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<ul class="chapter"><li><a href="ch01-00-introduction.html"><strong>1.</strong> Introduction</a></li><li><ul class="section"><li><a href="ch01-01-installation.html"><strong>1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong>1.2.</strong> Hello, World!</a></li></ul></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong>2.</strong> Guessing Game Tutorial</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong>3.</strong> Common Programming Concepts</a></li><li><ul class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong>3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong>3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong>3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong>3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong>3.5.</strong> Control Flow</a></li></ul></li><li><a href="ch04-00-understanding-ownership.html"><strong>4.</strong> Understanding Ownership</a></li><li><ul class="section"><li><a href="ch04-01-what-is-ownership.html"><strong>4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong>4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong>4.3.</strong> Slices</a></li></ul></li><li><a href="ch05-00-structs.html"><strong>5.</strong> Using Structs to Structure Related Data</a></li><li><ul class="section"><li><a href="ch05-01-defining-structs.html"><strong>5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong>5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html" class="active"><strong>5.3.</strong> Method Syntax</a></li></ul></li><li><a href="ch06-00-enums.html"><strong>6.</strong> Enums and Pattern Matching</a></li><li><ul class="section"><li><a href="ch06-01-defining-an-enum.html"><strong>6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong>6.2.</strong> The <code>match</code> Control Flow Operator</a></li><li><a href="ch06-03-if-let.html"><strong>6.3.</strong> Concise Control Flow with <code>if let</code></a></li></ul></li><li><a href="ch07-00-modules.html"><strong>7.</strong> Modules</a></li><li><ul class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong>7.1.</strong> <code>mod</code> and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong>7.2.</strong> Controlling Visibility with <code>pub</code></a></li><li><a href="ch07-03-importing-names-with-use.html"><strong>7.3.</strong> Importing Names with <code>use</code></a></li></ul></li><li><a href="ch08-00-common-collections.html"><strong>8.</strong> Common Collections</a></li><li><ul class="section"><li><a href="ch08-01-vectors.html"><strong>8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong>8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong>8.3.</strong> Hash Maps</a></li></ul></li><li><a href="ch09-00-error-handling.html"><strong>9.</strong> Error Handling</a></li><li><ul class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong>9.1.</strong> Unrecoverable Errors with <code>panic!</code></a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong>9.2.</strong> Recoverable Errors with <code>Result</code></a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong>9.3.</strong> To <code>panic!</code> or Not To <code>panic!</code></a></li></ul></li><li><a href="ch10-00-generics.html"><strong>10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ul class="section"><li><a href="ch10-01-syntax.html"><strong>10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong>10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong>10.3.</strong> Validating References with Lifetimes</a></li></ul></li><li><a href="ch11-00-testing.html"><strong>11.</strong> Testing</a></li><li><ul class="section"><li><a href="ch11-01-writing-tests.html"><strong>11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong>11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong>11.3.</strong> Test Organization</a></li></ul></li><li><a href="ch12-00-an-io-project.html"><strong>12.</strong> An I/O Project</a></li><li><ul class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong>12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong>12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong>12.3.</strong> Improving Error Handling and Modularity</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong>12.4.</strong> Testing the Library's Functionality</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong>12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong>12.6.</strong> Writing to <code>stderr</code> instead of <code>stdout</code></a></li></ul></li><li><a href="ch13-00-functional-features.html"><strong>13.</strong> Functional Language Features in Rust</a></li><li><ul class="section"><li><a href="ch13-01-closures.html"><strong>13.1.</strong> Closures</a></li><li><a href="ch13-02-iterators.html"><strong>13.2.</strong> Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong>13.3.</strong> Improving our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong>13.4.</strong> Performance</a></li></ul></li><li><a href="ch14-00-more-about-cargo.html"><strong>14.</strong> More about Cargo and Crates.io</a></li><li><ul class="section"><li><a href="ch14-01-release-profiles.html"><strong>14.1.</strong> Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong>14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong>14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong>14.4.</strong> Installing Binaries from Crates.io with <code>cargo install</code></a></li><li><a href="ch14-05-extending-cargo.html"><strong>14.5.</strong> Extending Cargo with Custom Commands</a></li></ul></li><li><a href="ch15-00-smart-pointers.html"><strong>15.</strong> Smart Pointers</a></li><li><ul class="section"><li><a href="ch15-01-box.html"><strong>15.1.</strong> <code>Box<T></code> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong>15.2.</strong> The <code>Deref</code> Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong>15.3.</strong> The <code>Drop</code> Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong>15.4.</strong> <code>Rc<T></code>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong>15.5.</strong> <code>RefCell<T></code> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong>15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ul></li><li><a href="ch16-00-concurrency.html"><strong>16.</strong> Fearless Concurrency</a></li><li><ul class="section"><li><a href="ch16-01-threads.html"><strong>16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong>16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong>16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong>16.4.</strong> Extensible Concurrency: <code>Sync</code> and <code>Send</code></a></li></ul></li><li><a href="ch17-00-oop.html"><strong>17.</strong> Is Rust an Object-Oriented Programming Language?</a></li><li><ul class="section"><li><a href="ch17-01-what-is-oo.html"><strong>17.1.</strong> What Does Object-Oriented Mean?</a></li><li><a href="ch17-02-trait-objects.html"><strong>17.2.</strong> Trait Objects for Using Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong>17.3.</strong> Object-Oriented Design Pattern Implementations</a></li></ul></li><li><a href="ch18-00-patterns.html"><strong>18.</strong> Patterns Match the Structure of Values</a></li><li><ul class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong>18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong>18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong>18.3.</strong> All the Pattern Syntax</a></li></ul></li><li><a href="ch19-00-advanced-features.html"><strong>19.</strong> Advanced Features</a></li><li><ul class="section"><li><a href="ch19-01-unsafe-rust.html"><strong>19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong>19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong>19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong>19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong>19.5.</strong> Advanced Functions & Closures</a></li></ul></li><li><a href="ch20-00-final-project-a-web-server.html"><strong>20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ul class="section"><li><a href="ch20-01-single-threaded.html"><strong>20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-slow-requests.html"><strong>20.2.</strong> How Slow Requests Affect Throughput</a></li><li><a href="ch20-03-designing-the-interface.html"><strong>20.3.</strong> Designing the Thread Pool Interface</a></li><li><a href="ch20-04-storing-threads.html"><strong>20.4.</strong> Creating the Thread Pool and Storing Threads</a></li><li><a href="ch20-05-sending-requests-via-channels.html"><strong>20.5.</strong> Sending Requests to Threads Via Channels</a></li><li><a href="ch20-06-graceful-shutdown-and-cleanup.html"><strong>20.6.</strong> Graceful Shutdown and Cleanup</a></li></ul></li><li><a href="appendix-00.html"><strong>21.</strong> Appendix</a></li><li><ul class="section"><li><a href="appendix-01-keywords.html"><strong>21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong>21.2.</strong> B - Operators</a></li><li><strong>21.3.</strong> C - Derivable Traits</li><li><strong>21.4.</strong> D - Nightly Rust</li><li><strong>21.5.</strong> E - Macros</li><li><strong>21.6.</strong> F - Translations</li><li><a href="appendix-07-newest-features.html"><strong>21.7.</strong> G - Newest Features</a></li></ul></li></ul>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<a class="header" href="ch05-03-method-syntax.html#method-syntax" id="method-syntax"><h2>Method Syntax</h2></a>
<p><em>Methods</em> are similar to functions: they’re declared with the <code>fn</code> keyword and
their name, they can have parameters and return values, and they contain some
code that is run when they’re called from somewhere else. However, methods are
different from functions in that they’re defined within the context of a struct
(or an enum or a trait object, which we cover in Chapters 6 and 17,
respectively), and their first parameter is always <code>self</code>, which represents the
instance of the struct the method is being called on.</p>
<a class="header" href="ch05-03-method-syntax.html#defining-methods" id="defining-methods"><h3>Defining Methods</h3></a>
<p>Let’s change the <code>area</code> function that has a <code>Rectangle</code> instance as a parameter
and instead make an <code>area</code> method defined on the <code>Rectangle</code> struct, 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 {
length: u32,
width: u32,
}
impl Rectangle {
fn area(&self) -> u32 {
self.length * self.width
}
}
fn main() {
let rect1 = Rectangle { length: 50, width: 30 };
println!(
"The area of the rectangle is {} square pixels.",
rect1.area()
);
}
</code></pre></pre>
<p><span class="caption">Listing 5-12: Defining an <code>area</code> method on the
<code>Rectangle</code> struct</span></p>
<p>To define the function within the context of <code>Rectangle</code>, we start an <code>impl</code>
(<em>implementation</em>) block. Then we move the <code>area</code> function within the <code>impl</code>
curly braces and change the first (and in this case, only) parameter to be
<code>self</code> in the signature and everywhere within the body. In <code>main</code> where we
called the <code>area</code> function and passed <code>rect1</code> as an argument, we can instead
use <em>method syntax</em> to call the <code>area</code> method on our <code>Rectangle</code> instance.
The method syntax goes after an instance: we add a dot followed by the method
name, parentheses, and any arguments.</p>
<p>In the signature for <code>area</code>, we use <code>&self</code> instead of <code>rectangle: &Rectangle</code>
because Rust knows the type of <code>self</code> is <code>Rectangle</code> due to this method being
inside the <code>impl Rectangle</code> context. Note that we still need to use the <code>&</code>
before <code>self</code>, just like we did in <code>&Rectangle</code>. Methods can take ownership of
<code>self</code>, borrow <code>self</code> immutably as we’ve done here, or borrow <code>self</code> mutably,
just like any other parameter.</p>
<p>We’ve chosen <code>&self</code> here for the same reason we used <code>&Rectangle</code> in the
function version: we don’t want to take ownership, and we just want to read the
data in the struct, not write to it. If we wanted to change the instance that
we’ve called the method on as part of what the method does, we’d use <code>&mut self</code> as the first parameter. Having a method that takes ownership of the
instance by using just <code>self</code> as the first parameter is rare; this technique is
usually used when the method transforms <code>self</code> into something else and we want
to prevent the caller from using the original instance after the transformation.</p>
<p>The main benefit of using methods instead of functions, in addition to using
method syntax and not having to repeat the type of <code>self</code> in every method’s
signature, is for organization. We’ve put all the things we can do with an
instance of a type in one <code>impl</code> block rather than making future users of our
code search for capabilities of <code>Rectangle</code> in various places in the library we
provide.</p>
<blockquote>
<a class="header" href="ch05-03-method-syntax.html#wheres-the---operator" id="wheres-the---operator"><h3>Where’s the <code>-></code> Operator?</h3></a>
<p>In languages like C++, two different operators are used for calling methods:
you use <code>.</code> if you’re calling a method on the object directly and <code>-></code> if
you’re calling the method on a pointer to the object and need to dereference
the pointer first. In other words, if <code>object</code> is a pointer,
<code>object->something()</code> is similar to <code>(*object).something()</code>.</p>
<p>Rust doesn’t have an equivalent to the <code>-></code> operator; instead, Rust has a
feature called <em>automatic referencing and dereferencing</em>. Calling methods is
one of the few places in Rust that has this behavior.</p>
<p>Here’s how it works: when you call a method with <code>object.something()</code>, Rust
automatically adds in <code>&</code>, <code>&mut</code>, or <code>*</code> so <code>object</code> matches the signature of
the method. In other words, the following are the same:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# #[derive(Debug,Copy,Clone)]
# struct Point {
# x: f64,
# y: f64,
# }
#
# impl Point {
# fn distance(&self, other: &Point) -> f64 {
# let x_squared = f64::powi(other.x - self.x, 2);
# let y_squared = f64::powi(other.y - self.y, 2);
#
# f64::sqrt(x_squared + y_squared)
# }
# }
# let p1 = Point { x: 0.0, y: 0.0 };
# let p2 = Point { x: 5.0, y: 6.5 };
p1.distance(&p2);
(&p1).distance(&p2);
#}</code></pre></pre>
<p>The first one looks much cleaner. This automatic referencing behavior works
because methods have a clear receiver—the type of <code>self</code>. Given the receiver
and name of a method, Rust can figure out definitively whether the method is
reading (<code>&self</code>), mutating (<code>&mut self</code>), or consuming (<code>self</code>). The fact
that Rust makes borrowing implicit for method receivers is a big part of
making ownership ergonomic in practice.</p>
</blockquote>
<a class="header" href="ch05-03-method-syntax.html#methods-with-more-parameters" id="methods-with-more-parameters"><h3>Methods with More Parameters</h3></a>
<p>Let’s practice using methods by implementing a second method on the <code>Rectangle</code>
struct. This time, we want an instance of <code>Rectangle</code> to take another instance
of <code>Rectangle</code> and return <code>true</code> if the second <code>Rectangle</code> can fit completely
within <code>self</code>; otherwise it should return <code>false</code>. That is, we want to be able
to write the program shown in Listing 5-13, once we’ve defined the <code>can_hold</code>
method:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore">fn main() {
let rect1 = Rectangle { length: 50, width: 30 };
let rect2 = Rectangle { length: 40, width: 10 };
let rect3 = Rectangle { length: 45, width: 60 };
println!("Can rect1 hold rect2? {}", rect1.can_hold(&rect2));
println!("Can rect1 hold rect3? {}", rect1.can_hold(&rect3));
}
</code></pre>
<p><span class="caption">Listing 5-13: Demonstration of using the as-yet-unwritten
<code>can_hold</code> method</span></p>
<p>And the expected output would look like the following, because both dimensions
of <code>rect2</code> are smaller than the dimensions of <code>rect1</code>, but <code>rect3</code> is wider
than <code>rect1</code>:</p>
<pre><code class="language-text">Can rect1 hold rect2? true
Can rect1 hold rect3? false
</code></pre>
<p>We know we want to define a method, so it will be within the <code>impl Rectangle</code>
block. The method name will be <code>can_hold</code>, and it will take an immutable borrow
of another <code>Rectangle</code> as a parameter. We can tell what the type of the
parameter will be by looking at the code that calls the method:
<code>rect1.can_hold(&rect2)</code> passes in <code>&rect2</code>, which is an immutable borrow to
<code>rect2</code>, an instance of <code>Rectangle</code>. This makes sense because we only need to
read <code>rect2</code> (rather than write, which would mean we’d need a mutable borrow),
and we want <code>main</code> to retain ownership of <code>rect2</code> so we can use it again after
calling the <code>can_hold</code> method. The return value of <code>can_hold</code> will be a
boolean, and the implementation will check whether the length and width of
<code>self</code> are both greater than the length and width of the other <code>Rectangle</code>,
respectively. Let’s add the new <code>can_hold</code> method to the <code>impl</code> block from
Listing 5-12, shown in Listing 5-14:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# #[derive(Debug)]
# struct Rectangle {
# length: u32,
# width: u32,
# }
#
impl Rectangle {
fn area(&self) -> u32 {
self.length * self.width
}
fn can_hold(&self, other: &Rectangle) -> bool {
self.length > other.length && self.width > other.width
}
}
#}</code></pre></pre>
<p><span class="caption">Listing 5-14: Implementing the <code>can_hold</code> method on
<code>Rectangle</code> that takes another <code>Rectangle</code> instance as a parameter</span></p>
<p>When we run this code with the <code>main</code> function in Listing 5-13, we’ll get our
desired output. Methods can take multiple parameters that we add to the
signature after the <code>self</code> parameter, and those parameters work just like
parameters in functions.</p>
<a class="header" href="ch05-03-method-syntax.html#associated-functions" id="associated-functions"><h3>Associated Functions</h3></a>
<p>Another useful feature of <code>impl</code> blocks is that we’re allowed to define
functions within <code>impl</code> blocks that <em>don’t</em> take <code>self</code> as a parameter. These
are called <em>associated functions</em> because they’re associated with the struct.
They’re still functions, not methods, because they don’t have an instance of
the struct to work with. You’ve already used the <code>String::from</code> associated
function.</p>
<p>Associated functions are often used for constructors that will return a new
instance of the struct. For example, we could provide an associated function
that would have one dimension parameter and use that as both length and width,
thus making it easier to create a square <code>Rectangle</code> rather than having to
specify the same value twice:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# #[derive(Debug)]
# struct Rectangle {
# length: u32,
# width: u32,
# }
#
impl Rectangle {
fn square(size: u32) -> Rectangle {
Rectangle { length: size, width: size }
}
}
#}</code></pre></pre>
<p>To call this associated function, we use the <code>::</code> syntax with the struct name,
like <code>let sq = Rectangle::square(3);</code>, for example. This function is
namespaced by the struct: the <code>::</code> syntax is used for both associated functions
and namespaces created by modules, which we'll discuss in Chapter 7.</p>
<a class="header" href="ch05-03-method-syntax.html#summary" id="summary"><h2>Summary</h2></a>
<p>Structs let us create custom types that are meaningful for our domain. By using
structs, we can keep associated pieces of data connected to each other and name
each piece to make our code clear. Methods let us specify the behavior that
instances of our structs have, and associated functions let us namespace
functionality that is particular to our struct without having an instance
available.</p>
<p>But structs aren’t the only way we can create custom types: let’s turn to
Rust’s enum feature to add another tool to our toolbox.</p>
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