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