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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../translations.html"><strong aria-hidden="true">1.1.</strong> Translations</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">2.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html" class="active"><strong aria-hidden="true">2.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">2.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">2.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">2.4.</strong> The Default Trait</a></li><li class="chapter-item expanded "><a href="../idioms/deref.html"><strong aria-hidden="true">2.5.</strong> Collections Are Smart Pointers</a></li><li class="chapter-item expanded "><a href="../idioms/dtor-finally.html"><strong aria-hidden="true">2.6.</strong> Finalisation in Destructors</a></li><li class="chapter-item expanded "><a href="../idioms/mem-replace.html"><strong aria-hidden="true">2.7.</strong> mem::{take(_), replace(_)}</a></li><li class="chapter-item expanded "><a href="../idioms/on-stack-dyn-dispatch.html"><strong aria-hidden="true">2.8.</strong> On-Stack Dynamic Dispatch</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/intro.html"><strong aria-hidden="true">2.9.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/ffi/errors.html"><strong aria-hidden="true">2.9.1.</strong> Idiomatic Errors</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/accepting-strings.html"><strong aria-hidden="true">2.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/passing-strings.html"><strong aria-hidden="true">2.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">2.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">2.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html"><strong aria-hidden="true">2.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">2.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">2.14.</strong> Temporary mutability</a></li><li class="chapter-item expanded "><a href="../idioms/return-consumed-arg-on-error.html"><strong aria-hidden="true">2.15.</strong> Return consumed arg on error</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">3.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/intro.html"><strong aria-hidden="true">3.1.</strong> Behavioural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/command.html"><strong aria-hidden="true">3.1.1.</strong> Command</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/interpreter.html"><strong aria-hidden="true">3.1.2.</strong> Interpreter</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/newtype.html"><strong aria-hidden="true">3.1.3.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/RAII.html"><strong aria-hidden="true">3.1.4.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/strategy.html"><strong aria-hidden="true">3.1.5.</strong> Strategy</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/visitor.html"><strong aria-hidden="true">3.1.6.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/creational/intro.html"><strong aria-hidden="true">3.2.</strong> Creational</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/creational/builder.html"><strong aria-hidden="true">3.2.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/creational/fold.html"><strong aria-hidden="true">3.2.2.</strong> Fold</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/structural/intro.html"><strong aria-hidden="true">3.3.</strong> Structural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/structural/compose-structs.html"><strong aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li><li class="chapter-item expanded "><a href="../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</a></li><li class="chapter-item expanded "><a href="../functional/lenses.html"><strong aria-hidden="true">5.3.</strong> Lenses and Prisms</a></li></ol></li><li class="chapter-item expanded "><a href="../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
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<h1 id="use-borrowed-types-for-arguments"><a class="header" href="#use-borrowed-types-for-arguments">Use borrowed types for arguments</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>Using a target of a deref coercion can increase the flexibility of your code
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when you are deciding which argument type to use for a function argument.
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In this way, the function will accept more input types.</p>
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<p>This is not limited to slice-able or fat pointer types.
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In fact, you should always prefer using the <strong>borrowed type</strong> over
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<strong>borrowing the owned type</strong>.
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Such as <code>&str</code> over <code>&String</code>, <code>&[T]</code> over <code>&Vec<T></code>, or <code>&T</code> over <code>&Box<T></code>.</p>
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<p>Using borrowed types you can avoid layers of indirection for those instances
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where the owned type already provides a layer of indirection. For instance, a
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<code>String</code> has a layer of indirection, so a <code>&String</code> will have two layers of
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indirection. We can avoid this by using <code>&str</code> instead, and letting <code>&String</code>
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coerce to a <code>&str</code> whenever the function is invoked.</p>
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<h2 id="example"><a class="header" href="#example">Example</a></h2>
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<p>For this example, we will illustrate some differences for using <code>&String</code> as a
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function argument versus using a <code>&str</code>, but the ideas apply as well to using
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<code>&Vec<T></code> versus using a <code>&[T]</code> or using a <code>&Box<T></code> versus a <code>&T</code>.</p>
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<p>Consider an example where we wish to determine if a word contains three
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consecutive vowels. We don't need to own the string to determine this, so we
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will take a reference.</p>
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<p>The code might look something like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2018">fn three_vowels(word: &String) -> bool {
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let mut vowel_count = 0;
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for c in word.chars() {
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match c {
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'a' | 'e' | 'i' | 'o' | 'u' => {
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vowel_count += 1;
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if vowel_count >= 3 {
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return true
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}
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}
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_ => vowel_count = 0
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}
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}
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false
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}
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fn main() {
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let ferris = "Ferris".to_string();
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let curious = "Curious".to_string();
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println!("{}: {}", ferris, three_vowels(&ferris));
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println!("{}: {}", curious, three_vowels(&curious));
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// This works fine, but the following two lines would fail:
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// println!("Ferris: {}", three_vowels("Ferris"));
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// println!("Curious: {}", three_vowels("Curious"));
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}</code></pre></pre>
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<p>This works fine because we are passing a <code>&String</code> type as a parameter.
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If we remove the comments on the last two lines, the example will fail. This
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is because a <code>&str</code> type will not coerce to a <code>&String</code> type. We can fix this
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by simply modifying the type for our argument.</p>
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<p>For instance, if we change our function declaration to:</p>
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<pre><code class="language-rust ignore">fn three_vowels(word: &str) -> bool {</code></pre>
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<p>then both versions will compile and print the same output.</p>
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<pre><code class="language-bash">Ferris: false
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Curious: true
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</code></pre>
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<p>But wait, that's not all! There is more to this story.
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It's likely that you may say to yourself: that doesn't matter, I will never be
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using a <code>&'static str</code> as an input anyways (as we did when we used <code>"Ferris"</code>).
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Even ignoring this special example, you may still find that using <code>&str</code> will
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give you more flexibility than using a <code>&String</code>.</p>
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<p>Let's now take an example where someone gives us a sentence, and we want to
|
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determine if any of the words in the sentence contain three consecutive vowels.
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We probably should make use of the function we have already defined and simply
|
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feed in each word from the sentence.</p>
|
|
<p>An example of this could look like this:</p>
|
|
<pre><pre class="playground"><code class="language-rust edition2018">fn three_vowels(word: &str) -> bool {
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let mut vowel_count = 0;
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|
for c in word.chars() {
|
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match c {
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'a' | 'e' | 'i' | 'o' | 'u' => {
|
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vowel_count += 1;
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if vowel_count >= 3 {
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return true
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}
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}
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_ => vowel_count = 0
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}
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}
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false
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}
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fn main() {
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let sentence_string =
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"Once upon a time, there was a friendly curious crab named Ferris".to_string();
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for word in sentence_string.split(' ') {
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if three_vowels(word) {
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println!("{} has three consecutive vowels!", word);
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}
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}
|
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}</code></pre></pre>
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<p>Running this example using our function declared with an argument type <code>&str</code>
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will yield</p>
|
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<pre><code class="language-bash">curious has three consecutive vowels!
|
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</code></pre>
|
|
<p>However, this example will not run when our function is declared with an
|
|
argument type <code>&String</code>. This is because string slices are a <code>&str</code> and not a
|
|
<code>&String</code> which would require an allocation to be converted to <code>&String</code> which
|
|
is not implicit, whereas converting from <code>String</code> to <code>&str</code> is cheap and implicit.</p>
|
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<h2 id="see-also"><a class="header" href="#see-also">See also</a></h2>
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<ul>
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<li><a href="https://doc.rust-lang.org/reference/type-coercions.html">Rust Language Reference on Type Coercions</a></li>
|
|
<li>For more discussion on how to handle <code>String</code> and <code>&str</code> see
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|
<a href="https://web.archive.org/web/20201112023149/https://hermanradtke.com/2015/05/03/string-vs-str-in-rust-functions.html">this blog series (2015)</a>
|
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by Herman J. Radtke III</li>
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</ul>
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