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<ol class="chapter"><li class="chapter-item expanded "><a href="motivation.html"><strong aria-hidden="true">1.</strong> Motivation</a></li><li class="chapter-item expanded "><a href="rust-basics.html"><strong aria-hidden="true">2.</strong> Rust Basics</a></li><li class="chapter-item expanded "><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li class="chapter-item expanded "><a href="borrowing.html" class="active"><strong aria-hidden="true">4.</strong> Borrowing</a></li><li class="chapter-item expanded "><a href="vectors.html"><strong aria-hidden="true">5.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="structs.html"><strong aria-hidden="true">6.</strong> Optional: Structures</a></li><li class="chapter-item expanded "><a href="additional_resources.html"><strong aria-hidden="true">7.</strong> Additional Resources</a></li></ol>
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<h1 id="borrowing"><a class="header" href="#borrowing">Borrowing</a></h1>
<p>In the previous section, we learned that each resource has a unique owner.
Ownership can be moved—for example, into a function.</p>
<p>In many situations, however, we do not want to permanently move a resource into
a function. Instead, we want to retain ownership but allow the function to temporarily
access the resource while it executes.</p>
<p>We could accomplish this by having each function agree to return resources of this
sort. For
example, <code>take_and_return_ownership</code> below takes ownership of a string
resource and returns ownership of that exact same resource. The caller, <code>main</code>,
assigns the returned resource to the same variable, <code>s</code>. </p>
<div class="flex-container vis_block" style="position:relative; margin-left:-75px; margin-right:-75px; display: flex;">
<object type="image/svg+xml" class="func_take_return_ownership code_panel" data="assets/code_examples/func_take_return_ownership/vis_code.svg"></object>
<object type="image/svg+xml" class="func_take_return_ownership tl_panel" data="assets/code_examples/func_take_return_ownership/vis_timeline.svg" style="width: auto;" onmouseenter="helpers('func_take_return_ownership')"></object>
</div>
<p>This code prints <code>hello</code> twice.</p>
<p>The type of
<code>take_and_return_ownership</code> does not guarantee that the returned resource is the
same as the provided resource. Instead, the programmer has to trust that it returns
the same resource.</p>
<p>As code becomes more complex, this pattern of returning all of the provided
resources explicitly becomes both syntactically and semantically unwieldy.</p>
<p>Fortunately, Rust offers a powerful solution: passing in arguments via a
reference. Taking a reference does <em>not</em> change the owner of a resource.
Instead, the reference simply <em>borrows</em> access to the resource temporarily.
Rust's <em>borrow checker</em> requires that references to resources do not outlive
their owner, to avoid the possibility of there being references to resources
that the ownership system has decided can be dropped.</p>
<p>There are two kinds of borrows in Rust, <em>immutable borrows</em> and <em>mutable
borrows</em>. These differ in how much access to the resource they provide. </p>
<h2 id="immutable-borrows"><a class="header" href="#immutable-borrows">Immutable Borrows</a></h2>
<p>In the following example, we define a function, <code>f</code>, that takes an <em>immutable
reference</em> to a <code>String</code>, which has type <code>&String</code>, as input. It then de-references
the immutable reference, written <code>*s</code>, in order to print it.</p>
<p>When the <code>main</code> function calls <code>f</code>, it must provide a reference to a <code>String</code> as
an argument. Here, we do so by taking a reference to the let-bound variable <code>x</code>
on Line 3, written <code>&x</code>. Taking a reference does <strong>not</strong> cause a change in
ownership, so <code>x</code> still owns the string resource in the remainder of <code>main</code>
and it can, for example, print <code>x</code> on Line 4. The resource will be dropped when
<code>x</code> goes out of scope at the end of <code>main</code> as we discussed previously. Because <code>f</code>
takes a reference, it is only <em>borrowing</em> access to the resource that the
reference points to. It does not need to explicitly return the resource because
it does not own it. Rust knows that the borrow does not outlive the owner
because the borrow is no longer accessible after <code>f</code> returns.</p>
<div class="flex-container vis_block" style="position:relative; margin-left:-75px; margin-right:-75px; display: flex;">
<object type="image/svg+xml" class="immutable_borrow code_panel" data="assets/code_examples/immutable_borrow/vis_code.svg"></object>
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<p>This code prints <code>hello</code> twice.</p>
<p>Note: you do not actually need to dereference <code>s</code> to pass it to <code>println!</code> in Rust:
it is a macro, so it will automatically dereference or borrow as needed
to ensure that a move is not needed. Indeed, Rust does a lot of implicit
borrowing and dereferencing to make its syntax simple, as we will see in other examples
below.</p>
<p>Methods of the <code>String</code> type, like <code>len</code> for computing the length, typically
take their arguments by reference. You can call a method explicitly with a
reference, e.g. <code>String::len(&s)</code>. As shorthand, you can use dot notation to
call a method, e.g. <code>s.len()</code>. This implicitly takes a reference to <code>s</code>. </p>
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</div>
<p>This code prints <code>len1 = 5 = len2 = 5</code>.</p>
<p>You can keep multiple immutable borrows live at the same time, e.g. <code>y</code> and <code>z</code>
in the following example are both live as shown in the visualization. For this
reason, immutable borrows are also sometimes called shared borrows: each
immutable reference shares access to the resource with the owner and with any
other immutable references that might be live.</p>
<div class="flex-container vis_block" style="position:relative; margin-left:-75px; margin-right:-75px; display: flex;">
<object type="image/svg+xml" class="multiple_immutable_borrow code_panel" data="assets/code_examples/multiple_immutable_borrow/vis_code.svg"></object>
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</div>
<p>This code prints <code>hello and hello</code>.</p>
<p>Ownership of a resource cannot be moved while it is borrowed. For example, the
following is erroneous:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let s = String::from("hello");
let x = &s;
let s2 = s; // ERROR: cannot move s while a borrow is live
println!("{}", String::len(x));
}
</code></pre></pre>
<p>The compiler error here is: <code>cannot move out of s because it is borrowed</code>.</p>
<h2 id="mutable-borrows"><a class="header" href="#mutable-borrows">Mutable Borrows</a></h2>
<p>Unlike immutable borrows, Rust's mutable borrows allow you to mutate the
borrowed resource. In the example below, we push (copy) the contents of a <code>String</code> <code>s2</code>
to the end of the heap-allocated <code>String</code> <code>s1</code> twice, first by explictly calling
the <code>String::push_str</code> method, and then using the equivalent shorthand method
call syntax. In both cases, the method takes a <em>mutable reference</em> to <code>s1</code>,
written explicitly <code>&mut s1</code>.</p>
<div class="flex-container vis_block" style="position:relative; margin-left:-75px; margin-right:-75px; display: flex;">
<object type="image/svg+xml" class="mutable_borrow_method_call code_panel" data="assets/code_examples/mutable_borrow_method_call/vis_code.svg"></object>
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</div>
<p>This code prints <code>Hello, world, world</code>.</p>
<p>Code that does a lot of mutation is notoriously difficult to reason about, so in
Rust, mutation is much more carefully controlled than in other imperative
languages.</p>
<p>First, you can only take a mutable borrow from a mutable variable, i.e. one
bound using <code>let mut</code> like <code>s1</code> in the example above. Immutability is the
default in Rust because it is considered easier to reason about.</p>
<p>Second, mutable borrows are unique—you cannot take a borrow, mutable or
immutable, if any mutable borrow is live. This means that you can be certain
that no other code will be mutating a resource when you have mutably borrowed it.
For this reason, mutable borrows are also sometimes called <em>unique borrows</em>.</p>
<p>For example, the following code is erroneous because a mutable borrow, <code>y</code>, is
live.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let mut x = String::from("hello");
let y = &mut x;
f(&x); // ERROR: y is still live
String::push_str(y, ", world");
}
fn f(x : &String) {
println!("{}", x);
}
</code></pre></pre>
<p>The compiler error here is: <code>cannot borrow x as immutable because it is also borrowed as mutable</code>.</p>
<p>The following code is erroneous for the same reason.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let mut x = String::from("Hello");
let y = &mut x;
let z = &mut x; // ERROR: y is still live
String::push_str(y, ", world");
String::push_str(z, ", friend");
println!("{}", x);
}
</code></pre></pre>
<p>The compiler error here is: <code>cannot borrow x as mutable more than once at a time</code>.</p>
<h3 id="optional-threading-in-rust"><a class="header" href="#optional-threading-in-rust">Optional: Threading in Rust</a></h3>
<p>In the example above, the two calls to <code>push_str</code> are sequenced. However, if we
wanted to execute them concurrently, we could do so by spawning a thread as
follows. Here, <code>|| { e }</code> is Rust's notation for an anonymous function taking
unit input.</p>
<pre><pre class="playground"><code class="language-rust">use std::thread;
fn main() {
let mut x = String::from("Hello");
let y = &mut x;
let z = &mut x; // NOT OK: y is still live
thread::spawn(|| { String::push_str(y, ", world"); });
String::push_str(z, ", friend");
println!("{}", x);
}
</code></pre></pre>
<p>If the borrow checker did not stop us, this program would have a race
condition—it could print either <code>Hello, world, friend</code> or <code>Hello, friend, world</code>
depending on the interleaving of the main thread and the newly spawned thread.
By tightly controlling mutation, Rust prevents races mediated by shared mutable state.
(The topic of parallelism and concurrency in Rust will be explored further in A9!)</p>
<h2 id="non-lexical-lifetimes"><a class="header" href="#non-lexical-lifetimes">Non-Lexical Lifetimes</a></h2>
<p>Above, we use the phrase "live borrow". A borrow is <em>live</em> if it is in scope and
there remain future uses of the borrow. A borrow dies as soon it is no longer
needed. So the following code works, even though there are two mutable borrows
in the same scope:</p>
<div class="flex-container vis_block" style="position:relative; margin-left:-75px; margin-right:-75px; display: flex;">
<object type="image/svg+xml" class="nll_lexical_scope_different code_panel" data="assets/code_examples/nll_lexical_scope_different/vis_code.svg"></object>
<object type="image/svg+xml" class="nll_lexical_scope_different tl_panel" data="assets/code_examples/nll_lexical_scope_different/vis_timeline.svg" style="width: auto;" onmouseenter="helpers('nll_lexical_scope_different')"></object>
</div>
<p>This code prints <code>Hello, world, world!!</code>.</p>
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