Expand description
Bind by reference during pattern matching.
ref annotates pattern bindings to make them borrow rather than move.
It is not a part of the pattern as far as matching is concerned: it does
not affect whether a value is matched, only how it is matched.
By default, match statements consume all they can, which can sometimes
be a problem, when you don’t really need the value to be moved and owned:
let maybe_name = Some(String::from("Alice"));
// The variable 'maybe_name' is consumed here ...
match maybe_name {
Some(n) => println!("Hello, {n}"),
_ => println!("Hello, world"),
}
// ... and is now unavailable.
println!("Hello again, {}", maybe_name.unwrap_or("world".into()));Using the ref keyword, the value is only borrowed, not moved, making it
available for use after the match statement:
let maybe_name = Some(String::from("Alice"));
// Using `ref`, the value is borrowed, not moved ...
match maybe_name {
Some(ref n) => println!("Hello, {n}"),
_ => println!("Hello, world"),
}
// ... so it's available here!
println!("Hello again, {}", maybe_name.unwrap_or("world".into()));§& vs ref
-
&denotes that your pattern expects a reference to an object. Hence&is a part of said pattern:&Foomatches different objects thanFoodoes. -
refindicates that you want a reference to an unpacked value. It is not matched against:Foo(ref foo)matches the same objects asFoo(foo).
See also the Reference for more information.