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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"><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" class="active"><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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<h1 class="menu-title">The Rust Programming Language</h1>
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<h2><a class="header" href="#characteristics-of-object-oriented-languages" id="characteristics-of-object-oriented-languages">Characteristics of Object-Oriented Languages</a></h2>
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<p>There is no consensus in the programming community about what features a
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language must have to be considered object oriented. Rust is influenced by many
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programming paradigms, including OOP; for example, we explored the features
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that came from functional programming in Chapter 13. Arguably, OOP languages
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share certain common characteristics, namely objects, encapsulation, and
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inheritance. Let’s look at what each of those characteristics means and whether
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Rust supports it.</p>
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<h3><a class="header" href="#objects-contain-data-and-behavior" id="objects-contain-data-and-behavior">Objects Contain Data and Behavior</a></h3>
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<p>The book <em>Design Patterns: Elements of Reusable Object-Oriented Software</em> by
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Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley
|
||
Professional, 1994) colloquially referred to as <em>The Gang of Four</em> book, is a
|
||
catalog of object-oriented design patterns. It defines OOP this way:</p>
|
||
<blockquote>
|
||
<p>Object-oriented programs are made up of objects. An <em>object</em> packages both
|
||
data and the procedures that operate on that data. The procedures are
|
||
typically called <em>methods</em> or <em>operations</em>.</p>
|
||
</blockquote>
|
||
<p>Using this definition, Rust is object oriented: structs and enums have data,
|
||
and <code>impl</code> blocks provide methods on structs and enums. Even though structs and
|
||
enums with methods aren’t <em>called</em> objects, they provide the same
|
||
functionality, according to the Gang of Four’s definition of objects.</p>
|
||
<h3><a class="header" href="#encapsulation-that-hides-implementation-details" id="encapsulation-that-hides-implementation-details">Encapsulation that Hides Implementation Details</a></h3>
|
||
<p>Another aspect commonly associated with OOP is the idea of <em>encapsulation</em>,
|
||
which means that the implementation details of an object aren’t accessible to
|
||
code using that object. Therefore, the only way to interact with an object is
|
||
through its public API; code using the object shouldn’t be able to reach into
|
||
the object’s internals and change data or behavior directly. This enables the
|
||
programmer to change and refactor an object’s internals without needing to
|
||
change the code that uses the object.</p>
|
||
<p>We discussed how to control encapsulation in Chapter 7: we can use the <code>pub</code>
|
||
keyword to decide which modules, types, functions, and methods in our code
|
||
should be public, and by default everything else is private. For example, we
|
||
can define a struct <code>AveragedCollection</code> that has a field containing a vector
|
||
of <code>i32</code> values. The struct can also have a field that contains the average of
|
||
the values in the vector, meaning the average doesn’t have to be computed
|
||
on demand whenever anyone needs it. In other words, <code>AveragedCollection</code> will
|
||
cache the calculated average for us. Listing 17-1 has the definition of the
|
||
<code>AveragedCollection</code> struct:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>pub struct AveragedCollection {
|
||
list: Vec<i32>,
|
||
average: f64,
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 17-1: An <code>AveragedCollection</code> struct that
|
||
maintains a list of integers and the average of the items in the
|
||
collection</span></p>
|
||
<p>The struct is marked <code>pub</code> so that other code can use it, but the fields within
|
||
the struct remain private. This is important in this case because we want to
|
||
ensure that whenever a value is added or removed from the list, the average is
|
||
also updated. We do this by implementing <code>add</code>, <code>remove</code>, and <code>average</code> methods
|
||
on the struct, as shown in Listing 17-2:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></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">pub struct AveragedCollection {
|
||
</span><span class="boring"> list: Vec<i32>,
|
||
</span><span class="boring"> average: f64,
|
||
</span><span class="boring">}
|
||
</span>impl AveragedCollection {
|
||
pub fn add(&mut self, value: i32) {
|
||
self.list.push(value);
|
||
self.update_average();
|
||
}
|
||
|
||
pub fn remove(&mut self) -> Option<i32> {
|
||
let result = self.list.pop();
|
||
match result {
|
||
Some(value) => {
|
||
self.update_average();
|
||
Some(value)
|
||
},
|
||
None => None,
|
||
}
|
||
}
|
||
|
||
pub fn average(&self) -> f64 {
|
||
self.average
|
||
}
|
||
|
||
fn update_average(&mut self) {
|
||
let total: i32 = self.list.iter().sum();
|
||
self.average = total as f64 / self.list.len() as f64;
|
||
}
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 17-2: Implementations of the public methods
|
||
<code>add</code>, <code>remove</code>, and <code>average</code> on <code>AveragedCollection</code></span></p>
|
||
<p>The public methods <code>add</code>, <code>remove</code>, and <code>average</code> are the only ways to access
|
||
or modify data in an instance of <code>AveragedCollection</code>. When an item is added
|
||
to <code>list</code> using the <code>add</code> method or removed using the <code>remove</code> method, the
|
||
implementations of each call the private <code>update_average</code> method that handles
|
||
updating the <code>average</code> field as well.</p>
|
||
<p>We leave the <code>list</code> and <code>average</code> fields private so there is no way for
|
||
external code to add or remove items to the <code>list</code> field directly; otherwise,
|
||
the <code>average</code> field might become out of sync when the <code>list</code> changes. The
|
||
<code>average</code> method returns the value in the <code>average</code> field, allowing external
|
||
code to read the <code>average</code> but not modify it.</p>
|
||
<p>Because we’ve encapsulated the implementation details of the struct
|
||
<code>AveragedCollection</code>, we can easily change aspects, such as the data structure,
|
||
in the future. For instance, we could use a <code>HashSet<i32></code> instead of a
|
||
<code>Vec<i32></code> for the <code>list</code> field. As long as the signatures of the <code>add</code>,
|
||
<code>remove</code>, and <code>average</code> public methods stay the same, code using
|
||
<code>AveragedCollection</code> wouldn’t need to change. If we made <code>list</code> public instead,
|
||
this wouldn’t necessarily be the case: <code>HashSet<i32></code> and <code>Vec<i32></code> have
|
||
different methods for adding and removing items, so the external code would
|
||
likely have to change if it were modifying <code>list</code> directly.</p>
|
||
<p>If encapsulation is a required aspect for a language to be considered object
|
||
oriented, then Rust meets that requirement. The option to use <code>pub</code> or not for
|
||
different parts of code enables encapsulation of implementation details.</p>
|
||
<h3><a class="header" href="#inheritance-as-a-type-system-and-as-code-sharing" id="inheritance-as-a-type-system-and-as-code-sharing">Inheritance as a Type System and as Code Sharing</a></h3>
|
||
<p><em>Inheritance</em> is a mechanism whereby an object can inherit from another
|
||
object’s definition, thus gaining the parent object’s data and behavior without
|
||
you having to define them again.</p>
|
||
<p>If a language must have inheritance to be an object-oriented language, then
|
||
Rust is not one. There is no way to define a struct that inherits the parent
|
||
struct’s fields and method implementations. However, if you’re used to having
|
||
inheritance in your programming toolbox, you can use other solutions in Rust,
|
||
depending on your reason for reaching for inheritance in the first place.</p>
|
||
<p>You choose inheritance for two main reasons. One is for reuse of code: you can
|
||
implement particular behavior for one type, and inheritance enables you to
|
||
reuse that implementation for a different type. You can share Rust code using
|
||
default trait method implementations instead, which you saw in Listing 10-14
|
||
when we added a default implementation of the <code>summarize</code> method on the
|
||
<code>Summary</code> trait. Any type implementing the <code>Summary</code> trait would have the
|
||
<code>summarize</code> method available on it without any further code. This is similar to
|
||
a parent class having an implementation of a method and an inheriting child
|
||
class also having the implementation of the method. We can also override the
|
||
default implementation of the <code>summarize</code> method when we implement the
|
||
<code>Summary</code> trait, which is similar to a child class overriding the
|
||
implementation of a method inherited from a parent class.</p>
|
||
<p>The other reason to use inheritance relates to the type system: to enable a
|
||
child type to be used in the same places as the parent type. This is also
|
||
called <em>polymorphism</em>, which means that you can substitute multiple objects for
|
||
each other at runtime if they share certain characteristics.</p>
|
||
<blockquote>
|
||
<h3><a class="header" href="#polymorphism" id="polymorphism">Polymorphism</a></h3>
|
||
<p>To many people, polymorphism is synonymous with inheritance. But it’s
|
||
actually a more general concept that refers to code that can work with data
|
||
of multiple types. For inheritance, those types are generally subclasses.</p>
|
||
<p>Rust instead uses generics to abstract over different possible types and
|
||
trait bounds to impose constraints on what those types must provide. This is
|
||
sometimes called <em>bounded parametric polymorphism</em>.</p>
|
||
</blockquote>
|
||
<p>Inheritance has recently fallen out of favor as a programming design solution
|
||
in many programming languages because it’s often at risk of sharing more code
|
||
than necessary. Subclasses shouldn’t always share all characteristics of their
|
||
parent class but will do so with inheritance. This can make a program’s design
|
||
less flexible. It also introduces the possibility of calling methods on
|
||
subclasses that don’t make sense or that cause errors because the methods don’t
|
||
apply to the subclass. In addition, some languages will only allow a subclass
|
||
to inherit from one class, further restricting the flexibility of a program’s
|
||
design.</p>
|
||
<p>For these reasons, Rust takes a different approach, using trait objects instead
|
||
of inheritance. Let’s look at how trait objects enable polymorphism in Rust.</p>
|
||
|
||
</main>
|
||
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