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<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 href="bibliography.html"><strong aria-hidden="true">7.</strong> Bibliography</a></li></ol>
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<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>
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