Module std::ref_keyword[][src]

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()));
Run

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()));
Run

& vs ref

  • & denotes that your pattern expects a reference to an object. Hence & is a part of said pattern: &Foo matches different objects than Foo does.

  • ref indicates that you want a reference to an unpacked value. It is not matched against: Foo(ref foo) matches the same objects as Foo(foo).

See also the Reference for more information.