# 1.0.0[−][src]Trait core::borrow::Borrow

    pub trait Borrow<Borrowed: ?Sized> {
        fn borrow(&self) -> &Borrowed;
    }

A trait for borrowing data.

In Rust, it is common to provide different representations of a type for different use cases. For instance, storage location and management for a value can be specifically chosen as appropriate for a particular use via pointer types such as `Box<T>` or `Rc<T>`. Beyond these generic wrappers that can be used with any type, some types provide optional facets providing potentially costly functionality. An example for such a type is `String` which adds the ability to extend a string to the basic `str`. This requires keeping additional information unnecessary for a simple, immutable string.

These types provide access to the underlying data through references to the type of that data. They are said to be ‘borrowed as’ that type. For instance, a `Box<T>` can be borrowed as `T` while a `String` can be borrowed as `str`.

Types express that they can be borrowed as some type `T` by implementing `Borrow<T>`, providing a reference to a `T` in the trait’s `borrow` method. A type is free to borrow as several different types. If it wishes to mutably borrow as the type – allowing the underlying data to be modified, it can additionally implement `BorrowMut<T>`.

Further, when providing implementations for additional traits, it needs to be considered whether they should behave identical to those of the underlying type as a consequence of acting as a representation of that underlying type. Generic code typically uses `Borrow<T>` when it relies on the identical behavior of these additional trait implementations. These traits will likely appear as additional trait bounds.

If generic code merely needs to work for all types that can provide a reference to related type `T`, it is often better to use `AsRef<T>` as more types can safely implement it.

# Examples

As a data collection, `HashMap<K, V>` owns both keys and values. If the key’s actual data is wrapped in a managing type of some kind, it should, however, still be possible to search for a value using a reference to the key’s data. For instance, if the key is a string, then it is likely stored with the hash map as a `String`, while it should be possible to search using a `&str`. Thus, `insert` needs to operate on a `String` while `get` needs to be able to use a `&str`.

Slightly simplified, the relevant parts of `HashMap<K, V>` look like this:

    use std::borrow::Borrow;
    use std::hash::Hash;

    pub struct HashMap<K, V> {
        // fields omitted
    }

    impl<K, V> HashMap<K, V> {
        pub fn insert(&self, key: K, value: V) -> Option<V>
        where K: Hash + Eq
        {
            // ...
        }

        pub fn get<Q>(&self, k: &Q) -> Option<&V>
        where
            K: Borrow<Q>,
            Q: Hash + Eq + ?Sized
        {
            // ...
        }
    }Run

The entire hash map is generic over a key type `K`. Because these keys are stored with the hash map, this type has to own the key’s data. When inserting a key-value pair, the map is given such a `K` and needs to find the correct hash bucket and check if the key is already present based on that `K`. It therefore requires `K: Hash + Eq`.

When searching for a value in the map, however, having to provide a reference to a `K` as the key to search for would require to always create such an owned value. For string keys, this would mean a `String` value needs to be created just for the search for cases where only a `str` is available.

Instead, the `get` method is generic over the type of the underlying key data, called `Q` in the method signature above. It states that `K` borrows as a `Q` by requiring that `K: Borrow<Q>`. By additionally requiring `Q: Hash + Eq`, it signals the requirement that `K` and `Q` have implementations of the `Hash` and `Eq` traits that produce identical results.

The implementation of `get` relies in particular on identical implementations of `Hash` by determining the key’s hash bucket by calling `Hash::hash` on the `Q` value even though it inserted the key based on the hash value calculated from the `K` value.

As a consequence, the hash map breaks if a `K` wrapping a `Q` value produces a different hash than `Q`. For instance, imagine you have a type that wraps a string but compares ASCII letters ignoring their case:

    pub struct CaseInsensitiveString(String);

    impl PartialEq for CaseInsensitiveString {
        fn eq(&self, other: &Self) -> bool {
            self.0.eq_ignore_ascii_case(&other.0)
        }
    }

    impl Eq for CaseInsensitiveString { }Run

Because two equal values need to produce the same hash value, the implementation of `Hash` needs to ignore ASCII case, too:

    impl Hash for CaseInsensitiveString {
        fn hash<H: Hasher>(&self, state: &mut H) {
            for c in self.0.as_bytes() {
                c.to_ascii_lowercase().hash(state)
            }
        }
    }Run

Can `CaseInsensitiveString` implement `Borrow<str>`? It certainly can provide a reference to a string slice via its contained owned string. But because its `Hash` implementation differs, it behaves differently from `str` and therefore must not, in fact, implement `Borrow<str>`. If it wants to allow others access to the underlying `str`, it can do that via `AsRef<str>` which doesn’t carry any extra requirements.

## Required Methods

### `fn borrow(&self) -> &Borrowed`

Immutably borrows from an owned value.

# Examples

    use std::borrow::Borrow;

    fn check<T: Borrow<str>>(s: T) {
        assert_eq!("Hello", s.borrow());
    }

    let s = "Hello".to_string();

    check(s);

    let s = "Hello";

    check(s);Run

## Implementors

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`impl<'a, T: ?Sized> Borrow<T> for &'a T`

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`fn borrow(&self) -> &T`

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`impl<'a, T: ?Sized> Borrow<T> for &'a mut T`

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`fn borrow(&self) -> &T`

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`impl<T> Borrow<[T]> for [T; 0]`

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`impl<T> Borrow<[T]> for [T; 18]`

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`impl<T> Borrow<[T]> for [T; 25]`

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`impl<T> Borrow<[T]> for [T; 26]`

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`impl<T: ?Sized> Borrow<T> for T`

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`fn borrow(&self) -> &T`

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