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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Updates</a></li><li class="chapter-item expanded "><a href="Chapter_1.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="Chapter_2.html"><strong aria-hidden="true">3.</strong> Who am I?</a></li><li class="chapter-item expanded "><a href="Chapter_3.html"><strong aria-hidden="true">4.</strong> Writing Rust in Easy English</a></li><li class="chapter-item expanded "><a href="Chapter_4.html"><strong aria-hidden="true">5.</strong> Rust Playground</a></li><li class="chapter-item expanded "><a href="Chapter_5.html"><strong aria-hidden="true">6.</strong> 🚧 and ⚠️</a></li><li class="chapter-item expanded "><a href="Chapter_6.html"><strong aria-hidden="true">7.</strong> Comments</a></li><li class="chapter-item expanded "><a href="Chapter_7.html"><strong aria-hidden="true">8.</strong> Types</a></li><li class="chapter-item expanded "><a href="Chapter_8.html"><strong aria-hidden="true">9.</strong> Type inference</a></li><li class="chapter-item expanded "><a href="Chapter_9.html"><strong aria-hidden="true">10.</strong> Printing 'hello, world!'</a></li><li class="chapter-item expanded "><a href="Chapter_10.html"><strong aria-hidden="true">11.</strong> Display and debug</a></li><li class="chapter-item expanded "><a href="Chapter_11.html"><strong aria-hidden="true">12.</strong> Mutability (changing)</a></li><li class="chapter-item expanded "><a href="Chapter_12.html"><strong aria-hidden="true">13.</strong> The stack, the heap, and pointers</a></li><li class="chapter-item expanded "><a href="Chapter_13.html"><strong aria-hidden="true">14.</strong> More about printing</a></li><li class="chapter-item expanded "><a href="Chapter_14.html"><strong aria-hidden="true">15.</strong> Strings</a></li><li class="chapter-item expanded "><a href="Chapter_15.html"><strong aria-hidden="true">16.</strong> const and static</a></li><li class="chapter-item expanded "><a href="Chapter_16.html"><strong aria-hidden="true">17.</strong> More on references</a></li><li class="chapter-item expanded "><a href="Chapter_17.html"><strong aria-hidden="true">18.</strong> Mutable references</a></li><li class="chapter-item expanded "><a href="Chapter_18.html"><strong aria-hidden="true">19.</strong> Giving references to functions</a></li><li class="chapter-item expanded "><a href="Chapter_19.html"><strong aria-hidden="true">20.</strong> Copy types</a></li><li class="chapter-item expanded "><a href="Chapter_20.html"><strong aria-hidden="true">21.</strong> Collection types</a></li><li class="chapter-item expanded "><a href="Chapter_21.html"><strong aria-hidden="true">22.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="Chapter_22.html"><strong aria-hidden="true">23.</strong> Tuples</a></li><li class="chapter-item expanded "><a href="Chapter_23.html"><strong aria-hidden="true">24.</strong> Control flow</a></li><li class="chapter-item expanded "><a href="Chapter_24.html"><strong aria-hidden="true">25.</strong> Structs</a></li><li class="chapter-item expanded "><a href="Chapter_25.html"><strong aria-hidden="true">26.</strong> Enums</a></li><li class="chapter-item expanded "><a href="Chapter_26.html"><strong aria-hidden="true">27.</strong> Loops</a></li><li class="chapter-item expanded "><a href="Chapter_27.html"><strong aria-hidden="true">28.</strong> Implementing structs and enums</a></li><li class="chapter-item expanded "><a href="Chapter_28.html"><strong aria-hidden="true">29.</strong> Destructuring</a></li><li class="chapter-item expanded "><a href="Chapter_29.html"><strong aria-hidden="true">30.</strong> References and the dot operator</a></li><li class="chapter-item expanded "><a href="Chapter_30.html"><strong aria-hidden="true">31.</strong> Generics</a></li><li class="chapter-item expanded "><a href="Chapter_31.html"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html"><strong aria-hidden="true">33.</strong> Other collections</a></li><li class="chapter-item expanded "><a href="Chapter_33.html"><strong aria-hidden="true">34.</strong> The ? operator</a></li><li class="chapter-item expanded "><a href="Chapter_34.html"><strong aria-hidden="true">35.</strong> Traits</a></li><li class="chapter-item expanded "><a href="Chapter_35.html"><strong aria-hidden="true">36.</strong> Chaining methods</a></li><li class="chapter-item expanded "><a href="Chapter_36.html"><strong aria-hidden="true">37.</strong> Iterators</a></li><li class="chapter-item expanded "><a href="Chapter_37.html"><strong aria-hidden="true">38.</strong> Closures</a></li><li class="chapter-item expanded "><a href="Chapter_38.html"><strong aria-hidden="true">39.</strong> The dbg! macro and .inspect</a></li><li class="chapter-item expanded "><a href="Chapter_39.html"><strong aria-hidden="true">40.</strong> Types of &amp;str</a></li><li class="chapter-item expanded "><a href="Chapter_40.html"><strong aria-hidden="true">41.</strong> Lifetimes</a></li><li class="chapter-item expanded "><a href="Chapter_41.html"><strong aria-hidden="true">42.</strong> Interior mutability</a></li><li class="chapter-item expanded "><a href="Chapter_42.html"><strong aria-hidden="true">43.</strong> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_56.html" class="active"><strong aria-hidden="true">57.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> The end?</a></li></ol>
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<h2 id="deref-and-derefmut"><a class="header" href="#deref-and-derefmut">Deref and DerefMut</a></h2>
<p><code>Deref</code> is the trait that lets you use <code>*</code> to dereference something. We saw the word <code>Deref</code> before when using a tuple struct to make a new type, and now it's time to learn it.</p>
<p>We know that a reference is not the same as a value:</p>
<pre><pre class="playground"><code class="language-rust">// ⚠️
fn main() {
let value = 7; // This is an i32
let reference = &amp;7; // This is a &amp;i32
println!(&quot;{}&quot;, value == reference);
}
</code></pre></pre>
<p>And Rust won't even give a <code>false</code> because it won't even compare the two.</p>
<pre><code class="language-text">error[E0277]: can't compare `{integer}` with `&amp;{integer}`
--&gt; src\main.rs:4:26
|
4 | println!(&quot;{}&quot;, value == reference);
| ^^ no implementation for `{integer} == &amp;{integer}`
</code></pre>
<p>Of course, the solution here is <code>*</code>. So this will print <code>true</code>:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let value = 7;
let reference = &amp;7;
println!(&quot;{}&quot;, value == *reference);
}
</code></pre></pre>
<p>Now let's imagine a simple type that just holds a number. It will be like a <code>Box</code>, and we have some ideas for some extra functions for it. But if we just give it a number, it won't be able to do much with it.</p>
<p>We can't use <code>*</code> like we can with <code>Box</code>:</p>
<pre><pre class="playground"><code class="language-rust">// ⚠️
struct HoldsANumber(u8);
fn main() {
let my_number = HoldsANumber(20);
println!(&quot;{}&quot;, *my_number + 20);
}
</code></pre></pre>
<p>The error is:</p>
<pre><code class="language-text">error[E0614]: type `HoldsANumber` cannot be dereferenced
--&gt; src\main.rs:24:22
|
24 | println!(&quot;{:?}&quot;, *my_number + 20);
</code></pre>
<p>We can of course do this: <code>println!(&quot;{:?}&quot;, my_number.0 + 20);</code>. But then we are just adding a separate <code>u8</code> to the 20. It would be nice if we could just add them together. The message <code>cannot be dereferenced</code> gives us a clue: we need to implement <code>Deref</code>. Something simple that implements <code>Deref</code> is sometimes called a &quot;smart pointer&quot;. A smart pointer can point to its item, has information about it, and can use its methods. Because right now we can add <code>my_number.0</code>, which is a <code>u8</code>, but we can't do much else with a <code>HoldsANumber</code>: all it has so far is <code>Debug</code>.</p>
<p>Interesting fact: <code>String</code> is actually a smart pointer to <code>&amp;str</code> and <code>Vec</code> is a smart pointer to array (or other types). So we have actually been using smart pointers since the beginning.</p>
<p>Implementing <code>Deref</code> is not too hard and the examples in the standard library are easy. <a href="https://doc.rust-lang.org/std/ops/trait.Deref.html">Here's the sample code from the standard library</a>:</p>
<pre><pre class="playground"><code class="language-rust">use std::ops::Deref;
struct DerefExample&lt;T&gt; {
value: T
}
impl&lt;T&gt; Deref for DerefExample&lt;T&gt; {
type Target = T;
fn deref(&amp;self) -&gt; &amp;Self::Target {
&amp;self.value
}
}
fn main() {
let x = DerefExample { value: 'a' };
assert_eq!('a', *x);
}
</code></pre></pre>
<p>So we follow that and now our <code>Deref</code> looks like this:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// 🚧
impl Deref for HoldsANumber {
type Target = u8; // Remember, this is the &quot;associated type&quot;: the type that goes together.
// You have to use the right type Target = (the type you want to return)
fn deref(&amp;self) -&gt; &amp;Self::Target { // Rust calls .deref() when you use *. We just defined Target as a u8 so this is easy to understand
&amp;self.0 // We chose &amp;self.0 because it's a tuple struct. In a named struct it would be something like &quot;&amp;self.number&quot;
}
}
<span class="boring">}
</span></code></pre></pre>
<p>So now we can do this with <code>*</code>:</p>
<pre><pre class="playground"><code class="language-rust">use std::ops::Deref;
#[derive(Debug)]
struct HoldsANumber(u8);
impl Deref for HoldsANumber {
type Target = u8;
fn deref(&amp;self) -&gt; &amp;Self::Target {
&amp;self.0
}
}
fn main() {
let my_number = HoldsANumber(20);
println!(&quot;{:?}&quot;, *my_number + 20);
}
</code></pre></pre>
<p>So that will print <code>40</code> and we didn't need to write <code>my_number.0</code>. That means we get the methods of <code>u8</code> and we can write our own methods for <code>HoldsANumber</code>. We will add our own simple method and use another method we get from <code>u8</code> called <code>.checked_sub()</code>. The <code>.checked_sub()</code> method is a safe subtraction that returns an <code>Option</code>. If it can do the subtraction then it gives it to you inside <code>Some</code>, and if it can't do it then it gives a <code>None</code>. Remember, a <code>u8</code> can't be negative so it's safer to do <code>.checked_sub()</code> so we don't panic.</p>
<pre><pre class="playground"><code class="language-rust">use std::ops::Deref;
struct HoldsANumber(u8);
impl HoldsANumber {
fn prints_the_number_times_two(&amp;self) {
println!(&quot;{}&quot;, self.0 * 2);
}
}
impl Deref for HoldsANumber {
type Target = u8;
fn deref(&amp;self) -&gt; &amp;Self::Target {
&amp;self.0
}
}
fn main() {
let my_number = HoldsANumber(20);
println!(&quot;{:?}&quot;, my_number.checked_sub(100)); // This method comes from u8
my_number.prints_the_number_times_two(); // This is our own method
}
</code></pre></pre>
<p>This prints:</p>
<pre><code class="language-text">None
40
</code></pre>
<p>We can also implement <code>DerefMut</code> so we can change the values through <code>*</code>. It looks almost the same. You need <code>Deref</code> before you can implement <code>DerefMut</code>.</p>
<pre><pre class="playground"><code class="language-rust">use std::ops::{Deref, DerefMut};
struct HoldsANumber(u8);
impl HoldsANumber {
fn prints_the_number_times_two(&amp;self) {
println!(&quot;{}&quot;, self.0 * 2);
}
}
impl Deref for HoldsANumber {
type Target = u8;
fn deref(&amp;self) -&gt; &amp;Self::Target {
&amp;self.0
}
}
impl DerefMut for HoldsANumber { // You don't need type Target = u8; here because it already knows thanks to Deref
fn deref_mut(&amp;mut self) -&gt; &amp;mut Self::Target { // Everything else is the same except it says mut everywhere
&amp;mut self.0
}
}
fn main() {
let mut my_number = HoldsANumber(20);
*my_number = 30; // DerefMut lets us do this
println!(&quot;{:?}&quot;, my_number.checked_sub(100));
my_number.prints_the_number_times_two();
}
</code></pre></pre>
<p>So you can see that <code>Deref</code> gives your type a lot of power.</p>
<p>This is also why the standard library says: <code>Deref should only be implemented for smart pointers to avoid confusion</code>. That's because you can do some strange things with <code>Deref</code> for a complicated type. Let's imagine a really confusing example to understand what they mean. We'll start with <code>Character</code> struct for a game. A new <code>Character</code> needs some stats like intelligence and strength. So here is our first character:</p>
<pre><pre class="playground"><code class="language-rust">struct Character {
name: String,
strength: u8,
dexterity: u8,
health: u8,
intelligence: u8,
wisdom: u8,
charm: u8,
hit_points: i8,
alignment: Alignment,
}
impl Character {
fn new(
name: String,
strength: u8,
dexterity: u8,
health: u8,
intelligence: u8,
wisdom: u8,
charm: u8,
hit_points: i8,
alignment: Alignment,
) -&gt; Self {
Self {
name,
strength,
dexterity,
health,
intelligence,
wisdom,
charm,
hit_points,
alignment,
}
}
}
enum Alignment {
Good,
Neutral,
Evil,
}
fn main() {
let billy = Character::new(&quot;Billy&quot;.to_string(), 9, 8, 7, 10, 19, 19, 5, Alignment::Good);
}
</code></pre></pre>
<p>Now let's imagine that we want to keep character hit points in a big vec. Maybe we'll put monster data in there too, and keep it all together. Since <code>hit_points</code> is an <code>i8</code>, we implement <code>Deref</code> so we can do all sorts of math on it. But look at how strange it looks in our <code>main()</code> function now:</p>
<pre><pre class="playground"><code class="language-rust">use std::ops::Deref;
// All the other code is the same until after the enum Alignment
struct Character {
name: String,
strength: u8,
dexterity: u8,
health: u8,
intelligence: u8,
wisdom: u8,
charm: u8,
hit_points: i8,
alignment: Alignment,
}
impl Character {
fn new(
name: String,
strength: u8,
dexterity: u8,
health: u8,
intelligence: u8,
wisdom: u8,
charm: u8,
hit_points: i8,
alignment: Alignment,
) -&gt; Self {
Self {
name,
strength,
dexterity,
health,
intelligence,
wisdom,
charm,
hit_points,
alignment,
}
}
}
enum Alignment {
Good,
Neutral,
Evil,
}
impl Deref for Character { // impl Deref for Character. Now we can do any integer math we want!
type Target = i8;
fn deref(&amp;self) -&gt; &amp;Self::Target {
&amp;self.hit_points
}
}
fn main() {
let billy = Character::new(&quot;Billy&quot;.to_string(), 9, 8, 7, 10, 19, 19, 5, Alignment::Good); // Create two characters, billy and brandy
let brandy = Character::new(&quot;Brandy&quot;.to_string(), 9, 8, 7, 10, 19, 19, 5, Alignment::Good);
let mut hit_points_vec = vec![]; // Put our hit points data in here
hit_points_vec.push(*billy); // Push *billy?
hit_points_vec.push(*brandy); // Push *brandy?
println!(&quot;{:?}&quot;, hit_points_vec);
}
</code></pre></pre>
<p>This just prints <code>[5, 5]</code>. Our code is now very strange for someone to read. We can read <code>Deref</code> just above <code>main()</code> and figure out that <code>*billy</code> means <code>i8</code>, but what if there was a lot of code? Maybe our code is 2000 lines long, and suddenly we have to figure out why we are <code>.push()</code>ing <code>*billy</code>. <code>Character</code> is certainly more than just a smart pointer for <code>i8</code>.</p>
<p>Of course, it is not illegal to write <code>hit_points_vec.push(*billy)</code>, but it makes the code look very strange. Probably a simple <code>.get_hp()</code> method would be much better, or another struct that holds the characters. Then you could iterate through and push the <code>hit_points</code> for each one. <code>Deref</code> gives a lot of power but it's good to make sure that the code is logical.</p>
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