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<title>Running Code on Cleanup with the Drop Trait - The Rust Programming Language</title>
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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 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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 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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" class="active"><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="#running-code-on-cleanup-with-the-drop-trait" id="running-code-on-cleanup-with-the-drop-trait">Running Code on Cleanup with the <code>Drop</code> Trait</a></h2>
<p>The second trait important to the smart pointer pattern is <code>Drop</code>, which lets
you customize what happens when a value is about to go out of scope. You can
provide an implementation for the <code>Drop</code> trait on any type, and the code you
specify can be used to release resources like files or network connections.
We’re introducing <code>Drop</code> in the context of smart pointers because the
functionality of the <code>Drop</code> trait is almost always used when implementing a
smart pointer. For example, <code>Box&lt;T&gt;</code> customizes <code>Drop</code> to deallocate the space
on the heap that the box points to.</p>
<p>In some languages, the programmer must call code to free memory or resources
every time they finish using an instance of a smart pointer. If they forget,
the system might become overloaded and crash. In Rust, you can specify that a
particular bit of code be run whenever a value goes out of scope, and the
compiler will insert this code automatically. As a result, you don’t need to be
careful about placing cleanup code everywhere in a program that an instance of
a particular type is finished with—you still won’t leak resources!</p>
<p>Specify the code to run when a value goes out of scope by implementing the
<code>Drop</code> trait. The <code>Drop</code> trait requires you to implement one method named
<code>drop</code> that takes a mutable reference to <code>self</code>. To see when Rust calls <code>drop</code>,
let’s implement <code>drop</code> with <code>println!</code> statements for now.</p>
<p>Listing 15-14 shows a <code>CustomSmartPointer</code> struct whose only custom
functionality is that it will print <code>Dropping CustomSmartPointer!</code> when the
instance goes out of scope. This example demonstrates when Rust runs the <code>drop</code>
function.</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">struct CustomSmartPointer {
data: String,
}
impl Drop for CustomSmartPointer {
fn drop(&amp;mut self) {
println!(&quot;Dropping CustomSmartPointer with data `{}`!&quot;, self.data);
}
}
fn main() {
let c = CustomSmartPointer { data: String::from(&quot;my stuff&quot;) };
let d = CustomSmartPointer { data: String::from(&quot;other stuff&quot;) };
println!(&quot;CustomSmartPointers created.&quot;);
}
</code></pre></pre>
<p><span class="caption">Listing 15-14: A <code>CustomSmartPointer</code> struct that
implements the <code>Drop</code> trait where we would put our cleanup code</span></p>
<p>The <code>Drop</code> trait is included in the prelude, so we don’t need to bring it into
scope. We implement the <code>Drop</code> trait on <code>CustomSmartPointer</code> and provide an
implementation for the <code>drop</code> method that calls <code>println!</code>. The body of the
<code>drop</code> function is where you would place any logic that you wanted to run when
an instance of your type goes out of scope. We’re printing some text here to
demonstrate when Rust will call <code>drop</code>.</p>
<p>In <code>main</code>, we create two instances of <code>CustomSmartPointer</code> and then print
<code>CustomSmartPointers created</code>. At the end of <code>main</code>, our instances of
<code>CustomSmartPointer</code> will go out of scope, and Rust will call the code we put
in the <code>drop</code> method, printing our final message. Note that we didn’t need to
call the <code>drop</code> method explicitly.</p>
<p>When we run this program, we’ll see the following output:</p>
<pre><code class="language-text">CustomSmartPointers created.
Dropping CustomSmartPointer with data `other stuff`!
Dropping CustomSmartPointer with data `my stuff`!
</code></pre>
<p>Rust automatically called <code>drop</code> for us when our instances went out of scope,
calling the code we specified. Variables are dropped in the reverse order of
their creation, so <code>d</code> was dropped before <code>c</code>. This example gives you a visual
guide to how the <code>drop</code> method works; usually you would specify the cleanup
code that your type needs to run rather than a print message.</p>
<h3><a class="header" href="#dropping-a-value-early-with-stdmemdrop" id="dropping-a-value-early-with-stdmemdrop">Dropping a Value Early with <code>std::mem::drop</code></a></h3>
<p>Unfortunately, it’s not straightforward to disable the automatic <code>drop</code>
functionality. Disabling <code>drop</code> isn’t usually necessary; the whole point of the
<code>Drop</code> trait is that it’s taken care of automatically. Occasionally, however,
you might want to clean up a value early. One example is when using smart
pointers that manage locks: you might want to force the <code>drop</code> method that
releases the lock so that other code in the same scope can acquire the lock.
Rust doesn’t let you call the <code>Drop</code> trait’s <code>drop</code> method manually; instead
you have to call the <code>std::mem::drop</code> function provided by the standard library
if you want to force a value to be dropped before the end of its scope.</p>
<p>If we try to call the <code>Drop</code> trait’s <code>drop</code> method manually by modifying the
<code>main</code> function from Listing 15-14, as shown in Listing 15-15, we’ll get a
compiler error:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore does_not_compile">fn main() {
let c = CustomSmartPointer { data: String::from(&quot;some data&quot;) };
println!(&quot;CustomSmartPointer created.&quot;);
c.drop();
println!(&quot;CustomSmartPointer dropped before the end of main.&quot;);
}
</code></pre>
<p><span class="caption">Listing 15-15: Attempting to call the <code>drop</code> method from
the <code>Drop</code> trait manually to clean up early</span></p>
<p>When we try to compile this code, we’ll get this error:</p>
<pre><code class="language-text">error[E0040]: explicit use of destructor method
--&gt; src/main.rs:14:7
|
14 | c.drop();
| ^^^^ explicit destructor calls not allowed
</code></pre>
<p>This error message states that we’re not allowed to explicitly call <code>drop</code>. The
error message uses the term <em>destructor</em>, which is the general programming term
for a function that cleans up an instance. A <em>destructor</em> is analogous to a
<em>constructor</em>, which creates an instance. The <code>drop</code> function in Rust is one
particular destructor.</p>
<p>Rust doesn’t let us call <code>drop</code> explicitly because Rust would still
automatically call <code>drop</code> on the value at the end of <code>main</code>. This would be a
<em>double free</em> error because Rust would be trying to clean up the same value
twice.</p>
<p>We can’t disable the automatic insertion of <code>drop</code> when a value goes out of
scope, and we can’t call the <code>drop</code> method explicitly. So, if we need to force
a value to be cleaned up early, we can use the <code>std::mem::drop</code> function.</p>
<p>The <code>std::mem::drop</code> function is different from the <code>drop</code> method in the <code>Drop</code>
trait. We call it by passing the value we want to force to be dropped early as
an argument. The function is in the prelude, so we can modify <code>main</code> in Listing
15-15 to call the <code>drop</code> function, as shown in Listing 15-16:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust"><span class="boring">struct CustomSmartPointer {
</span><span class="boring"> data: String,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Drop for CustomSmartPointer {
</span><span class="boring"> fn drop(&amp;mut self) {
</span><span class="boring"> println!(&quot;Dropping CustomSmartPointer with data `{}`!&quot;, self.data);
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span>fn main() {
let c = CustomSmartPointer { data: String::from(&quot;some data&quot;) };
println!(&quot;CustomSmartPointer created.&quot;);
drop(c);
println!(&quot;CustomSmartPointer dropped before the end of main.&quot;);
}
</code></pre></pre>
<p><span class="caption">Listing 15-16: Calling <code>std::mem::drop</code> to explicitly
drop a value before it goes out of scope</span></p>
<p>Running this code will print the following:</p>
<pre><code class="language-text">CustomSmartPointer created.
Dropping CustomSmartPointer with data `some data`!
CustomSmartPointer dropped before the end of main.
</code></pre>
<p>The text <code>Dropping CustomSmartPointer with data `some data`!</code> is printed
between the <code>CustomSmartPointer created.</code> and <code>CustomSmartPointer dropped before the end of main.</code> text, showing that the <code>drop</code> method code is called to
drop <code>c</code> at that point.</p>
<p>You can use code specified in a <code>Drop</code> trait implementation in many ways to
make cleanup convenient and safe: for instance, you could use it to create your
own memory allocator! With the <code>Drop</code> trait and Rust’s ownership system, you
don’t have to remember to clean up because Rust does it automatically.</p>
<p>You also don’t have to worry about problems resulting from accidentally
cleaning up values still in use: the ownership system that makes sure
references are always valid also ensures that <code>drop</code> gets called only once when
the value is no longer being used.</p>
<p>Now that we’ve examined <code>Box&lt;T&gt;</code> and some of the characteristics of smart
pointers, let’s look at a few other smart pointers defined in the standard
library.</p>
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