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<main>
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<h2><a class="header" href="#giving-references-to-functions" id="giving-references-to-functions">Giving references to functions</a></h2>
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<p>References are very useful for functions. The rule in Rust on values is: a value can only have one owner.</p>
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<p>This code will not work:</p>
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<pre><pre class="playground"><code class="language-rust">fn print_country(country_name: String) {
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println!("{}", country_name);
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}
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fn main() {
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let country = String::from("Austria");
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print_country(country); // We print "Austria"
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print_country(country); // ⚠️ That was fun, let's do it again!
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}
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</code></pre></pre>
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<p>It does not work because <code>country</code> is destroyed. Here's how:</p>
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<ul>
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<li>Step 1: We create the <code>String</code> called <code>country</code>. <code>country</code> is the owner.</li>
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<li>Step 2: We give <code>country</code> to <code>print_country</code>. <code>print_country</code> doesn't have an <code>-></code>, so it doesn't return anything. After <code>print_country</code> finishes, our <code>String</code> is now dead.</li>
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<li>Step 3: We try to give <code>country</code> to <code>print_country</code>, but we already did that. We don't have <code>country</code> to give anymore.</li>
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</ul>
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<p>We can make <code>print_country</code> give the <code>String</code> back, but it is a bit awkward.</p>
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<pre><pre class="playground"><code class="language-rust">fn print_country(country_name: String) -> String {
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println!("{}", country_name);
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country_name // return it here
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}
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fn main() {
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let country = String::from("Austria");
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let country = print_country(country); // we have to use let here now to get the String back
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print_country(country);
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}
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</code></pre></pre>
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<p>Now it prints:</p>
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<pre><code class="language-text">Austria
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Austria
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</code></pre>
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<p>The much better way to fix this is by adding <code>&</code>.</p>
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<pre><pre class="playground"><code class="language-rust">fn print_country(country_name: &String) {
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println!("{}", country_name);
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}
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fn main() {
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let country = String::from("Austria");
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print_country(&country); // We print "Austria"
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print_country(&country); // That was fun, let's do it again!
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}
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</code></pre></pre>
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<p>Now <code>print_country()</code> is a function that takes a reference to a <code>String</code>: a <code>&String</code>. Also, we give it a reference to country by writing <code>&country</code>. This says "you can look at it, but I will keep it".</p>
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<p>Now let's do something similar with a mutable reference. Here is an example of a function that uses a mutable variable.</p>
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<pre><pre class="playground"><code class="language-rust">fn add_hungary(country_name: &mut String) { // first we say that the function takes a mutable reference
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country_name.push_str("-Hungary"); // push_str() adds a &str to a String
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println!("Now it says: {}", country_name);
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}
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fn main() {
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let mut country = String::from("Austria");
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add_hungary(&mut country); // we also need to give it a mutable reference.
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}
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</code></pre></pre>
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<p>This prints <code>Now it says: Austria-Hungary</code>.</p>
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<p>So to conclude:</p>
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<ul>
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<li><code>fn function_name(variable: String)</code> takes a <code>String</code> and owns it. If it doesn't return anything, then the variable dies inside the function.</li>
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<li><code>fn function_name(variable: &String)</code> borrows a <code>String</code> and can look at it</li>
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<li><code>fn function_name(variable: &mut String)</code> borrows a <code>String</code> and can change it</li>
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</ul>
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<p>Here is an example that looks like a mutable reference, but it is different.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let country = String::from("Austria"); // country is not mutable, but we are going to print Austria-Hungary. How?
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adds_hungary(country);
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}
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fn adds_hungary(mut country: String) { // Here's how: adds_hungary takes the String and declares it mutable!
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country.push_str("-Hungary");
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println!("{}", country);
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}
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</code></pre></pre>
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<p>How is this possible? It is because <code>mut country</code> is not a reference: <code>adds_hungary</code> owns <code>country</code> now. (Remember, it takes <code>String</code> and not <code>&String</code>). The moment you call <code>adds_hungary</code>, it becomes the full owner. <code>country</code> has nothing to do with <code>String::from("Austria")</code> anymore. So <code>adds_hungary</code> can take <code>country</code> as mutable, and it is perfectly safe to do so.</p>
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<p>Remember our employee Powerpoint and manager situation above? In this situation it is like the employee just giving his whole computer to the manager. The employee won't ever touch it again, so the manager can do anything he wants to it.</p>
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<i class="fa fa-angle-right"></i>
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</a>
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</nav>
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</div>
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<script type="text/javascript">
|
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window.playground_copyable = true;
|
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</script>
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<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
|
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<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
|
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<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
|
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<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
|
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<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
|
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<script src="book.js" type="text/javascript" charset="utf-8"></script>
|
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<!-- Custom JS scripts -->
|
||
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</body>
|
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</html>
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