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<ol class="chapter"><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><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><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><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><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><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><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><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><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><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><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><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><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><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><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><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><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><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><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><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><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><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><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><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><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><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><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><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><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><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><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><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><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-nightly-rust.html"><strong aria-hidden="true">21.6.</strong> F - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<a class="header" href="ch15-04-rc.html#rct-the-reference-counted-smart-pointer" id="rct-the-reference-counted-smart-pointer"><h2><code>Rc<T></code>, the Reference Counted Smart Pointer</h2></a>
<p>In the majority of cases, ownership is clear: you know exactly which variable
owns a given value. However, there are cases when a single value might have
multiple owners. For example, in graph data structures, multiple edges might
point to the same node, and that node is conceptually owned by all of the edges
that point to it. A node shouldn’t be cleaned up unless it doesn’t have any
edges pointing to it.</p>
<p>To enable multiple ownership, Rust has a type called <code>Rc<T></code>, which is an
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>
<a class="header" href="ch15-04-rc.html#using-rct-to-share-data" id="using-rct-to-share-data"><h3>Using <code>Rc<T></code> to Share Data</h3></a>
<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>
<p><img alt="Two lists that share ownership of a third list" src="img/trpl15-03.svg" class="center" /></p>
<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">enum List {
Cons(i32, Box<List>),
Nil,
}
use 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 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>
<a class="header" href="ch15-04-rc.html#cloning-an-rct-increases-the-reference-count" id="cloning-an-rct-increases-the-reference-count"><h3>Cloning an <code>Rc<T></code> Increases the Reference Count</h3></a>
<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"># enum List {
# Cons(i32, Rc<List>),
# Nil,
# }
#
# use List::{Cons, Nil};
# use std::rc::Rc;
#
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
“Preventing Reference Cycles” 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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