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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> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_54.html" class="active"><strong aria-hidden="true">55.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> The end?</a></li></ol>
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<h2 id="box-around-traits"><a class="header" href="#box-around-traits">Box around traits</a></h2>
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<p><code>Box</code> is very useful for returning traits. You know that you can write traits in generic functions like in this example:</p>
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<pre><pre class="playground"><code class="language-rust">use std::fmt::Display;
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struct DoesntImplementDisplay {}
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fn displays_it<T: Display>(input: T) {
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println!("{}", input);
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}
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fn main() {}
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</code></pre></pre>
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<p>This only takes something with <code>Display</code>, so it can't accept our struct <code>DoesntImplementDisplay</code>. But it can take in a lot of others like <code>String</code>.</p>
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<p>You also saw that we can use <code>impl Trait</code> to return other traits, or closures. <code>Box</code> can be used in a similar way. You can use a <code>Box</code> because otherwise the compiler won't know the size of the value. This example shows that a trait can be used on something of any size:</p>
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<pre><pre class="playground"><code class="language-rust">#![allow(dead_code)] // Tell the compiler to be quiet
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use std::mem::size_of; // This gives the size of a type
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trait JustATrait {} // We will implement this on everything
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enum EnumOfNumbers {
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I8(i8),
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AnotherI8(i8),
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OneMoreI8(i8),
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}
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impl JustATrait for EnumOfNumbers {}
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struct StructOfNumbers {
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an_i8: i8,
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another_i8: i8,
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one_more_i8: i8,
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}
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impl JustATrait for StructOfNumbers {}
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enum EnumOfOtherTypes {
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I8(i8),
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AnotherI8(i8),
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Collection(Vec<String>),
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}
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impl JustATrait for EnumOfOtherTypes {}
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struct StructOfOtherTypes {
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an_i8: i8,
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another_i8: i8,
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a_collection: Vec<String>,
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}
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impl JustATrait for StructOfOtherTypes {}
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struct ArrayAndI8 {
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array: [i8; 1000], // This one will be very large
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an_i8: i8,
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in_u8: u8,
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}
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impl JustATrait for ArrayAndI8 {}
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fn main() {
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println!(
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"{}, {}, {}, {}, {}",
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size_of::<EnumOfNumbers>(),
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size_of::<StructOfNumbers>(),
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size_of::<EnumOfOtherTypes>(),
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size_of::<StructOfOtherTypes>(),
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size_of::<ArrayAndI8>(),
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);
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}
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</code></pre></pre>
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<p>When we print the size of these, we get <code>2, 3, 32, 32, 1002</code>. So if you were to do this, it would give an error:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// ⚠️
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fn returns_just_a_trait() -> JustATrait {
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let some_enum = EnumOfNumbers::I8(8);
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some_enum
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>It says:</p>
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<pre><code class="language-text">error[E0746]: return type cannot have an unboxed trait object
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--> src\main.rs:53:30
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53 | fn returns_just_a_trait() -> JustATrait {
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| ^^^^^^^^^^ doesn't have a size known at compile-time
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</code></pre>
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<p>And this is true, because the size could be 2, 3, 32, 1002, or anything else. So we put it in a <code>Box</code> instead. Here we also add the keyword <code>dyn</code>. <code>dyn</code> is a word that shows you that you are talking about a trait, not a struct or anything else.</p>
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<p>So you can change the function to this:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// 🚧
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fn returns_just_a_trait() -> Box<dyn JustATrait> {
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let some_enum = EnumOfNumbers::I8(8);
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Box::new(some_enum)
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>And now it works, because on the stack is just a <code>Box</code> and we know the size of <code>Box</code>.</p>
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<p>You see this a lot in the form <code>Box<dyn Error></code>, because sometimes you can have more than one possible error.</p>
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<p>We can quickly create two error types to show this. To make an official error type, you have to implement <code>std::error::Error</code> for it. That part is easy: just write impl <code>std::error::Error {}</code>. But errors also need <code>Debug</code> and <code>Display</code> so they can give information on the problem. <code>Debug</code> is easy with <code>#[derive(Debug)]</code> but <code>Display</code> needs the <code>.fmt()</code> method. We did this once before.</p>
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<p>The code looks like this:</p>
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<pre><pre class="playground"><code class="language-rust">use std::error::Error;
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use std::fmt;
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#[derive(Debug)]
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struct ErrorOne;
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impl Error for ErrorOne {} // Now it is an error type with Debug. Time for Display:
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impl fmt::Display for ErrorOne {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "You got the first error!") // All it does is write this message
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}
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}
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#[derive(Debug)] // Do the same thing with ErrorTwo
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struct ErrorTwo;
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impl Error for ErrorTwo {}
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impl fmt::Display for ErrorTwo {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "You got the second error!")
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}
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}
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// Make a function that just returns a String or an error
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fn returns_errors(input: u8) -> Result<String, Box<dyn Error>> { // With Box<dyn Error> you can return anything that has the Error trait
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match input {
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0 => Err(Box::new(ErrorOne)), // Don't forget to put it in a box
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1 => Err(Box::new(ErrorTwo)),
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_ => Ok("Looks fine to me".to_string()), // This is the success type
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}
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}
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fn main() {
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let vec_of_u8s = vec![0_u8, 1, 80]; // Three numbers to try out
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for number in vec_of_u8s {
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match returns_errors(number) {
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Ok(input) => println!("{}", input),
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Err(message) => println!("{}", 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 will print:</p>
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<pre><code class="language-text">You got the first error!
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You got the second error!
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Looks fine to me
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</code></pre>
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<p>If we didn't have a <code>Box<dyn Error></code> and wrote this, we would have a problem:</p>
|
|
<pre><pre class="playground"><code class="language-rust">
|
|
<span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>// ⚠️
|
|
fn returns_errors(input: u8) -> Result<String, Error> {
|
|
match input {
|
|
0 => Err(ErrorOne),
|
|
1 => Err(ErrorTwo),
|
|
_ => Ok("Looks fine to me".to_string()),
|
|
}
|
|
}
|
|
<span class="boring">}
|
|
</span></code></pre></pre>
|
|
<p>It will tell you:</p>
|
|
<pre><code class="language-text">21 | fn returns_errors(input: u8) -> Result<String, Error> {
|
|
| ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
|
|
</code></pre>
|
|
<p>This is not surprising, because we know that a trait can work on many things, and they each have different sizes.</p>
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