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<ol class="chapter"><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html" class="active"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects that Allow for Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning our Single Threaded Server into a Multithreaded Server</a></li><li><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-nightly-rust.html"><strong aria-hidden="true">21.6.</strong> F - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<a class="header" href="ch05-02-example-structs.html#an-example-program-using-structs" id="an-example-program-using-structs"><h2>An Example Program Using Structs</h2></a>
<p>To understand when we might want to use structs, let’s write a program that
calculates the area of a rectangle. We’ll start with single variables, and then
refactor the program until we’re using structs instead.</p>
<p>Let’s make a new binary project with Cargo called <em>rectangles</em> that will take
the width and height of a rectangle specified in pixels and calculate the area
of the rectangle. Listing 5-8 shows a short program with one way of doing
exactly that in our project’s <em>src/main.rs</em>:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let width1 = 30;
let height1 = 50;
println!(
"The area of the rectangle is {} square pixels.",
area(width1, height1)
);
}
fn area(width: u32, height: u32) -> u32 {
width * height
}
</code></pre></pre>
<p><span class="caption">Listing 5-8: Calculating the area of a rectangle
specified by separate width and height variables</span></p>
<p>Now, run this program using <code>cargo run</code>:</p>
<pre><code class="language-text">The area of the rectangle is 1500 square pixels.
</code></pre>
<p>Even though Listing 5-8 works and figures out the area of the rectangle by
calling the <code>area</code> function with each dimension, we can do better. The width
and the height are related to each other because together they describe one
rectangle.</p>
<p>The issue with this code is evident in the signature of <code>area</code>:</p>
<pre><code class="language-rust ignore">fn area(width: u32, height: u32) -> u32 {
</code></pre>
<p>The <code>area</code> function is supposed to calculate the area of one rectangle, but the
function we wrote has two parameters. The parameters are related, but that’s
not expressed anywhere in our program. It would be more readable and more
manageable to group width and height together. We’ve already discussed one way
we might do that in “The Tuple Type” section of Chapter 3: by using tuples.</p>
<a class="header" href="ch05-02-example-structs.html#refactoring-with-tuples" id="refactoring-with-tuples"><h3>Refactoring with Tuples</h3></a>
<p>Listing 5-9 shows another version of our program that uses tuples:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let rect1 = (30, 50);
println!(
"The area of the rectangle is {} square pixels.",
area(rect1)
);
}
fn area(dimensions: (u32, u32)) -> u32 {
dimensions.0 * dimensions.1
}
</code></pre></pre>
<p><span class="caption">Listing 5-9: Specifying the width and height of the
rectangle with a tuple</span></p>
<p>In one way, this program is better. Tuples let us add a bit of structure, and
we’re now passing just one argument. But in another way, this version is less
clear: tuples don’t name their elements, so our calculation has become more
confusing because we have to index into the parts of the tuple.</p>
<p>It doesn’t matter if we mix up width and height for the area calculation, but
if we want to draw the rectangle on the screen, it would matter! We would have
to keep in mind that <code>width</code> is the tuple index <code>0</code> and <code>height</code> is the tuple
index <code>1</code>. If someone else worked on this code, they would have to figure this
out and keep it in mind as well. It would be easy to forget or mix up these
values and cause errors, because we haven’t conveyed the meaning of our data in
our code.</p>
<a class="header" href="ch05-02-example-structs.html#refactoring-with-structs-adding-more-meaning" id="refactoring-with-structs-adding-more-meaning"><h3>Refactoring with Structs: Adding More Meaning</h3></a>
<p>We use structs to add meaning by labeling the data. We can transform the tuple
we’re using into a data type with a name for the whole as well as names for the
parts, as shown in Listing 5-10:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">struct Rectangle {
width: u32,
height: u32,
}
fn main() {
let rect1 = Rectangle { width: 30, height: 50 };
println!(
"The area of the rectangle is {} square pixels.",
area(&rect1)
);
}
fn area(rectangle: &Rectangle) -> u32 {
rectangle.width * rectangle.height
}
</code></pre></pre>
<p><span class="caption">Listing 5-10: Defining a <code>Rectangle</code> struct</span></p>
<p>Here we’ve defined a struct and named it <code>Rectangle</code>. Inside the curly
brackets, we defined the fields as <code>width</code> and <code>height</code>, both of which have
type <code>u32</code>. Then in <code>main</code>, we created a particular instance of <code>Rectangle</code>
that has a width of 30 and a height of 50.</p>
<p>Our <code>area</code> function is now defined with one parameter, which we’ve named
<code>rectangle</code>, whose type is an immutable borrow of a struct <code>Rectangle</code>
instance. As mentioned in Chapter 4, we want to borrow the struct rather than
take ownership of it. This way, <code>main</code> retains its ownership and can continue
using <code>rect1</code>, which is the reason we use the <code>&</code> in the function signature and
where we call the function.</p>
<p>The <code>area</code> function accesses the <code>width</code> and <code>height</code> fields of the <code>Rectangle</code>
instance. Our function signature for <code>area</code> now says exactly what we mean:
calculate the area of <code>Rectangle</code>, using its <code>width</code> and <code>height</code> fields. This
conveys that the width and height are related to each other, and it gives
descriptive names to the values rather than using the tuple index values of <code>0</code>
and <code>1</code>. This is a win for clarity.</p>
<a class="header" href="ch05-02-example-structs.html#adding-useful-functionality-with-derived-traits" id="adding-useful-functionality-with-derived-traits"><h3>Adding Useful Functionality with Derived Traits</h3></a>
<p>It’d be nice to be able to print an instance of <code>Rectangle</code> while we’re
debugging our program and see the values for all its fields. Listing 5-11 tries
using the <code>println!</code> macro as we have used in previous chapters. This won’t
work, however:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore">struct Rectangle {
width: u32,
height: u32,
}
fn main() {
let rect1 = Rectangle { width: 30, height: 50 };
println!("rect1 is {}", rect1);
}
</code></pre>
<p><span class="caption">Listing 5-11: Attempting to print a <code>Rectangle</code>
instance</span></p>
<p>When we run this code, we get an error with this core message:</p>
<pre><code class="language-text">error[E0277]: the trait bound `Rectangle: std::fmt::Display` is not satisfied
</code></pre>
<p>The <code>println!</code> macro can do many kinds of formatting, and by default, curly
brackets tell <code>println!</code> to use formatting known as <code>Display</code>: output intended
for direct end user consumption. The primitive types we’ve seen so far
implement <code>Display</code> by default, because there’s only one way you’d want to show
a <code>1</code> or any other primitive type to a user. But with structs, the way
<code>println!</code> should format the output is less clear because there are more
display possibilities: Do you want commas or not? Do you want to print the
curly brackets? Should all the fields be shown? Due to this ambiguity, Rust
doesn’t try to guess what we want, and structs don’t have a provided
implementation of <code>Display</code>.</p>
<p>If we continue reading the errors, we’ll find this helpful note:</p>
<pre><code class="language-text">`Rectangle` cannot be formatted with the default formatter; try using
`:?` instead if you are using a format string
</code></pre>
<p>Let’s try it! The <code>println!</code> macro call will now look like <code>println!("rect1 is {:?}", rect1);</code>. Putting the specifier <code>:?</code> inside the curly brackets tells
<code>println!</code> we want to use an output format called <code>Debug</code>. <code>Debug</code> is a trait
that enables us to print our struct in a way that is useful for developers so
we can see its value while we’re debugging our code.</p>
<p>Run the code with this change. Drat! We still get an error:</p>
<pre><code class="language-text">error[E0277]: the trait bound `Rectangle: std::fmt::Debug` is not satisfied
</code></pre>
<p>But again, the compiler gives us a helpful note:</p>
<pre><code class="language-text">`Rectangle` cannot be formatted using `:?`; if it is defined in your
crate, add `#[derive(Debug)]` or manually implement it
</code></pre>
<p>Rust <em>does</em> include functionality to print out debugging information, but we
have to explicitly opt in to make that functionality available for our struct.
To do that, we add the annotation <code>#[derive(Debug)]</code> just before the struct
definition, as shown in Listing 5-12:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">#[derive(Debug)]
struct Rectangle {
width: u32,
height: u32,
}
fn main() {
let rect1 = Rectangle { width: 30, height: 50 };
println!("rect1 is {:?}", rect1);
}
</code></pre></pre>
<p><span class="caption">Listing 5-12: Adding the annotation to derive the <code>Debug</code>
trait and printing the <code>Rectangle</code> instance using debug formatting</span></p>
<p>Now when we run the program, we won’t get any errors, and we’ll see the
following output:</p>
<pre><code class="language-text">rect1 is Rectangle { width: 30, height: 50 }
</code></pre>
<p>Nice! It’s not the prettiest output, but it shows the values of all the fields
for this instance, which would definitely help during debugging. When we have
larger structs, it’s useful to have output that’s a bit easier to read; in
those cases, we can use <code>{:#?}</code> instead of <code>{:?}</code> in the <code>println!</code> string.
When we use the <code>{:#?}</code> style in the example, the output will look like this:</p>
<pre><code class="language-text">rect1 is Rectangle {
width: 30,
height: 50
}
</code></pre>
<p>Rust has provided a number of traits for us to use with the <code>derive</code> annotation
that can add useful behavior to our custom types. Those traits and their
behaviors are listed in Appendix C, “Derivable Traits.” We’ll cover how to
implement these traits with custom behavior as well as how to create your own
traits in Chapter 10.</p>
<p>Our <code>area</code> function is very specific: it only computes the area of rectangles.
It would be helpful to tie this behavior more closely to our <code>Rectangle</code>
struct, because it won’t work with any other type. Let’s look at how we can
continue to refactor this code by turning the <code>area</code> function into an <code>area</code>
<em>method</em> defined on our <code>Rectangle</code> type.</p>
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