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<title>An Example Program Using Structs - The Rust Programming Language</title>
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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="chapter-item expanded affix "><a href="foreword.html">Foreword</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html" class="active"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="chapter-item expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li class="chapter-item expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="chapter-item expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="chapter-item expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="chapter-item expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="chapter-item expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="chapter-item expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="chapter-item expanded "><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 class="chapter-item expanded "><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 class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<h2 id="an-example-program-using-structs"><a class="header" href="#an-example-program-using-structs">An Example Program Using Structs</a></h2>
<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="playground"><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-console">$ cargo run
Compiling rectangles v0.1.0 (file:///projects/rectangles)
Finished dev [unoptimized + debuginfo] target(s) in 0.42s
Running `target/debug/rectangles`
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"><span class="boring">fn main() {
</span><span class="boring"> let width1 = 30;
</span><span class="boring"> let height1 = 50;
</span><span class="boring">
</span><span class="boring"> println!(
</span><span class="boring"> "The area of the rectangle is {} square pixels.",
</span><span class="boring"> area(width1, height1)
</span><span class="boring"> );
</span><span class="boring">}
</span><span class="boring">
</span>fn area(width: u32, height: u32) -> u32 {
<span class="boring"> width * height
</span><span class="boring">}
</span></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 <a href="ch03-02-data-types.html#the-tuple-type">“The Tuple Type”</a><!-- ignore --> section
of Chapter 3: by using tuples.</p>
<h3 id="refactoring-with-tuples"><a class="header" href="#refactoring-with-tuples">Refactoring with Tuples</a></h3>
<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="playground"><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>
<h3 id="refactoring-with-structs-adding-more-meaning"><a class="header" href="#refactoring-with-structs-adding-more-meaning">Refactoring with Structs: Adding More Meaning</a></h3>
<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="playground"><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>
<h3 id="adding-useful-functionality-with-derived-traits"><a class="header" href="#adding-useful-functionality-with-derived-traits">Adding Useful Functionality with Derived Traits</a></h3>
<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 <a href="../std/macro.println.html"><code>println!</code> macro</a><!-- ignore --> 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 does_not_compile">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 compile this code, we get an error with this core message:</p>
<pre><code class="language-text">error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`
</code></pre>
<p>The <code>println!</code> macro can do many kinds of formatting, and by default, the 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"> = help: the trait `std::fmt::Display` is not implemented for `Rectangle`
= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
</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>. The <code>Debug</code> trait
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>Compile the code with this change. Drat! We still get an error:</p>
<pre><code class="language-text">error[E0277]: `Rectangle` doesn't implement `Debug`
</code></pre>
<p>But again, the compiler gives us a helpful note:</p>
<pre><code class="language-text"> = help: the trait `Debug` is not implemented for `Rectangle`
= note: add `#[derive(Debug)]` or manually implement `Debug`
</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 outer attribute <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="playground"><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 attribute 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-console">$ cargo run
Compiling rectangles v0.1.0 (file:///projects/rectangles)
Finished dev [unoptimized + debuginfo] target(s) in 0.48s
Running `target/debug/rectangles`
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-console">$ cargo run
Compiling rectangles v0.1.0 (file:///projects/rectangles)
Finished dev [unoptimized + debuginfo] target(s) in 0.48s
Running `target/debug/rectangles`
rect1 is Rectangle {
width: 30,
height: 50,
}
</code></pre>
<p>Another way to print out a value using the <code>Debug</code> format is by using the
<a href="../std/macro.dbg.html"><code>dbg!</code> macro</a> <!-- ignore -->. The <code>dbg!</code> macro takes ownership of an
expression, prints the file and line number of where that <code>dbg!</code> macro call
occurs in your code along with the resulting value of that expression, and
returns ownership of the value. Calling the <code>dbg!</code> macro prints to the standard
error console stream (<code>stderr</code>), as opposed to <code>println!</code> which prints to the
standard output console stream (<code>stdout</code>). We’ll talk more about <code>stderr</code> and
<code>stdout</code> in the <a href="ch12-06-writing-to-stderr-instead-of-stdout.html">“Writing Error Messages to Standard Error Instead of Standard
Output” section in Chapter 12</a><!-- ignore -->. Here’s an example where
we’re interested in the value that gets assigned to the <code>width</code> field, as well
as the value of the whole struct in <code>rect1</code>:</p>
<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
struct Rectangle {
width: u32,
height: u32,
}
fn main() {
let scale = 2;
let rect1 = Rectangle {
width: dbg!(30 * scale),
height: 50,
};
dbg!(&rect1);
}
</code></pre></pre>
<p>We can put <code>dbg!</code> around the expression <code>30 * scale</code> and, because <code>dbg!</code>
returns ownership of the expression’s value, the <code>width</code> field will get the
same value as if we didn’t have the <code>dbg!</code> call there. We don’t want <code>dbg!</code> to
take ownership of <code>rect1</code>, so we use a reference to <code>dbg!</code> in the next call.
Here’s what the output of this example looks like:</p>
<pre><code class="language-console">$ cargo run
Compiling rectangles v0.1.0 (file:///projects/rectangles)
Finished dev [unoptimized + debuginfo] target(s) in 0.61s
Running `target/debug/rectangles`
[src/main.rs:10] 30 * scale = 60
[src/main.rs:14] &rect1 = Rectangle {
width: 60,
height: 50,
}
</code></pre>
<p>We can see the first bit of output came from <em>src/main.rs</em> line 10, where we’re
debugging the expression <code>30 * scale</code>, and its resulting value is 60 (the
<code>Debug</code> formatting implemented for integers is to print only their value). The
<code>dbg!</code> call on line 14 of <em>src/main.rs</em> outputs the value of <code>&rect1</code>, which is
the <code>Rectangle</code> struct. This output uses the pretty <code>Debug</code> formatting of the
<code>Rectangle</code> type. The <code>dbg!</code> macro can be really helpful when you’re trying to
figure out what your code is doing!</p>
<p>In addition to the <code>Debug</code> trait, Rust has provided a number of traits for us
to use with the <code>derive</code> attribute that can add useful behavior to our custom
types. Those traits and their behaviors are listed in <a href="appendix-03-derivable-traits.html">Appendix C</a><!--
ignore -->. We’ll cover how to implement these traits with custom behavior as
well as how to create your own traits in Chapter 10. There are also many
attributes other than <code>derive</code>; for more information, see <a href="../reference/attributes.html">the “Attributes”
section of the Rust Reference</a>.</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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