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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" class="active"><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-modu
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<h2><a class="header" href="#the-slice-type" id="the-slice-type">The Slice Type</a></h2>
<p>Another data type that does not have ownership is the <em>slice</em>. Slices let you
reference a contiguous sequence of elements in a collection rather than the
whole collection.</p>
<p>Here’s a small programming problem: write a function that takes a string and
returns the first word it finds in that string. If the function doesn’t find a
space in the string, the whole string must be one word, so the entire string
should be returned.</p>
<p>Let’s think about the signature of this function:</p>
<pre><code class="language-rust ignore">fn first_word(s: &amp;String) -&gt; ?
</code></pre>
<p>This function, <code>first_word</code>, has a <code>&amp;String</code> as a parameter. We don’t want
ownership, so this is fine. But what should we return? We don’t really have a
way to talk about <em>part</em> of a string. However, we could return the index of the
end of the word. Let’s try that, as shown in Listing 4-7.</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>fn first_word(s: &amp;String) -&gt; usize {
let bytes = s.as_bytes();
for (i, &amp;item) in bytes.iter().enumerate() {
if item == b' ' {
return i;
}
}
s.len()
}
<span class="boring">}
</span></code></pre></pre>
<p><span class="caption">Listing 4-7: The <code>first_word</code> function that returns a
byte index value into the <code>String</code> parameter</span></p>
<p>Because we need to go through the <code>String</code> element by element and check whether
a value is a space, we’ll convert our <code>String</code> to an array of bytes using the
<code>as_bytes</code> method:</p>
<pre><code class="language-rust ignore">let bytes = s.as_bytes();
</code></pre>
<p>Next, we create an iterator over the array of bytes using the <code>iter</code> method:</p>
<pre><code class="language-rust ignore">for (i, &amp;item) in bytes.iter().enumerate() {
</code></pre>
<p>We’ll discuss iterators in more detail in Chapter 13. For now, know that <code>iter</code>
is a method that returns each element in a collection and that <code>enumerate</code>
wraps the result of <code>iter</code> and returns each element as part of a tuple instead.
The first element of the tuple returned from <code>enumerate</code> is the index, and the
second element is a reference to the element. This is a bit more convenient
than calculating the index ourselves.</p>
<p>Because the <code>enumerate</code> method returns a tuple, we can use patterns to
destructure that tuple, just like everywhere else in Rust. So in the <code>for</code>
loop, we specify a pattern that has <code>i</code> for the index in the tuple and <code>&amp;item</code>
for the single byte in the tuple. Because we get a reference to the element
from <code>.iter().enumerate()</code>, we use <code>&amp;</code> in the pattern.</p>
<p>Inside the <code>for</code> loop, we search for the byte that represents the space by
using the byte literal syntax. If we find a space, we return the position.
Otherwise, we return the length of the string by using <code>s.len()</code>:</p>
<pre><code class="language-rust ignore"> if item == b' ' {
return i;
}
}
s.len()
</code></pre>
<p>We now have a way to find out the index of the end of the first word in the
string, but there’s a problem. We’re returning a <code>usize</code> on its own, but it’s
only a meaningful number in the context of the <code>&amp;String</code>. In other words,
because it’s a separate value from the <code>String</code>, there’s no guarantee that it
will still be valid in the future. Consider the program in Listing 4-8 that
uses the <code>first_word</code> function from Listing 4-7.</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust"><span class="boring">fn first_word(s: &amp;String) -&gt; usize {
</span><span class="boring"> let bytes = s.as_bytes();
</span><span class="boring">
</span><span class="boring"> for (i, &amp;item) in bytes.iter().enumerate() {
</span><span class="boring"> if item == b' ' {
</span><span class="boring"> return i;
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> s.len()
</span><span class="boring">}
</span><span class="boring">
</span>fn main() {
let mut s = String::from(&quot;hello world&quot;);
let word = first_word(&amp;s); // word will get the value 5
s.clear(); // this empties the String, making it equal to &quot;&quot;
// word still has the value 5 here, but there's no more string that
// we could meaningfully use the value 5 with. word is now totally invalid!
}
</code></pre></pre>
<p><span class="caption">Listing 4-8: Storing the result from calling the
<code>first_word</code> function and then changing the <code>String</code> contents</span></p>
<p>This program compiles without any errors and would also do so if we used <code>word</code>
after calling <code>s.clear()</code>. Because <code>word</code> isn’t connected to the state of <code>s</code>
at all, <code>word</code> still contains the value <code>5</code>. We could use that value <code>5</code> with
the variable <code>s</code> to try to extract the first word out, but this would be a bug
because the contents of <code>s</code> have changed since we saved <code>5</code> in <code>word</code>.</p>
<p>Having to worry about the index in <code>word</code> getting out of sync with the data in
<code>s</code> is tedious and error prone! Managing these indices is even more brittle if
we write a <code>second_word</code> function. Its signature would have to look like this:</p>
<pre><code class="language-rust ignore">fn second_word(s: &amp;String) -&gt; (usize, usize) {
</code></pre>
<p>Now we’re tracking a starting <em>and</em> an ending index, and we have even more
values that were calculated from data in a particular state but aren’t tied to
that state at all. We now have three unrelated variables floating around that
need to be kept in sync.</p>
<p>Luckily, Rust has a solution to this problem: string slices.</p>
<h3><a class="header" href="#string-slices" id="string-slices">String Slices</a></h3>
<p>A <em>string slice</em> is a reference to part of a <code>String</code>, and it looks like this:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let s = String::from(&quot;hello world&quot;);
let hello = &amp;s[0..5];
let world = &amp;s[6..11];
<span class="boring">}
</span></code></pre></pre>
<p>This is similar to taking a reference to the whole <code>String</code> but with the extra
<code>[0..5]</code> bit. Rather than a reference to the entire <code>String</code>, it’s a reference
to a portion of the <code>String</code>.</p>
<p>We can create slices using a range within brackets by specifying
<code>[starting_index..ending_index]</code>, where <code>starting_index</code> is the first position
in the slice and <code>ending_index</code> is one more than the last position in the
slice. Internally, the slice data structure stores the starting position and
the length of the slice, which corresponds to <code>ending_index</code> minus
<code>starting_index</code>. So in the case of <code>let world = &amp;s[6..11];</code>, <code>world</code> would be
a slice that contains a pointer to the 7th byte of <code>s</code> with a length value of 5.</p>
<p>Figure 4-6 shows this in a diagram.</p>
<img alt="world containing a pointer to the 6th byte of String s and a length 5" src="img/trpl04-06.svg" class="center" style="width: 50%;" />
<p><span class="caption">Figure 4-6: String slice referring to part of a
<code>String</code></span></p>
<p>With Rust’s <code>..</code> range syntax, if you want to start at the first index (zero),
you can drop the value before the two periods. In other words, these are equal:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let s = String::from(&quot;hello&quot;);
let slice = &amp;s[0..2];
let slice = &amp;s[..2];
<span class="boring">}
</span></code></pre></pre>
<p>By the same token, if your slice includes the last byte of the <code>String</code>, you
can drop the trailing number. That means these are equal:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let s = String::from(&quot;hello&quot;);
let len = s.len();
let slice = &amp;s[3..len];
let slice = &amp;s[3..];
<span class="boring">}
</span></code></pre></pre>
<p>You can also drop both values to take a slice of the entire string. So these
are equal:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let s = String::from(&quot;hello&quot;);
let len = s.len();
let slice = &amp;s[0..len];
let slice = &amp;s[..];
<span class="boring">}
</span></code></pre></pre>
<blockquote>
<p>Note: String slice range indices must occur at valid UTF-8 character
boundaries. If you attempt to create a string slice in the middle of a
multibyte character, your program will exit with an error. For the purposes
of introducing string slices, we are assuming ASCII only in this section; a
more thorough discussion of UTF-8 handling is in the <a href="ch08-02-strings.html#storing-utf-8-encoded-text-with-strings">“Storing UTF-8 Encoded
Text with Strings”</a><!-- ignore --> section of Chapter 8.</p>
</blockquote>
<p>With all this information in mind, let’s rewrite <code>first_word</code> to return a
slice. The type that signifies “string slice” is written as <code>&amp;str</code>:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>fn first_word(s: &amp;String) -&gt; &amp;str {
let bytes = s.as_bytes();
for (i, &amp;item) in bytes.iter().enumerate() {
if item == b' ' {
return &amp;s[0..i];
}
}
&amp;s[..]
}
<span class="boring">}
</span></code></pre></pre>
<p>We get the index for the end of the word in the same way as we did in Listing
4-7, by looking for the first occurrence of a space. When we find a space, we
return a string slice using the start of the string and the index of the space
as the starting and ending indices.</p>
<p>Now when we call <code>first_word</code>, we get back a single value that is tied to the
underlying data. The value is made up of a reference to the starting point of
the slice and the number of elements in the slice.</p>
<p>Returning a slice would also work for a <code>second_word</code> function:</p>
<pre><code class="language-rust ignore">fn second_word(s: &amp;String) -&gt; &amp;str {
</code></pre>
<p>We now have a straightforward API that’s much harder to mess up, because the
compiler will ensure the references into the <code>String</code> remain valid. Remember
the bug in the program in Listing 4-8, when we got the index to the end of the
first word but then cleared the string so our index was invalid? That code was
logically incorrect but didn’t show any immediate errors. The problems would
show up later if we kept trying to use the first word index with an emptied
string. Slices make this bug impossible and let us know we have a problem with
our code much sooner. Using the slice version of <code>first_word</code> will throw a
compile-time error:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore does_not_compile">fn main() {
let mut s = String::from(&quot;hello world&quot;);
let word = first_word(&amp;s);
s.clear(); // error!
println!(&quot;the first word is: {}&quot;, word);
}
</code></pre>
<p>Here’s the compiler error:</p>
<pre><code class="language-text">error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable
--&gt; src/main.rs:18:5
|
16 | let word = first_word(&amp;s);
| -- immutable borrow occurs here
17 |
18 | s.clear(); // error!
| ^^^^^^^^^ mutable borrow occurs here
19 |
20 | println!(&quot;the first word is: {}&quot;, word);
| ---- immutable borrow later used here
</code></pre>
<p>Recall from the borrowing rules that if we have an immutable reference to
something, we cannot also take a mutable reference. Because <code>clear</code> needs to
truncate the <code>String</code>, it needs to get a mutable reference. Rust disallows
this, and compilation fails. Not only has Rust made our API easier to use, but
it has also eliminated an entire class of errors at compile time!</p>
<h4><a class="header" href="#string-literals-are-slices" id="string-literals-are-slices">String Literals Are Slices</a></h4>
<p>Recall that we talked about string literals being stored inside the binary. Now
that we know about slices, we can properly understand string literals:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let s = &quot;Hello, world!&quot;;
<span class="boring">}
</span></code></pre></pre>
<p>The type of <code>s</code> here is <code>&amp;str</code>: it’s a slice pointing to that specific point of
the binary. This is also why string literals are immutable; <code>&amp;str</code> is an
immutable reference.</p>
<h4><a class="header" href="#string-slices-as-parameters" id="string-slices-as-parameters">String Slices as Parameters</a></h4>
<p>Knowing that you can take slices of literals and <code>String</code> values leads us to
one more improvement on <code>first_word</code>, and that’s its signature:</p>
<pre><code class="language-rust ignore">fn first_word(s: &amp;String) -&gt; &amp;str {
</code></pre>
<p>A more experienced Rustacean would write the signature shown in Listing 4-9
instead because it allows us to use the same function on both <code>&amp;String</code> values
and <code>&amp;str</code> values.</p>
<pre><code class="language-rust ignore">fn first_word(s: &amp;str) -&gt; &amp;str {
</code></pre>
<p><span class="caption">Listing 4-9: Improving the <code>first_word</code> function by using
a string slice for the type of the <code>s</code> parameter</span></p>
<p>If we have a string slice, we can pass that directly. If we have a <code>String</code>, we
can pass a slice of the entire <code>String</code>. Defining a function to take a string
slice instead of a reference to a <code>String</code> makes our API more general and useful
without losing any functionality:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust"><span class="boring">fn first_word(s: &amp;str) -&gt; &amp;str {
</span><span class="boring"> let bytes = s.as_bytes();
</span><span class="boring">
</span><span class="boring"> for (i, &amp;item) in bytes.iter().enumerate() {
</span><span class="boring"> if item == b' ' {
</span><span class="boring"> return &amp;s[0..i];
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> &amp;s[..]
</span><span class="boring">}
</span>fn main() {
let my_string = String::from(&quot;hello world&quot;);
// first_word works on slices of `String`s
let word = first_word(&amp;my_string[..]);
let my_string_literal = &quot;hello world&quot;;
// first_word works on slices of string literals
let word = first_word(&amp;my_string_literal[..]);
// Because string literals *are* string slices already,
// this works too, without the slice syntax!
let word = first_word(my_string_literal);
}
</code></pre></pre>
<h3><a class="header" href="#other-slices" id="other-slices">Other Slices</a></h3>
<p>String slices, as you might imagine, are specific to strings. But there’s a
more general slice type, too. Consider this array:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let a = [1, 2, 3, 4, 5];
<span class="boring">}
</span></code></pre></pre>
<p>Just as we might want to refer to a part of a string, we might want to refer
to part of an array. We’d do so like this:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let a = [1, 2, 3, 4, 5];
let slice = &amp;a[1..3];
<span class="boring">}
</span></code></pre></pre>
<p>This slice has the type <code>&amp;[i32]</code>. It works the same way as string slices do, by
storing a reference to the first element and a length. You’ll use this kind of
slice for all sorts of other collections. We’ll discuss these collections in
detail when we talk about vectors in Chapter 8.</p>
<h2><a class="header" href="#summary" id="summary">Summary</a></h2>
<p>The concepts of ownership, borrowing, and slices ensure memory safety in Rust
programs at compile time. The Rust language gives you control over your memory
usage in the same way as other systems programming languages, but having the
owner of data automatically clean up that data when the owner goes out of scope
means you don’t have to write and debug extra code to get this control.</p>
<p>Ownership affects how lots of other parts of Rust work, so we’ll talk about
these concepts further throughout the rest of the book. Let’s move on to
Chapter 5 and look at grouping pieces of data together in a <code>struct</code>.</p>
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