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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" class="active"><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="#rct-the-reference-counted-smart-pointer" id="rct-the-reference-counted-smart-pointer"><code>Rc<T></code>, the Reference Counted Smart Pointer</a></h2>
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<p>In the majority of cases, ownership is clear: you know exactly which variable
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owns a given value. However, there are cases when a single value might have
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multiple owners. For example, in graph data structures, multiple edges might
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point to the same node, and that node is conceptually owned by all of the edges
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that point to it. A node shouldn’t be cleaned up unless it doesn’t have any
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edges pointing to it.</p>
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<p>To enable multiple ownership, Rust has a type called <code>Rc<T></code>, which is an
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||
abbreviation for <em>reference counting</em>. The <code>Rc<T></code> type keeps track of the
|
||
number of references to a value which determines whether or not a value is
|
||
still in use. If there are zero references to a value, the value can be cleaned
|
||
up without any references becoming invalid.</p>
|
||
<p>Imagine <code>Rc<T></code> as a TV in a family room. When one person enters to watch TV,
|
||
they turn it on. Others can come into the room and watch the TV. When the last
|
||
person leaves the room, they turn off the TV because it’s no longer being used.
|
||
If someone turns off the TV while others are still watching it, there would be
|
||
uproar from the remaining TV watchers!</p>
|
||
<p>We use the <code>Rc<T></code> type when we want to allocate some data on the heap for
|
||
multiple parts of our program to read and we can’t determine at compile time
|
||
which part will finish using the data last. If we knew which part would finish
|
||
last, we could just make that part the data’s owner, and the normal ownership
|
||
rules enforced at compile time would take effect.</p>
|
||
<p>Note that <code>Rc<T></code> is only for use in single-threaded scenarios. When we discuss
|
||
concurrency in Chapter 16, we’ll cover how to do reference counting in
|
||
multithreaded programs.</p>
|
||
<h3><a class="header" href="#using-rct-to-share-data" id="using-rct-to-share-data">Using <code>Rc<T></code> to Share Data</a></h3>
|
||
<p>Let’s return to our cons list example in Listing 15-5. Recall that we defined
|
||
it using <code>Box<T></code>. This time, we’ll create two lists that both share ownership
|
||
of a third list. Conceptually, this looks similar to Figure 15-3:</p>
|
||
<img alt="Two lists that share ownership of a third list" src="img/trpl15-03.svg" class="center" />
|
||
<p><span class="caption">Figure 15-3: Two lists, <code>b</code> and <code>c</code>, sharing ownership of
|
||
a third list, <code>a</code></span></p>
|
||
<p>We’ll create list <code>a</code> that contains 5 and then 10. Then we’ll make two more
|
||
lists: <code>b</code> that starts with 3 and <code>c</code> that starts with 4. Both <code>b</code> and <code>c</code>
|
||
lists will then continue on to the first <code>a</code> list containing 5 and 10. In other
|
||
words, both lists will share the first list containing 5 and 10.</p>
|
||
<p>Trying to implement this scenario using our definition of <code>List</code> with <code>Box<T></code>
|
||
won’t work, as shown in Listing 15-17:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">enum List {
|
||
Cons(i32, Box<List>),
|
||
Nil,
|
||
}
|
||
|
||
use crate::List::{Cons, Nil};
|
||
|
||
fn main() {
|
||
let a = Cons(5,
|
||
Box::new(Cons(10,
|
||
Box::new(Nil))));
|
||
let b = Cons(3, Box::new(a));
|
||
let c = Cons(4, Box::new(a));
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 15-17: Demonstrating we’re not allowed to have
|
||
two lists using <code>Box<T></code> that try to share ownership of a third list</span></p>
|
||
<p>When we compile this code, we get this error:</p>
|
||
<pre><code class="language-text">error[E0382]: use of moved value: `a`
|
||
--> src/main.rs:13:30
|
||
|
|
||
12 | let b = Cons(3, Box::new(a));
|
||
| - value moved here
|
||
13 | let c = Cons(4, Box::new(a));
|
||
| ^ value used here after move
|
||
|
|
||
= note: move occurs because `a` has type `List`, which does not implement
|
||
the `Copy` trait
|
||
</code></pre>
|
||
<p>The <code>Cons</code> variants own the data they hold, so when we create the <code>b</code> list, <code>a</code>
|
||
is moved into <code>b</code> and <code>b</code> owns <code>a</code>. Then, when we try to use <code>a</code> again when
|
||
creating <code>c</code>, we’re not allowed to because <code>a</code> has been moved.</p>
|
||
<p>We could change the definition of <code>Cons</code> to hold references instead, but then
|
||
we would have to specify lifetime parameters. By specifying lifetime
|
||
parameters, we would be specifying that every element in the list will live at
|
||
least as long as the entire list. The borrow checker wouldn’t let us compile
|
||
<code>let a = Cons(10, &Nil);</code> for example, because the temporary <code>Nil</code> value would
|
||
be dropped before <code>a</code> could take a reference to it.</p>
|
||
<p>Instead, we’ll change our definition of <code>List</code> to use <code>Rc<T></code> in place of
|
||
<code>Box<T></code>, as shown in Listing 15-18. Each <code>Cons</code> variant will now hold a value
|
||
and an <code>Rc<T></code> pointing to a <code>List</code>. When we create <code>b</code>, instead of taking
|
||
ownership of <code>a</code>, we’ll clone the <code>Rc<List></code> that <code>a</code> is holding, thereby
|
||
increasing the number of references from one to two and letting <code>a</code> and <code>b</code>
|
||
share ownership of the data in that <code>Rc<List></code>. We’ll also clone <code>a</code> when
|
||
creating <code>c</code>, increasing the number of references from two to three. Every time
|
||
we call <code>Rc::clone</code>, the reference count to the data within the <code>Rc<List></code> will
|
||
increase, and the data won’t be cleaned up unless there are zero references to
|
||
it.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">enum List {
|
||
Cons(i32, Rc<List>),
|
||
Nil,
|
||
}
|
||
|
||
use crate::List::{Cons, Nil};
|
||
use std::rc::Rc;
|
||
|
||
fn main() {
|
||
let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
|
||
let b = Cons(3, Rc::clone(&a));
|
||
let c = Cons(4, Rc::clone(&a));
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 15-18: A definition of <code>List</code> that uses
|
||
<code>Rc<T></code></span></p>
|
||
<p>We need to add a <code>use</code> statement to bring <code>Rc<T></code> into scope because it’s not
|
||
in the prelude. In <code>main</code>, we create the list holding 5 and 10 and store it in
|
||
a new <code>Rc<List></code> in <code>a</code>. Then when we create <code>b</code> and <code>c</code>, we call the
|
||
<code>Rc::clone</code> function and pass a reference to the <code>Rc<List></code> in <code>a</code> as an
|
||
argument.</p>
|
||
<p>We could have called <code>a.clone()</code> rather than <code>Rc::clone(&a)</code>, but Rust’s
|
||
convention is to use <code>Rc::clone</code> in this case. The implementation of
|
||
<code>Rc::clone</code> doesn’t make a deep copy of all the data like most types’
|
||
implementations of <code>clone</code> do. The call to <code>Rc::clone</code> only increments the
|
||
reference count, which doesn’t take much time. Deep copies of data can take a
|
||
lot of time. By using <code>Rc::clone</code> for reference counting, we can visually
|
||
distinguish between the deep-copy kinds of clones and the kinds of clones that
|
||
increase the reference count. When looking for performance problems in the
|
||
code, we only need to consider the deep-copy clones and can disregard calls to
|
||
<code>Rc::clone</code>.</p>
|
||
<h3><a class="header" href="#cloning-an-rct-increases-the-reference-count" id="cloning-an-rct-increases-the-reference-count">Cloning an <code>Rc<T></code> Increases the Reference Count</a></h3>
|
||
<p>Let’s change our working example in Listing 15-18 so we can see the reference
|
||
counts changing as we create and drop references to the <code>Rc<List></code> in <code>a</code>.</p>
|
||
<p>In Listing 15-19, we’ll change <code>main</code> so it has an inner scope around list <code>c</code>;
|
||
then we can see how the reference count changes when <code>c</code> goes out of scope.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust"><span class="boring">enum List {
|
||
</span><span class="boring"> Cons(i32, Rc<List>),
|
||
</span><span class="boring"> Nil,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">use crate::List::{Cons, Nil};
|
||
</span><span class="boring">use std::rc::Rc;
|
||
</span><span class="boring">
|
||
</span>fn main() {
|
||
let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
|
||
println!("count after creating a = {}", Rc::strong_count(&a));
|
||
let b = Cons(3, Rc::clone(&a));
|
||
println!("count after creating b = {}", Rc::strong_count(&a));
|
||
{
|
||
let c = Cons(4, Rc::clone(&a));
|
||
println!("count after creating c = {}", Rc::strong_count(&a));
|
||
}
|
||
println!("count after c goes out of scope = {}", Rc::strong_count(&a));
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 15-19: Printing the reference count</span></p>
|
||
<p>At each point in the program where the reference count changes, we print the
|
||
reference count, which we can get by calling the <code>Rc::strong_count</code> function.
|
||
This function is named <code>strong_count</code> rather than <code>count</code> because the <code>Rc<T></code>
|
||
type also has a <code>weak_count</code>; we’ll see what <code>weak_count</code> is used for in the
|
||
<a href="ch15-06-reference-cycles.html#preventing-reference-cycles-turning-an-rct-into-a-weakt">“Preventing Reference Cycles: Turning an <code>Rc<T></code> into a
|
||
<code>Weak<T></code>”</a><!-- ignore --> section.</p>
|
||
<p>This code prints the following:</p>
|
||
<pre><code class="language-text">count after creating a = 1
|
||
count after creating b = 2
|
||
count after creating c = 3
|
||
count after c goes out of scope = 2
|
||
</code></pre>
|
||
<p>We can see that the <code>Rc<List></code> in <code>a</code> has an initial reference count of 1; then
|
||
each time we call <code>clone</code>, the count goes up by 1. When <code>c</code> goes out of scope,
|
||
the count goes down by 1. We don’t have to call a function to decrease the
|
||
reference count like we have to call <code>Rc::clone</code> to increase the reference
|
||
count: the implementation of the <code>Drop</code> trait decreases the reference count
|
||
automatically when an <code>Rc<T></code> value goes out of scope.</p>
|
||
<p>What we can’t see in this example is that when <code>b</code> and then <code>a</code> go out of scope
|
||
at the end of <code>main</code>, the count is then 0, and the <code>Rc<List></code> is cleaned up
|
||
completely at that point. Using <code>Rc<T></code> allows a single value to have
|
||
multiple owners, and the count ensures that the value remains valid as long as
|
||
any of the owners still exist.</p>
|
||
<p>Via immutable references, <code>Rc<T></code> allows you to share data between multiple
|
||
parts of your program for reading only. If <code>Rc<T></code> allowed you to have multiple
|
||
mutable references too, you might violate one of the borrowing rules discussed
|
||
in Chapter 4: multiple mutable borrows to the same place can cause data races
|
||
and inconsistencies. But being able to mutate data is very useful! In the next
|
||
section, we’ll discuss the interior mutability pattern and the <code>RefCell<T></code>
|
||
type that you can use in conjunction with an <code>Rc<T></code> to work with this
|
||
immutability restriction.</p>
|
||
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