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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="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="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="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="expanded "><a href="ch05-01-defining-structs.html" class="active"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="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="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="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="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="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="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="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-modu
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<main>
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<h2><a class="header" href="#defining-and-instantiating-structs" id="defining-and-instantiating-structs">Defining and Instantiating Structs</a></h2>
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<p>Structs are similar to tuples, which were discussed in Chapter 3. Like tuples,
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the pieces of a struct can be different types. Unlike with tuples, you’ll name
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each piece of data so it’s clear what the values mean. As a result of these
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names, structs are more flexible than tuples: you don’t have to rely on the
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order of the data to specify or access the values of an instance.</p>
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<p>To define a struct, we enter the keyword <code>struct</code> and name the entire struct. A
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struct’s name should describe the significance of the pieces of data being
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grouped together. Then, inside curly brackets, we define the names and types of
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the pieces of data, which we call <em>fields</em>. For example, Listing 5-1 shows a
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struct that stores information about a user account.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>struct User {
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username: String,
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email: String,
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sign_in_count: u64,
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active: bool,
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 5-1: A <code>User</code> struct definition</span></p>
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<p>To use a struct after we’ve defined it, we create an <em>instance</em> of that struct
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by specifying concrete values for each of the fields. We create an instance by
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stating the name of the struct and then add curly brackets containing <code>key: value</code> pairs, where the keys are the names of the fields and the values are the
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data we want to store in those fields. We don’t have to specify the fields in
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the same order in which we declared them in the struct. In other words, the
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struct definition is like a general template for the type, and instances fill
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in that template with particular data to create values of the type. For
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example, we can declare a particular user as shown in Listing 5-2.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span><span class="boring">struct User {
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</span><span class="boring"> username: String,
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</span><span class="boring"> email: String,
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</span><span class="boring"> sign_in_count: u64,
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</span><span class="boring"> active: bool,
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</span><span class="boring">}
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</span><span class="boring">
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</span>let user1 = User {
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email: String::from("someone@example.com"),
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username: String::from("someusername123"),
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active: true,
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sign_in_count: 1,
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};
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 5-2: Creating an instance of the <code>User</code>
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struct</span></p>
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<p>To get a specific value from a struct, we can use dot notation. If we wanted
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just this user’s email address, we could use <code>user1.email</code> wherever we wanted
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to use this value. If the instance is mutable, we can change a value by using
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the dot notation and assigning into a particular field. Listing 5-3 shows how
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to change the value in the <code>email</code> field of a mutable <code>User</code> instance.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span><span class="boring">struct User {
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</span><span class="boring"> username: String,
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</span><span class="boring"> email: String,
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</span><span class="boring"> sign_in_count: u64,
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</span><span class="boring"> active: bool,
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</span><span class="boring">}
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</span><span class="boring">
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</span>let mut user1 = User {
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email: String::from("someone@example.com"),
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username: String::from("someusername123"),
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active: true,
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sign_in_count: 1,
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};
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user1.email = String::from("anotheremail@example.com");
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 5-3: Changing the value in the <code>email</code> field of a
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<code>User</code> instance</span></p>
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<p>Note that the entire instance must be mutable; Rust doesn’t allow us to mark
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only certain fields as mutable. As with any expression, we can construct a new
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instance of the struct as the last expression in the function body to
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implicitly return that new instance.</p>
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<p>Listing 5-4 shows a <code>build_user</code> function that returns a <code>User</code> instance with
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the given email and username. The <code>active</code> field gets the value of <code>true</code>, and
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the <code>sign_in_count</code> gets a value of <code>1</code>.</p>
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<pre><pre class="playpen"><code class="language-rust">
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|||
|
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span><span class="boring">struct User {
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</span><span class="boring"> username: String,
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</span><span class="boring"> email: String,
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|
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</span><span class="boring"> sign_in_count: u64,
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</span><span class="boring"> active: bool,
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</span><span class="boring">}
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</span><span class="boring">
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</span>fn build_user(email: String, username: String) -> User {
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User {
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email: email,
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username: username,
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active: true,
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sign_in_count: 1,
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}
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|||
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}
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|||
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<span class="boring">}
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|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p><span class="caption">Listing 5-4: A <code>build_user</code> function that takes an email
|
|||
|
|
and username and returns a <code>User</code> instance</span></p>
|
|||
|
|
<p>It makes sense to name the function parameters with the same name as the struct
|
|||
|
|
fields, but having to repeat the <code>email</code> and <code>username</code> field names and
|
|||
|
|
variables is a bit tedious. If the struct had more fields, repeating each name
|
|||
|
|
would get even more annoying. Luckily, there’s a convenient shorthand!</p>
|
|||
|
|
<h3><a class="header" href="#using-the-field-init-shorthand-when-variables-and-fields-have-the-same-name" id="using-the-field-init-shorthand-when-variables-and-fields-have-the-same-name">Using the Field Init Shorthand when Variables and Fields Have the Same Name</a></h3>
|
|||
|
|
<p>Because the parameter names and the struct field names are exactly the same in
|
|||
|
|
Listing 5-4, we can use the <em>field init shorthand</em> syntax to rewrite
|
|||
|
|
<code>build_user</code> so that it behaves exactly the same but doesn’t have the
|
|||
|
|
repetition of <code>email</code> and <code>username</code>, as shown in Listing 5-5.</p>
|
|||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
|
</span><span class="boring">fn main() {
|
|||
|
|
</span><span class="boring">struct User {
|
|||
|
|
</span><span class="boring"> username: String,
|
|||
|
|
</span><span class="boring"> email: String,
|
|||
|
|
</span><span class="boring"> sign_in_count: u64,
|
|||
|
|
</span><span class="boring"> active: bool,
|
|||
|
|
</span><span class="boring">}
|
|||
|
|
</span><span class="boring">
|
|||
|
|
</span>fn build_user(email: String, username: String) -> User {
|
|||
|
|
User {
|
|||
|
|
email,
|
|||
|
|
username,
|
|||
|
|
active: true,
|
|||
|
|
sign_in_count: 1,
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
<span class="boring">}
|
|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p><span class="caption">Listing 5-5: A <code>build_user</code> function that uses field init
|
|||
|
|
shorthand because the <code>email</code> and <code>username</code> parameters have the same name as
|
|||
|
|
struct fields</span></p>
|
|||
|
|
<p>Here, we’re creating a new instance of the <code>User</code> struct, which has a field
|
|||
|
|
named <code>email</code>. We want to set the <code>email</code> field’s value to the value in the
|
|||
|
|
<code>email</code> parameter of the <code>build_user</code> function. Because the <code>email</code> field and
|
|||
|
|
the <code>email</code> parameter have the same name, we only need to write <code>email</code> rather
|
|||
|
|
than <code>email: email</code>.</p>
|
|||
|
|
<h3><a class="header" href="#creating-instances-from-other-instances-with-struct-update-syntax" id="creating-instances-from-other-instances-with-struct-update-syntax">Creating Instances From Other Instances With Struct Update Syntax</a></h3>
|
|||
|
|
<p>It’s often useful to create a new instance of a struct that uses most of an old
|
|||
|
|
instance’s values but changes some. You’ll do this using <em>struct update syntax</em>.</p>
|
|||
|
|
<p>First, Listing 5-6 shows how we create a new <code>User</code> instance in <code>user2</code> without
|
|||
|
|
the update syntax. We set new values for <code>email</code> and <code>username</code> but otherwise
|
|||
|
|
use the same values from <code>user1</code> that we created in Listing 5-2.</p>
|
|||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
|
</span><span class="boring">fn main() {
|
|||
|
|
</span><span class="boring">struct User {
|
|||
|
|
</span><span class="boring"> username: String,
|
|||
|
|
</span><span class="boring"> email: String,
|
|||
|
|
</span><span class="boring"> sign_in_count: u64,
|
|||
|
|
</span><span class="boring"> active: bool,
|
|||
|
|
</span><span class="boring">}
|
|||
|
|
</span><span class="boring">
|
|||
|
|
</span><span class="boring">let user1 = User {
|
|||
|
|
</span><span class="boring"> email: String::from("someone@example.com"),
|
|||
|
|
</span><span class="boring"> username: String::from("someusername123"),
|
|||
|
|
</span><span class="boring"> active: true,
|
|||
|
|
</span><span class="boring"> sign_in_count: 1,
|
|||
|
|
</span><span class="boring">};
|
|||
|
|
</span><span class="boring">
|
|||
|
|
</span>let user2 = User {
|
|||
|
|
email: String::from("another@example.com"),
|
|||
|
|
username: String::from("anotherusername567"),
|
|||
|
|
active: user1.active,
|
|||
|
|
sign_in_count: user1.sign_in_count,
|
|||
|
|
};
|
|||
|
|
<span class="boring">}
|
|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p><span class="caption">Listing 5-6: Creating a new <code>User</code> instance using some of
|
|||
|
|
the values from <code>user1</code></span></p>
|
|||
|
|
<p>Using struct update syntax, we can achieve the same effect with less code, as
|
|||
|
|
shown in Listing 5-7. The syntax <code>..</code> specifies that the remaining fields not
|
|||
|
|
explicitly set should have the same value as the fields in the given instance.</p>
|
|||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
|
</span><span class="boring">fn main() {
|
|||
|
|
</span><span class="boring">struct User {
|
|||
|
|
</span><span class="boring"> username: String,
|
|||
|
|
</span><span class="boring"> email: String,
|
|||
|
|
</span><span class="boring"> sign_in_count: u64,
|
|||
|
|
</span><span class="boring"> active: bool,
|
|||
|
|
</span><span class="boring">}
|
|||
|
|
</span><span class="boring">
|
|||
|
|
</span><span class="boring">let user1 = User {
|
|||
|
|
</span><span class="boring"> email: String::from("someone@example.com"),
|
|||
|
|
</span><span class="boring"> username: String::from("someusername123"),
|
|||
|
|
</span><span class="boring"> active: true,
|
|||
|
|
</span><span class="boring"> sign_in_count: 1,
|
|||
|
|
</span><span class="boring">};
|
|||
|
|
</span><span class="boring">
|
|||
|
|
</span>let user2 = User {
|
|||
|
|
email: String::from("another@example.com"),
|
|||
|
|
username: String::from("anotherusername567"),
|
|||
|
|
..user1
|
|||
|
|
};
|
|||
|
|
<span class="boring">}
|
|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p><span class="caption">Listing 5-7: Using struct update syntax to set new
|
|||
|
|
<code>email</code> and <code>username</code> values for a <code>User</code> instance but use the rest of the
|
|||
|
|
values from the fields of the instance in the <code>user1</code> variable</span></p>
|
|||
|
|
<p>The code in Listing 5-7 also creates an instance in <code>user2</code> that has a
|
|||
|
|
different value for <code>email</code> and <code>username</code> but has the same values for the
|
|||
|
|
<code>active</code> and <code>sign_in_count</code> fields from <code>user1</code>.</p>
|
|||
|
|
<h3><a class="header" href="#using-tuple-structs-without-named-fields-to-create-different-types" id="using-tuple-structs-without-named-fields-to-create-different-types">Using Tuple Structs without Named Fields to Create Different Types</a></h3>
|
|||
|
|
<p>You can also define structs that look similar to tuples, called <em>tuple
|
|||
|
|
structs</em>. Tuple structs have the added meaning the struct name provides but
|
|||
|
|
don’t have names associated with their fields; rather, they just have the types
|
|||
|
|
of the fields. Tuple structs are useful when you want to give the whole tuple a
|
|||
|
|
name and make the tuple be a different type from other tuples, and naming each
|
|||
|
|
field as in a regular struct would be verbose or redundant.</p>
|
|||
|
|
<p>To define a tuple struct, start with the <code>struct</code> keyword and the struct name
|
|||
|
|
followed by the types in the tuple. For example, here are definitions and
|
|||
|
|
usages of two tuple structs named <code>Color</code> and <code>Point</code>:</p>
|
|||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
|
</span><span class="boring">fn main() {
|
|||
|
|
</span>struct Color(i32, i32, i32);
|
|||
|
|
struct Point(i32, i32, i32);
|
|||
|
|
|
|||
|
|
let black = Color(0, 0, 0);
|
|||
|
|
let origin = Point(0, 0, 0);
|
|||
|
|
<span class="boring">}
|
|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p>Note that the <code>black</code> and <code>origin</code> values are different types, because they’re
|
|||
|
|
instances of different tuple structs. Each struct you define is its own type,
|
|||
|
|
even though the fields within the struct have the same types. For example, a
|
|||
|
|
function that takes a parameter of type <code>Color</code> cannot take a <code>Point</code> as an
|
|||
|
|
argument, even though both types are made up of three <code>i32</code> values. Otherwise,
|
|||
|
|
tuple struct instances behave like tuples: you can destructure them into their
|
|||
|
|
individual pieces, you can use a <code>.</code> followed by the index to access an
|
|||
|
|
individual value, and so on.</p>
|
|||
|
|
<h3><a class="header" href="#unit-like-structs-without-any-fields" id="unit-like-structs-without-any-fields">Unit-Like Structs Without Any Fields</a></h3>
|
|||
|
|
<p>You can also define structs that don’t have any fields! These are called
|
|||
|
|
<em>unit-like structs</em> because they behave similarly to <code>()</code>, the unit type.
|
|||
|
|
Unit-like structs can be useful in situations in which you need to implement a
|
|||
|
|
trait on some type but don’t have any data that you want to store in the type
|
|||
|
|
itself. We’ll discuss traits in Chapter 10.</p>
|
|||
|
|
<blockquote>
|
|||
|
|
<h3><a class="header" href="#ownership-of-struct-data" id="ownership-of-struct-data">Ownership of Struct Data</a></h3>
|
|||
|
|
<p>In the <code>User</code> struct definition in Listing 5-1, we used the owned <code>String</code>
|
|||
|
|
type rather than the <code>&str</code> string slice type. This is a deliberate choice
|
|||
|
|
because we want instances of this struct to own all of its data and for that
|
|||
|
|
data to be valid for as long as the entire struct is valid.</p>
|
|||
|
|
<p>It’s possible for structs to store references to data owned by something else,
|
|||
|
|
but to do so requires the use of <em>lifetimes</em>, a Rust feature that we’ll
|
|||
|
|
discuss in Chapter 10. Lifetimes ensure that the data referenced by a struct
|
|||
|
|
is valid for as long as the struct is. Let’s say you try to store a reference
|
|||
|
|
in a struct without specifying lifetimes, like this, which won’t work:</p>
|
|||
|
|
<p><span class="filename">Filename: src/main.rs</span></p>
|
|||
|
|
<pre><code class="language-rust ignore does_not_compile">struct User {
|
|||
|
|
username: &str,
|
|||
|
|
email: &str,
|
|||
|
|
sign_in_count: u64,
|
|||
|
|
active: bool,
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
fn main() {
|
|||
|
|
let user1 = User {
|
|||
|
|
email: "someone@example.com",
|
|||
|
|
username: "someusername123",
|
|||
|
|
active: true,
|
|||
|
|
sign_in_count: 1,
|
|||
|
|
};
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>The compiler will complain that it needs lifetime specifiers:</p>
|
|||
|
|
<pre><code class="language-text">error[E0106]: missing lifetime specifier
|
|||
|
|
-->
|
|||
|
|
|
|
|||
|
|
2 | username: &str,
|
|||
|
|
| ^ expected lifetime parameter
|
|||
|
|
|
|||
|
|
error[E0106]: missing lifetime specifier
|
|||
|
|
-->
|
|||
|
|
|
|
|||
|
|
3 | email: &str,
|
|||
|
|
| ^ expected lifetime parameter
|
|||
|
|
</code></pre>
|
|||
|
|
<p>In Chapter 10, we’ll discuss how to fix these errors so you can store
|
|||
|
|
references in structs, but for now, we’ll fix errors like these using owned
|
|||
|
|
types like <code>String</code> instead of references like <code>&str</code>.</p>
|
|||
|
|
</blockquote>
|
|||
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