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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-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="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="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="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="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="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="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="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="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="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="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="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="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="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="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="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="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="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="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="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="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="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="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="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="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="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="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="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="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="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="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="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="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="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="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="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="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="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="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="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="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="expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="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="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="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="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="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="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="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="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="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="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="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="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="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="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="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="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="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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<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>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>struct User {
|
||
username: String,
|
||
email: String,
|
||
sign_in_count: u64,
|
||
active: bool,
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 5-1: A <code>User</code> struct definition</span></p>
|
||
<p>To use a struct after we’ve defined it, we create an <em>instance</em> of that struct
|
||
by specifying concrete values for each of the fields. We create an instance by
|
||
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
|
||
data we want to store in those fields. We don’t have to specify the fields in
|
||
the same order in which we declared them in the struct. In other words, the
|
||
struct definition is like a general template for the type, and instances fill
|
||
in that template with particular data to create values of the type. For
|
||
example, we can declare a particular user as shown 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>let user1 = User {
|
||
email: String::from("someone@example.com"),
|
||
username: String::from("someusername123"),
|
||
active: true,
|
||
sign_in_count: 1,
|
||
};
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 5-2: Creating an instance of the <code>User</code>
|
||
struct</span></p>
|
||
<p>To get a specific value from a struct, we can use dot notation. If we wanted
|
||
just this user’s email address, we could use <code>user1.email</code> wherever we wanted
|
||
to use this value. If the instance is mutable, we can change a value by using
|
||
the dot notation and assigning into a particular field. Listing 5-3 shows how
|
||
to change the value in the <code>email</code> field of a mutable <code>User</code> 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>let mut user1 = User {
|
||
email: String::from("someone@example.com"),
|
||
username: String::from("someusername123"),
|
||
active: true,
|
||
sign_in_count: 1,
|
||
};
|
||
|
||
user1.email = String::from("anotheremail@example.com");
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 5-3: Changing the value in the <code>email</code> field of a
|
||
<code>User</code> instance</span></p>
|
||
<p>Note that the entire instance must be mutable; Rust doesn’t allow us to mark
|
||
only certain fields as mutable. As with any expression, we can construct a new
|
||
instance of the struct as the last expression in the function body to
|
||
implicitly return that new instance.</p>
|
||
<p>Listing 5-4 shows a <code>build_user</code> function that returns a <code>User</code> instance with
|
||
the given email and username. The <code>active</code> field gets the value of <code>true</code>, and
|
||
the <code>sign_in_count</code> gets a value of <code>1</code>.</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: email,
|
||
username: username,
|
||
active: true,
|
||
sign_in_count: 1,
|
||
}
|
||
}
|
||
<span class="boring">}
|
||
</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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|
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