% Borrow and AsRef
The Borrow
and AsRef
traits are very similar, but
different. Here’s a quick refresher on what these two traits mean.
Borrow
The Borrow
trait is used when you’re writing a data structure, and you want to
use either an owned or borrowed type as synonymous for some purpose.
For example, HashMap
has a get
method which uses Borrow
:
fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
where K: Borrow<Q>,
Q: Hash + Eq
This signature is pretty complicated. The K
parameter is what we’re interested
in here. It refers to a parameter of the HashMap
itself:
struct HashMap<K, V, S = RandomState> {
The K
parameter is the type of key the HashMap
uses. So, looking at
the signature of get()
again, we can use get()
when the key implements
Borrow<Q>
. That way, we can make a HashMap
which uses String
keys,
but use &str
s when we’re searching:
use std::collections::HashMap;
let mut map = HashMap::new();
map.insert("Foo".to_string(), 42);
assert_eq!(map.get("Foo"), Some(&42));
This is because the standard library has impl Borrow<str> for String
.
For most types, when you want to take an owned or borrowed type, a &T
is
enough. But one area where Borrow
is effective is when there’s more than one
kind of borrowed value. This is especially true of references and slices: you
can have both an &T
or a &mut T
. If we wanted to accept both of these types,
Borrow
is up for it:
use std::borrow::Borrow;
use std::fmt::Display;
fn foo<T: Borrow<i32> + Display>(a: T) {
println!("a is borrowed: {}", a);
}
let mut i = 5;
foo(&i);
foo(&mut i);
This will print out a is borrowed: 5
twice.
AsRef
The AsRef
trait is a conversion trait. It’s used for converting some value to
a reference in generic code. Like this:
let s = "Hello".to_string();
fn foo<T: AsRef<str>>(s: T) {
let slice = s.as_ref();
}
Which should I use?
We can see how they’re kind of the same: they both deal with owned and borrowed versions of some type. However, they’re a bit different.
Choose Borrow
when you want to abstract over different kinds of borrowing, or
when you’re building a data structure that treats owned and borrowed values in
equivalent ways, such as hashing and comparison.
Choose AsRef
when you want to convert something to a reference directly, and
you’re writing generic code.