<!DOCTYPE HTML> <html lang="en" class="sidebar-visible no-js"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>Method Syntax - The Rust Programming Language</title> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <meta name="description" content=""> <meta name="viewport" content="width=device-width, initial-scale=1"> <meta name="theme-color" content="#ffffff" /> <base href=""> <link rel="stylesheet" href="book.css"> <link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css"> <link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css"> <link rel="shortcut icon" href="favicon.png"> <!-- Font Awesome --> <link rel="stylesheet" href="_FontAwesome/css/font-awesome.css"> <link rel="stylesheet" href="highlight.css"> <link rel="stylesheet" href="tomorrow-night.css"> <link rel="stylesheet" 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<!-- Hide / unhide sidebar before it is displayed --> <script type="text/javascript"> var html = document.querySelector('html'); var sidebar = 'hidden'; if (document.body.clientWidth >= 1080) { try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } sidebar = sidebar || 'visible'; } html.classList.remove('sidebar-visible'); html.classList.add("sidebar-" + sidebar); </script> <nav id="sidebar" class="sidebar" aria-label="Table of contents"> <ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong aria-hidden="true">2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong aria-hidden="true">3.</strong> Syntax and Semantics</a></li><li><ol class="section"><li><a href="variable-bindings.html"><strong aria-hidden="true">3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong aria-hidden="true">3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong aria-hidden="true">3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="if.html"><strong aria-hidden="true">3.5.</strong> if</a></li><li><a href="loops.html"><strong aria-hidden="true">3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong aria-hidden="true">3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong aria-hidden="true">3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong aria-hidden="true">3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong aria-hidden="true">3.12.</strong> Structs</a></li><li><a href="enums.html"><strong aria-hidden="true">3.13.</strong> Enums</a></li><li><a href="match.html"><strong aria-hidden="true">3.14.</strong> Match</a></li><li><a href="patterns.html"><strong aria-hidden="true">3.15.</strong> Patterns</a></li><li><a href="method-syntax.html" class="active"><strong aria-hidden="true">3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong aria-hidden="true">3.17.</strong> Strings</a></li><li><a href="generics.html"><strong aria-hidden="true">3.18.</strong> Generics</a></li><li><a href="traits.html"><strong aria-hidden="true">3.19.</strong> Traits</a></li><li><a href="drop.html"><strong aria-hidden="true">3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong aria-hidden="true">3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong aria-hidden="true">3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong aria-hidden="true">3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong aria-hidden="true">3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong aria-hidden="true">3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong aria-hidden="true">3.26.</strong> const and static</a></li><li><a href="attributes.html"><strong aria-hidden="true">3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong aria-hidden="true">3.28.</strong> type aliases</a></li><li><a href="casting-between-types.html"><strong aria-hidden="true">3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong aria-hidden="true">3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong aria-hidden="true">3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong aria-hidden="true">3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong aria-hidden="true">3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong aria-hidden="true">3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong aria-hidden="true">3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong aria-hidden="true">3.36.</strong> unsafe</a></li></ol></li><li><a href="effective-rust.html"><strong aria-hidden="true">4.</strong> Effective Rust</a></li><li><ol class="section"><li><a href="the-stack-and-the-heap.html"><strong aria-hidden="true">4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong aria-hidden="true">4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong aria-hidden="true">4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong aria-hidden="true">4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong aria-hidden="true">4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong aria-hidden="true">4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong aria-hidden="true">4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong aria-hidden="true">4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong aria-hidden="true">4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong aria-hidden="true">4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong aria-hidden="true">4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">4.13.</strong> Procedural Macros (and custom derive)</a></li></ol></li><li><a href="glossary.html"><strong aria-hidden="true">5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong aria-hidden="true">6.</strong> Syntax Index</a></li><li><a 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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> <i class="fa fa-search"></i> </button> </div> <h1 class="menu-title">The Rust Programming Language</h1> <div class="right-buttons"> <a href="print.html" title="Print this book" aria-label="Print this book"> <i id="print-button" class="fa fa-print"></i> </a> </div> </div> </div> <div id="search-wrapper" class="hidden"> <form id="searchbar-outer" class="searchbar-outer"> <input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> </form> <div id="searchresults-outer" class="searchresults-outer hidden"> <div id="searchresults-header" class="searchresults-header"></div> <ul id="searchresults"> </ul> </div> </div> <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> <script type="text/javascript"> document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); }); </script> <div id="content" class="content"> <main> <a class="header" href="method-syntax.html#method-syntax" id="method-syntax"><h1>Method Syntax</h1></a> <p>Functions are great, but if you want to call a bunch of them on some data, it can be awkward. Consider this code:</p> <pre><code class="language-rust ignore">baz(bar(foo)); </code></pre> <p>We would read this left-to-right, and so we see ‘baz bar foo’. But this isn’t the order that the functions would get called in, that’s inside-out: ‘foo bar baz’. Wouldn’t it be nice if we could do this instead?</p> <pre><code class="language-rust ignore">foo.bar().baz(); </code></pre> <p>Luckily, as you may have guessed with the leading question, you can! Rust provides the ability to use this ‘method call syntax’ via the <code>impl</code> keyword.</p> <a class="header" href="method-syntax.html#method-calls" id="method-calls"><h1>Method calls</h1></a> <p>Here’s how it works:</p> <pre><pre class="playpen"><code class="language-rust">struct Circle { x: f64, y: f64, radius: f64, } impl Circle { fn area(&self) -> f64 { std::f64::consts::PI * (self.radius * self.radius) } } fn main() { let c = Circle { x: 0.0, y: 0.0, radius: 2.0 }; println!("{}", c.area()); } </code></pre></pre> <p>This will print <code>12.566371</code>.</p> <p>We’ve made a <code>struct</code> that represents a circle. We then write an <code>impl</code> block, and inside it, define a method, <code>area</code>.</p> <p>Methods take a special first parameter, of which there are three variants: <code>self</code>, <code>&self</code>, and <code>&mut self</code>. You can think of this first parameter as being the <code>foo</code> in <code>foo.bar()</code>. The three variants correspond to the three kinds of things <code>foo</code> could be: <code>self</code> if it’s a value on the stack, <code>&self</code> if it’s a reference, and <code>&mut self</code> if it’s a mutable reference. Because we took the <code>&self</code> parameter to <code>area</code>, we can use it like any other parameter. Because we know it’s a <code>Circle</code>, we can access the <code>radius</code> like we would with any other <code>struct</code>.</p> <p>We should default to using <code>&self</code>, as you should prefer borrowing over taking ownership, as well as taking immutable references over mutable ones. Here’s an example of all three variants:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { struct Circle { x: f64, y: f64, radius: f64, } impl Circle { fn reference(&self) { println!("taking self by reference!"); } fn mutable_reference(&mut self) { println!("taking self by mutable reference!"); } fn takes_ownership(self) { println!("taking ownership of self!"); } } #}</code></pre></pre> <p>You can use as many <code>impl</code> blocks as you’d like. The previous example could have also been written like this:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { struct Circle { x: f64, y: f64, radius: f64, } impl Circle { fn reference(&self) { println!("taking self by reference!"); } } impl Circle { fn mutable_reference(&mut self) { println!("taking self by mutable reference!"); } } impl Circle { fn takes_ownership(self) { println!("taking ownership of self!"); } } #}</code></pre></pre> <a class="header" href="method-syntax.html#chaining-method-calls" id="chaining-method-calls"><h1>Chaining method calls</h1></a> <p>So, now we know how to call a method, such as <code>foo.bar()</code>. But what about our original example, <code>foo.bar().baz()</code>? This is called ‘method chaining’. Let’s look at an example:</p> <pre><pre class="playpen"><code class="language-rust">struct Circle { x: f64, y: f64, radius: f64, } impl Circle { fn area(&self) -> f64 { std::f64::consts::PI * (self.radius * self.radius) } fn grow(&self, increment: f64) -> Circle { Circle { x: self.x, y: self.y, radius: self.radius + increment } } } fn main() { let c = Circle { x: 0.0, y: 0.0, radius: 2.0 }; println!("{}", c.area()); let d = c.grow(2.0).area(); println!("{}", d); } </code></pre></pre> <p>Check the return type:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { # struct Circle; # impl Circle { fn grow(&self, increment: f64) -> Circle { # Circle } } #}</code></pre></pre> <p>We say we’re returning a <code>Circle</code>. With this method, we can grow a new <code>Circle</code> to any arbitrary size.</p> <a class="header" href="method-syntax.html#associated-functions" id="associated-functions"><h1>Associated functions</h1></a> <p>You can also define associated functions that do not take a <code>self</code> parameter. Here’s a pattern that’s very common in Rust code:</p> <pre><pre class="playpen"><code class="language-rust">struct Circle { x: f64, y: f64, radius: f64, } impl Circle { fn new(x: f64, y: f64, radius: f64) -> Circle { Circle { x: x, y: y, radius: radius, } } } fn main() { let c = Circle::new(0.0, 0.0, 2.0); } </code></pre></pre> <p>This ‘associated function’ builds a new <code>Circle</code> for us. Note that associated functions are called with the <code>Struct::function()</code> syntax, rather than the <code>ref.method()</code> syntax. Some other languages call associated functions ‘static methods’.</p> <a class="header" href="method-syntax.html#builder-pattern" id="builder-pattern"><h1>Builder Pattern</h1></a> <p>Let’s say that we want our users to be able to create <code>Circle</code>s, but we will allow them to only set the properties they care about. Otherwise, the <code>x</code> and <code>y</code> attributes will be <code>0.0</code>, and the <code>radius</code> will be <code>1.0</code>. Rust doesn’t have method overloading, named arguments, or variable arguments. We employ the builder pattern instead. It looks like this:</p> <pre><pre class="playpen"><code class="language-rust">struct Circle { x: f64, y: f64, radius: f64, } impl Circle { fn area(&self) -> f64 { std::f64::consts::PI * (self.radius * self.radius) } } struct CircleBuilder { x: f64, y: f64, radius: f64, } impl CircleBuilder { fn new() -> CircleBuilder { CircleBuilder { x: 0.0, y: 0.0, radius: 1.0, } } fn x(&mut self, coordinate: f64) -> &mut CircleBuilder { self.x = coordinate; self } fn y(&mut self, coordinate: f64) -> &mut CircleBuilder { self.y = coordinate; self } fn radius(&mut self, radius: f64) -> &mut CircleBuilder { self.radius = radius; self } fn finalize(&self) -> Circle { Circle { x: self.x, y: self.y, radius: self.radius } } } fn main() { let c = CircleBuilder::new() .x(1.0) .y(2.0) .radius(2.0) .finalize(); println!("area: {}", c.area()); println!("x: {}", c.x); println!("y: {}", c.y); } </code></pre></pre> <p>What we’ve done here is make another <code>struct</code>, <code>CircleBuilder</code>. We’ve defined our builder methods on it. We’ve also defined our <code>area()</code> method on <code>Circle</code>. We also made one more method on <code>CircleBuilder</code>: <code>finalize()</code>. This method creates our final <code>Circle</code> from the builder. Now, we’ve used the type system to enforce our concerns: we can use the methods on <code>CircleBuilder</code> to constrain making <code>Circle</code>s in any way we choose.</p> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="patterns.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="strings.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> <div style="clear: both"></div> </nav> </div> </div> <nav class="nav-wide-wrapper" aria-label="Page navigation"> <a href="patterns.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a href="strings.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> </nav> </div> <script src="searchindex.js" type="text/javascript" charset="utf-8"></script> <script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> <script src="mark.min.js" type="text/javascript" charset="utf-8"></script> <script src="searcher.js" type="text/javascript" charset="utf-8"></script> <script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script> <script src="highlight.js" type="text/javascript" charset="utf-8"></script> <script src="book.js" type="text/javascript" charset="utf-8"></script> <!-- Custom JS scripts --> </body> </html>