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<ul class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong>1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong>2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong>3.</strong> Syntax and Semantics</a></li><li><ul class="section"><li><a href="variable-bindings.html"><strong>3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong>3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong>3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong>3.4.</strong> Comments</a></li><li><a href="if.html"><strong>3.5.</strong> if</a></li><li><a href="loops.html"><strong>3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong>3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong>3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong>3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong>3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong>3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong>3.12.</strong> Structs</a></li><li><a href="enums.html"><strong>3.13.</strong> Enums</a></li><li><a href="match.html"><strong>3.14.</strong> Match</a></li><li><a href="patterns.html"><strong>3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong>3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong>3.17.</strong> Strings</a></li><li><a href="generics.html"><strong>3.18.</strong> Generics</a></li><li><a href="traits.html"><strong>3.19.</strong> Traits</a></li><li><a href="drop.html"><strong>3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong>3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong>3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong>3.23.</strong> Closures</a></li><li><a href="ufcs.html" class="active"><strong>3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong>3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong>3.26.</strong> <code>const</code> and <code>static</code></a></li><li><a href="attributes.html"><strong>3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong>3.28.</strong> <code>type</code> aliases</a></li><li><a href="casting-between-types.html"><strong>3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong>3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong>3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong>3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong>3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong>3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong>3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong>3.36.</strong> <code>unsafe</code></a></li></ul></li><li><a href="effective-rust.html"><strong>4.</strong> Effective Rust</a></li><li><ul class="section"><li><a href="the-stack-and-the-heap.html"><strong>4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong>4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong>4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong>4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong>4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong>4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong>4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong>4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong>4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong>4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong>4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong>4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong>4.13.</strong> Procedural Macros (and custom derive)</a></li></ul></li><li><a href="glossary.html"><strong>5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong>6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong>7.</strong> Bibliography</a></li></ul>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<a class="header" href="ufcs.html#universal-function-call-syntax" id="universal-function-call-syntax"><h1>Universal Function Call Syntax</h1></a>
<p>Sometimes, functions can have the same names. Consider this code:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
trait Foo {
fn f(&self);
}
trait Bar {
fn f(&self);
}
struct Baz;
impl Foo for Baz {
fn f(&self) { println!("Baz’s impl of Foo"); }
}
impl Bar for Baz {
fn f(&self) { println!("Baz’s impl of Bar"); }
}
let b = Baz;
#}</code></pre></pre>
<p>If we were to try to call <code>b.f()</code>, we’d get an error:</p>
<pre><code class="language-text">error: multiple applicable methods in scope [E0034]
b.f();
^~~
note: candidate #1 is defined in an impl of the trait `main::Foo` for the type
`main::Baz`
fn f(&self) { println!("Baz’s impl of Foo"); }
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: candidate #2 is defined in an impl of the trait `main::Bar` for the type
`main::Baz`
fn f(&self) { println!("Baz’s impl of Bar"); }
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
</code></pre>
<p>We need a way to disambiguate which method we need. This feature is called
‘universal function call syntax’, and it looks like this:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# trait Foo {
# fn f(&self);
# }
# trait Bar {
# fn f(&self);
# }
# struct Baz;
# impl Foo for Baz {
# fn f(&self) { println!("Baz’s impl of Foo"); }
# }
# impl Bar for Baz {
# fn f(&self) { println!("Baz’s impl of Bar"); }
# }
# let b = Baz;
Foo::f(&b);
Bar::f(&b);
#}</code></pre></pre>
<p>Let’s break it down.</p>
<pre><code class="language-rust ignore">Foo::
Bar::
</code></pre>
<p>These halves of the invocation are the types of the two traits: <code>Foo</code> and
<code>Bar</code>. This is what ends up actually doing the disambiguation between the two:
Rust calls the one from the trait name you use.</p>
<pre><code class="language-rust ignore">f(&b)
</code></pre>
<p>When we call a method like <code>b.f()</code> using <a href="method-syntax.html">method syntax</a>, Rust
will automatically borrow <code>b</code> if <code>f()</code> takes <code>&self</code>. In this case, Rust will
not, and so we need to pass an explicit <code>&b</code>.</p>
<a class="header" href="ufcs.html#angle-bracket-form" id="angle-bracket-form"><h1>Angle-bracket Form</h1></a>
<p>The form of UFCS we just talked about:</p>
<pre><code class="language-rust ignore">Trait::method(args);
</code></pre>
<p>Is a short-hand. There’s an expanded form of this that’s needed in some
situations:</p>
<pre><code class="language-rust ignore"><Type as Trait>::method(args);
</code></pre>
<p>The <code><>::</code> syntax is a means of providing a type hint. The type goes inside
the <code><></code>s. In this case, the type is <code>Type as Trait</code>, indicating that we want
<code>Trait</code>’s version of <code>method</code> to be called here. The <code>as Trait</code> part is
optional if it’s not ambiguous. Same with the angle brackets, hence the
shorter form.</p>
<p>Here’s an example of using the longer form.</p>
<pre><pre class="playpen"><code class="language-rust">trait Foo {
fn foo() -> i32;
}
struct Bar;
impl Bar {
fn foo() -> i32 {
20
}
}
impl Foo for Bar {
fn foo() -> i32 {
10
}
}
fn main() {
assert_eq!(10, <Bar as Foo>::foo());
assert_eq!(20, Bar::foo());
}
</code></pre></pre>
<p>Using the angle bracket syntax lets you call the trait method instead of the
inherent one.</p>
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