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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" class="active"><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="#treating-smart-pointers-like-regular-references-with-the-deref-trait" id="treating-smart-pointers-like-regular-references-with-the-deref-trait">Treating Smart Pointers Like Regular References with the <code>Deref</code> Trait</a></h2>
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<p>Implementing the <code>Deref</code> trait allows you to customize the behavior of the
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<em>dereference operator</em>, <code>*</code> (as opposed to the multiplication or glob
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operator). By implementing <code>Deref</code> in such a way that a smart pointer can be
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treated like a regular reference, you can write code that operates on
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references and use that code with smart pointers too.</p>
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<p>Let’s first look at how the dereference operator works with regular references.
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Then we’ll try to define a custom type that behaves like <code>Box<T></code>, and see why
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the dereference operator doesn’t work like a reference on our newly defined
|
||
type. We’ll explore how implementing the <code>Deref</code> trait makes it possible for
|
||
smart pointers to work in ways similar to references. Then we’ll look at
|
||
Rust’s <em>deref coercion</em> feature and how it lets us work with either references
|
||
or smart pointers.</p>
|
||
<blockquote>
|
||
<p>Note: there’s one big difference between the <code>MyBox<T></code> type we’re about to
|
||
build and the real <code>Box<T></code>: our version will not store its data on the heap.
|
||
We are focusing this example on <code>Deref</code>, so where the data is actually stored
|
||
is less important than the pointer-like behavior.</p>
|
||
</blockquote>
|
||
<h3><a class="header" href="#following-the-pointer-to-the-value-with-the-dereference-operator" id="following-the-pointer-to-the-value-with-the-dereference-operator">Following the Pointer to the Value with the Dereference Operator</a></h3>
|
||
<p>A regular reference is a type of pointer, and one way to think of a pointer is
|
||
as an arrow to a value stored somewhere else. In Listing 15-6, we create a
|
||
reference to an <code>i32</code> value and then use the dereference operator to follow the
|
||
reference to the data:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let x = 5;
|
||
let y = &x;
|
||
|
||
assert_eq!(5, x);
|
||
assert_eq!(5, *y);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 15-6: Using the dereference operator to follow a
|
||
reference to an <code>i32</code> value</span></p>
|
||
<p>The variable <code>x</code> holds an <code>i32</code> value, <code>5</code>. We set <code>y</code> equal to a reference to
|
||
<code>x</code>. We can assert that <code>x</code> is equal to <code>5</code>. However, if we want to make an
|
||
assertion about the value in <code>y</code>, we have to use <code>*y</code> to follow the reference
|
||
to the value it’s pointing to (hence <em>dereference</em>). Once we dereference <code>y</code>,
|
||
we have access to the integer value <code>y</code> is pointing to that we can compare with
|
||
<code>5</code>.</p>
|
||
<p>If we tried to write <code>assert_eq!(5, y);</code> instead, we would get this compilation
|
||
error:</p>
|
||
<pre><code class="language-text">error[E0277]: can't compare `{integer}` with `&{integer}`
|
||
--> src/main.rs:6:5
|
||
|
|
||
6 | assert_eq!(5, y);
|
||
| ^^^^^^^^^^^^^^^^^ no implementation for `{integer} == &{integer}`
|
||
|
|
||
= help: the trait `std::cmp::PartialEq<&{integer}>` is not implemented for
|
||
`{integer}`
|
||
</code></pre>
|
||
<p>Comparing a number and a reference to a number isn’t allowed because they’re
|
||
different types. We must use the dereference operator to follow the reference
|
||
to the value it’s pointing to.</p>
|
||
<h3><a class="header" href="#using-boxt-like-a-reference" id="using-boxt-like-a-reference">Using <code>Box<T></code> Like a Reference</a></h3>
|
||
<p>We can rewrite the code in Listing 15-6 to use a <code>Box<T></code> instead of a
|
||
reference; the dereference operator will work as shown in Listing 15-7:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let x = 5;
|
||
let y = Box::new(x);
|
||
|
||
assert_eq!(5, x);
|
||
assert_eq!(5, *y);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 15-7: Using the dereference operator on a
|
||
<code>Box<i32></code></span></p>
|
||
<p>The only difference between Listing 15-7 and Listing 15-6 is that here we set
|
||
<code>y</code> to be an instance of a box pointing to the value in <code>x</code> rather than a
|
||
reference pointing to the value of <code>x</code>. In the last assertion, we can use the
|
||
dereference operator to follow the box’s pointer in the same way that we did
|
||
when <code>y</code> was a reference. Next, we’ll explore what is special about <code>Box<T></code>
|
||
that enables us to use the dereference operator by defining our own box type.</p>
|
||
<h3><a class="header" href="#defining-our-own-smart-pointer" id="defining-our-own-smart-pointer">Defining Our Own Smart Pointer</a></h3>
|
||
<p>Let’s build a smart pointer similar to the <code>Box<T></code> type provided by the
|
||
standard library to experience how smart pointers behave differently from
|
||
references by default. Then we’ll look at how to add the ability to use the
|
||
dereference operator.</p>
|
||
<p>The <code>Box<T></code> type is ultimately defined as a tuple struct with one element, so
|
||
Listing 15-8 defines a <code>MyBox<T></code> type in the same way. We’ll also define a
|
||
<code>new</code> function to match the <code>new</code> function defined on <code>Box<T></code>.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>struct MyBox<T>(T);
|
||
|
||
impl<T> MyBox<T> {
|
||
fn new(x: T) -> MyBox<T> {
|
||
MyBox(x)
|
||
}
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 15-8: Defining a <code>MyBox<T></code> type</span></p>
|
||
<p>We define a struct named <code>MyBox</code> and declare a generic parameter <code>T</code>, because
|
||
we want our type to hold values of any type. The <code>MyBox</code> type is a tuple struct
|
||
with one element of type <code>T</code>. The <code>MyBox::new</code> function takes one parameter of
|
||
type <code>T</code> and returns a <code>MyBox</code> instance that holds the value passed in.</p>
|
||
<p>Let’s try adding the <code>main</code> function in Listing 15-7 to Listing 15-8 and
|
||
changing it to use the <code>MyBox<T></code> type we’ve defined instead of <code>Box<T></code>. The
|
||
code in Listing 15-9 won’t compile because Rust doesn’t know how to dereference
|
||
<code>MyBox</code>.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">fn main() {
|
||
let x = 5;
|
||
let y = MyBox::new(x);
|
||
|
||
assert_eq!(5, x);
|
||
assert_eq!(5, *y);
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 15-9: Attempting to use <code>MyBox<T></code> in the same
|
||
way we used references and <code>Box<T></code></span></p>
|
||
<p>Here’s the resulting compilation error:</p>
|
||
<pre><code class="language-text">error[E0614]: type `MyBox<{integer}>` cannot be dereferenced
|
||
--> src/main.rs:14:19
|
||
|
|
||
14 | assert_eq!(5, *y);
|
||
| ^^
|
||
</code></pre>
|
||
<p>Our <code>MyBox<T></code> type can’t be dereferenced because we haven’t implemented that
|
||
ability on our type. To enable dereferencing with the <code>*</code> operator, we
|
||
implement the <code>Deref</code> trait.</p>
|
||
<h3><a class="header" href="#treating-a-type-like-a-reference-by-implementing-the-deref-trait" id="treating-a-type-like-a-reference-by-implementing-the-deref-trait">Treating a Type Like a Reference by Implementing the <code>Deref</code> Trait</a></h3>
|
||
<p>As discussed in Chapter 10, to implement a trait, we need to provide
|
||
implementations for the trait’s required methods. The <code>Deref</code> trait, provided
|
||
by the standard library, requires us to implement one method named <code>deref</code> that
|
||
borrows <code>self</code> and returns a reference to the inner data. Listing 15-10
|
||
contains an implementation of <code>Deref</code> to add to the definition of <code>MyBox</code>:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>use std::ops::Deref;
|
||
|
||
<span class="boring">struct MyBox<T>(T);
|
||
</span>impl<T> Deref for MyBox<T> {
|
||
type Target = T;
|
||
|
||
fn deref(&self) -> &T {
|
||
&self.0
|
||
}
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 15-10: Implementing <code>Deref</code> on <code>MyBox<T></code></span></p>
|
||
<p>The <code>type Target = T;</code> syntax defines an associated type for the <code>Deref</code> trait
|
||
to use. Associated types are a slightly different way of declaring a generic
|
||
parameter, but you don’t need to worry about them for now; we’ll cover them in
|
||
more detail in Chapter 19.</p>
|
||
<p>We fill in the body of the <code>deref</code> method with <code>&self.0</code> so <code>deref</code> returns a
|
||
reference to the value we want to access with the <code>*</code> operator. The <code>main</code>
|
||
function in Listing 15-9 that calls <code>*</code> on the <code>MyBox<T></code> value now compiles,
|
||
and the assertions pass!</p>
|
||
<p>Without the <code>Deref</code> trait, the compiler can only dereference <code>&</code> references.
|
||
The <code>deref</code> method gives the compiler the ability to take a value of any type
|
||
that implements <code>Deref</code> and call the <code>deref</code> method to get a <code>&</code> reference that
|
||
it knows how to dereference.</p>
|
||
<p>When we entered <code>*y</code> in Listing 15-9, behind the scenes Rust actually ran this
|
||
code:</p>
|
||
<pre><code class="language-rust ignore">*(y.deref())
|
||
</code></pre>
|
||
<p>Rust substitutes the <code>*</code> operator with a call to the <code>deref</code> method and then a
|
||
plain dereference so we don’t have to think about whether or not we need to
|
||
call the <code>deref</code> method. This Rust feature lets us write code that functions
|
||
identically whether we have a regular reference or a type that implements
|
||
<code>Deref</code>.</p>
|
||
<p>The reason the <code>deref</code> method returns a reference to a value, and that the plain
|
||
dereference outside the parentheses in <code>*(y.deref())</code> is still necessary, is the
|
||
ownership system. If the <code>deref</code> method returned the value directly instead of
|
||
a reference to the value, the value would be moved out of <code>self</code>. We don’t want
|
||
to take ownership of the inner value inside <code>MyBox<T></code> in this case or in most
|
||
cases where we use the dereference operator.</p>
|
||
<p>Note that the <code>*</code> operator is replaced with a call to the <code>deref</code> method and
|
||
then a call to the <code>*</code> operator just once, each time we use a <code>*</code> in our code.
|
||
Because the substitution of the <code>*</code> operator does not recurse infinitely, we
|
||
end up with data of type <code>i32</code>, which matches the <code>5</code> in <code>assert_eq!</code> in
|
||
Listing 15-9.</p>
|
||
<h3><a class="header" href="#implicit-deref-coercions-with-functions-and-methods" id="implicit-deref-coercions-with-functions-and-methods">Implicit Deref Coercions with Functions and Methods</a></h3>
|
||
<p><em>Deref coercion</em> is a convenience that Rust performs on arguments to functions
|
||
and methods. Deref coercion converts a reference to a type that implements
|
||
<code>Deref</code> into a reference to a type that <code>Deref</code> can convert the original type
|
||
into. Deref coercion happens automatically when we pass a reference to a
|
||
particular type’s value as an argument to a function or method that doesn’t
|
||
match the parameter type in the function or method definition. A sequence of
|
||
calls to the <code>deref</code> method converts the type we provided into the type the
|
||
parameter needs.</p>
|
||
<p>Deref coercion was added to Rust so that programmers writing function and
|
||
method calls don’t need to add as many explicit references and dereferences
|
||
with <code>&</code> and <code>*</code>. The deref coercion feature also lets us write more code that
|
||
can work for either references or smart pointers.</p>
|
||
<p>To see deref coercion in action, let’s use the <code>MyBox<T></code> type we defined in
|
||
Listing 15-8 as well as the implementation of <code>Deref</code> that we added in Listing
|
||
15-10. Listing 15-11 shows the definition of a function that has a string slice
|
||
parameter:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>fn hello(name: &str) {
|
||
println!("Hello, {}!", name);
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 15-11: A <code>hello</code> function that has the parameter
|
||
<code>name</code> of type <code>&str</code></span></p>
|
||
<p>We can call the <code>hello</code> function with a string slice as an argument, such as
|
||
<code>hello("Rust");</code> for example. Deref coercion makes it possible to call <code>hello</code>
|
||
with a reference to a value of type <code>MyBox<String></code>, as shown in Listing 15-12:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust"><span class="boring">use std::ops::Deref;
|
||
</span><span class="boring">
|
||
</span><span class="boring">struct MyBox<T>(T);
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl<T> MyBox<T> {
|
||
</span><span class="boring"> fn new(x: T) -> MyBox<T> {
|
||
</span><span class="boring"> MyBox(x)
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl<T> Deref for MyBox<T> {
|
||
</span><span class="boring"> type Target = T;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> fn deref(&self) -> &T {
|
||
</span><span class="boring"> &self.0
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn hello(name: &str) {
|
||
</span><span class="boring"> println!("Hello, {}!", name);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>fn main() {
|
||
let m = MyBox::new(String::from("Rust"));
|
||
hello(&m);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 15-12: Calling <code>hello</code> with a reference to a
|
||
<code>MyBox<String></code> value, which works because of deref coercion</span></p>
|
||
<p>Here we’re calling the <code>hello</code> function with the argument <code>&m</code>, which is a
|
||
reference to a <code>MyBox<String></code> value. Because we implemented the <code>Deref</code> trait
|
||
on <code>MyBox<T></code> in Listing 15-10, Rust can turn <code>&MyBox<String></code> into <code>&String</code>
|
||
by calling <code>deref</code>. The standard library provides an implementation of <code>Deref</code>
|
||
on <code>String</code> that returns a string slice, and this is in the API documentation
|
||
for <code>Deref</code>. Rust calls <code>deref</code> again to turn the <code>&String</code> into <code>&str</code>, which
|
||
matches the <code>hello</code> function’s definition.</p>
|
||
<p>If Rust didn’t implement deref coercion, we would have to write the code in
|
||
Listing 15-13 instead of the code in Listing 15-12 to call <code>hello</code> with a value
|
||
of type <code>&MyBox<String></code>.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust"><span class="boring">use std::ops::Deref;
|
||
</span><span class="boring">
|
||
</span><span class="boring">struct MyBox<T>(T);
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl<T> MyBox<T> {
|
||
</span><span class="boring"> fn new(x: T) -> MyBox<T> {
|
||
</span><span class="boring"> MyBox(x)
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl<T> Deref for MyBox<T> {
|
||
</span><span class="boring"> type Target = T;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> fn deref(&self) -> &T {
|
||
</span><span class="boring"> &self.0
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn hello(name: &str) {
|
||
</span><span class="boring"> println!("Hello, {}!", name);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>fn main() {
|
||
let m = MyBox::new(String::from("Rust"));
|
||
hello(&(*m)[..]);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 15-13: The code we would have to write if Rust
|
||
didn’t have deref coercion</span></p>
|
||
<p>The <code>(*m)</code> dereferences the <code>MyBox<String></code> into a <code>String</code>. Then the <code>&</code> and
|
||
<code>[..]</code> take a string slice of the <code>String</code> that is equal to the whole string to
|
||
match the signature of <code>hello</code>. The code without deref coercions is harder to
|
||
read, write, and understand with all of these symbols involved. Deref coercion
|
||
allows Rust to handle these conversions for us automatically.</p>
|
||
<p>When the <code>Deref</code> trait is defined for the types involved, Rust will analyze the
|
||
types and use <code>Deref::deref</code> as many times as necessary to get a reference to
|
||
match the parameter’s type. The number of times that <code>Deref::deref</code> needs to be
|
||
inserted is resolved at compile time, so there is no runtime penalty for taking
|
||
advantage of deref coercion!</p>
|
||
<h3><a class="header" href="#how-deref-coercion-interacts-with-mutability" id="how-deref-coercion-interacts-with-mutability">How Deref Coercion Interacts with Mutability</a></h3>
|
||
<p>Similar to how you use the <code>Deref</code> trait to override the <code>*</code> operator on
|
||
immutable references, you can use the <code>DerefMut</code> trait to override the <code>*</code>
|
||
operator on mutable references.</p>
|
||
<p>Rust does deref coercion when it finds types and trait implementations in three
|
||
cases:</p>
|
||
<ul>
|
||
<li>From <code>&T</code> to <code>&U</code> when <code>T: Deref<Target=U></code></li>
|
||
<li>From <code>&mut T</code> to <code>&mut U</code> when <code>T: DerefMut<Target=U></code></li>
|
||
<li>From <code>&mut T</code> to <code>&U</code> when <code>T: Deref<Target=U></code></li>
|
||
</ul>
|
||
<p>The first two cases are the same except for mutability. The first case states
|
||
that if you have a <code>&T</code>, and <code>T</code> implements <code>Deref</code> to some type <code>U</code>, you can
|
||
get a <code>&U</code> transparently. The second case states that the same deref coercion
|
||
happens for mutable references.</p>
|
||
<p>The third case is trickier: Rust will also coerce a mutable reference to an
|
||
immutable one. But the reverse is <em>not</em> possible: immutable references will
|
||
never coerce to mutable references. Because of the borrowing rules, if you have
|
||
a mutable reference, that mutable reference must be the only reference to that
|
||
data (otherwise, the program wouldn’t compile). Converting one mutable
|
||
reference to one immutable reference will never break the borrowing rules.
|
||
Converting an immutable reference to a mutable reference would require that
|
||
there is only one immutable reference to that data, and the borrowing rules
|
||
don’t guarantee that. Therefore, Rust can’t make the assumption that converting
|
||
an immutable reference to a mutable reference is possible.</p>
|
||
|
||
</main>
|
||
|
||
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