter-item expanded "><a href="Chapter_4.html"><strong aria-hidden="true">5.</strong> Rust Playground</a></li><li class="chapter-item expanded "><a href="Chapter_5.html"><strong aria-hidden="true">6.</strong> 🚧 and ⚠️</a></li><li class="chapter-item expanded "><a href="Chapter_6.html"><strong aria-hidden="true">7.</strong> Comments</a></li><li class="chapter-item expanded "><a href="Chapter_7.html"><strong aria-hidden="true">8.</strong> Types</a></li><li class="chapter-item expanded "><a href="Chapter_8.html"><strong aria-hidden="true">9.</strong> Type inference</a></li><li class="chapter-item expanded "><a href="Chapter_9.html"><strong aria-hidden="true">10.</strong> Printing 'hello, world!'</a></li><li class="chapter-item expanded "><a href="Chapter_10.html"><strong aria-hidden="true">11.</strong> Display and debug</a></li><li class="chapter-item expanded "><a href="Chapter_11.html"><strong aria-hidden="true">12.</strong> Mutability (changing)</a></li><li class="chapter-item expanded "><a href="Chapter_12.html"><strong aria-hidden="true">13.</strong> The stack, the heap, and pointers</a></li><li class="chapter-item expanded "><a href="Chapter_13.html"><strong aria-hidden="true">14.</strong> More about printing</a></li><li class="chapter-item expanded "><a href="Chapter_14.html"><strong aria-hidden="true">15.</strong> Strings</a></li><li class="chapter-item expanded "><a href="Chapter_15.html"><strong aria-hidden="true">16.</strong> const and static</a></li><li class="chapter-item expanded "><a href="Chapter_16.html"><strong aria-hidden="true">17.</strong> More on references</a></li><li class="chapter-item expanded "><a href="Chapter_17.html"><strong aria-hidden="true">18.</strong> Mutable references</a></li><li class="chapter-item expanded "><a href="Chapter_18.html"><strong aria-hidden="true">19.</strong> Giving references to functions</a></li><li class="chapter-item expanded "><a href="Chapter_19.html"><strong aria-hidden="true">20.</strong> Copy types</a></li><li class="chapter-item expanded "><a href="Chapter_20.html"><strong aria-hidden="true">21.</strong> Collection types</a></li><li class="chapter-item expanded "><a href="Chapter_21.html"><strong aria-hidden="true">22.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="Chapter_22.html"><strong aria-hidden="true">23.</strong> Tuples</a></li><li class="chapter-item expanded "><a href="Chapter_23.html"><strong aria-hidden="true">24.</strong> Control flow</a></li><li class="chapter-item expanded "><a href="Chapter_24.html"><strong aria-hidden="true">25.</strong> Structs</a></li><li class="chapter-item expanded "><a href="Chapter_25.html"><strong aria-hidden="true">26.</strong> Enums</a></li><li class="chapter-item expanded "><a href="Chapter_26.html"><strong aria-hidden="true">27.</strong> Loops</a></li><li class="chapter-item expanded "><a href="Chapter_27.html"><strong aria-hidden="true">28.</strong> Implementing structs and enums</a></li><li class="chapter-item expanded "><a href="Chapter_28.html"><strong aria-hidden="true">29.</strong> Destructuring</a></li><li class="chapter-item expanded "><a href="Chapter_29.html"><strong aria-hidden="true">30.</strong> References and the dot operator</a></li><li class="chapter-item expanded "><a href="Chapter_30.html"><strong aria-hidden="true">31.</strong> Generics</a></li><li class="chapter-item expanded "><a href="Chapter_31.html"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html"><strong aria-hidden="true">33.</strong> Other collections</a></li><li class="chapter-item expanded "><a href="Chapter_33.html"><strong aria-hidden="true">34.</strong> The ? operator</a></li><li class="chapter-item expanded "><a href="Chapter_34.html"><strong aria-hidden="true">35.</strong> Traits</a></li><li class="chapter-item expanded "><a href="Chapter_35.html"><strong aria-hidden="true">36.</strong> Chaining methods</a></li><li class="chapter-item expanded "><a href="Chapter_36.html"><strong aria-hidden="true">37.</strong> Iterators</a></li><li class="chapter-item expanded "><a href="Chapter_37.html"><strong aria-hidden="true">38.</strong> Closures</a></li><li class="chapter-item expanded "><a href="Chapter_38.html"><strong aria-hidden="true">39.</strong> The dbg! macro and .inspect</a></li><li class="chapter-item expanded "><a href="Chapter_39.html"><strong aria-hidden="true">40.</strong> Types of &str</a></li><li class="chapter-item expanded "><a href="Chapter_40.html"><strong aria-hidden="true">41.</strong> Lifetimes</a></li><li class="chapter-item expanded "><a href="Chapter_41.html"><strong aria-hidden="true">42.</strong> Interior mutability</a></li><li class="chapter-item expanded "><a href="Chapter_42.html"><strong aria-hidden="true">43.</strong> RefCell</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Mutex</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> RwLock</a></li><li class="chapter-item expanded "><a href="Chapter_45.html" class="active"><strong aria-hidden="true">46.</strong> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_67.html"><strong aria-hidden="true">68.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_68.html"><strong aria-hidden="true">69.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_69.html"><strong aria-hidden="true">70.</strong> The end?</a></li></ol>
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<h2><a class="header" href="#cow" id="cow">Cow</a></h2>
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<p>Cow is a very convenient enum. It means "clone on write" and lets you return a <code>&str</code> if you don't need a <code>String</code>, and a <code>String</code> if you need it. (It can also do the same with arrays vs. Vecs, etc.)</p>
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<p>To understand it, let's look at the signature. It says:</p>
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<pre><pre class="playground"><code class="language-rust">pub enum Cow<'a, B>
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where
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B: 'a + ToOwned + ?Sized,
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{
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Borrowed(&'a B),
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Owned(<B as ToOwned>::Owned),
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}
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fn main() {}
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</code></pre></pre>
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<p>You know right away that <code>'a</code> means it works with references. The <code>ToOwned</code> trait means that it is a type that can be turned into an owned type. For example, <code>str</code> is usually a reference (<code>&str</code>) and you can turn it into an owned <code>String</code>.</p>
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<p>Next is <code>?Sized</code>. This means "maybe Sized, but maybe not". Almost every type in Rust is Sized, but types like <code>str</code> are not. That is why we need a <code>&</code> for a <code>str</code>, because the compiler doesn't know the size. So if you want a trait that can use something like a <code>str</code>, you add <code>?Sized.</code></p>
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<p>Next are the <code>enum</code> variants. They are <code>Borrowed</code> and <code>Owned</code>.</p>
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<p>Imagine that you have a function that returns <code>Cow<'static, str></code>. If you tell the function to return <code>"My message".into()</code>, it will look at the type: "My message" is a <code>str</code>. This is a <code>Borrowed</code> type, so it chooses <code>Borrowed(&'a B)</code>. So it becomes <code>Cow::Borrowed(&'static str)</code>.</p>
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<p>And if you give it a <code>format!("{}", "My message").into()</code> then it will look at the type. This time it is a <code>String</code>, because <code>format!</code> makes a <code>String</code>. So this time it will select "Owned".</p>
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<p>Here is an example to test <code>Cow</code>. We will put a number into a function that returns a <code>Cow<'static, str></code>. Depending on the number, it will create a <code>&str</code> or a <code>String</code>. Then it uses <code>.into()</code> to turn it into a <code>Cow</code>. When you do that, it will choose either <code>Cow:::Borrowed</code> or <code>Cow::Owned</code>. Then we will match to see which one it chose.</p>
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<pre><pre class="playground"><code class="language-rust">use std::borrow::Cow;
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fn modulo_3(input: u8) -> Cow<'static, str> {
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match input % 3 {
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0 => "Remainder is 0".into(),
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1 => "Remainder is 1".into(),
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remainder => format!("Remainder is {}", remainder).into(),
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}
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}
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fn main() {
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for number in 1..=6 {
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match modulo_3(number) {
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Cow::Borrowed(message) => println!("{} went in. The Cow is borrowed with this message: {}", number, message),
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Cow::Owned(message) => println!("{} went in. The Cow is owned with this message: {}", number, message),
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}
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}
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}
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</code></pre></pre>
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<p>This prints:</p>
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<pre><code class="language-text">1 went in. The Cow is borrowed with this message: Remainder is 1
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2 went in. The Cow is owned with this message: Remainder is 2
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3 went in. The Cow is borrowed with this message: Remainder is 0
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4 went in. The Cow is borrowed with this message: Remainder is 1
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5 went in. The Cow is owned with this message: Remainder is 2
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6 went in. The Cow is borrowed with this message: Remainder is 0
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</code></pre>
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<p><code>Cow</code> has some other methods like <code>into_owned</code> or <code>into_borrowed</code> so you can change it if you need to.</p>
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