128 lines
4.2 KiB
Rust
128 lines
4.2 KiB
Rust
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! A module for working with borrowed data.
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//!
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//! # The `BorrowFrom` traits
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//!
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//! In general, there may be several ways to "borrow" a piece of data. The
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//! typical ways of borrowing a type `T` are `&T` (a shared borrow) and `&mut T`
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//! (a mutable borrow). But types like `Vec<T>` provide additional kinds of
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//! borrows: the borrowed slices `&[T]` and `&mut [T]`.
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//!
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//! When writing generic code, it is often desirable to abstract over all ways
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//! of borrowing data from a given type. That is the role of the `BorrowFrom`
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//! trait: if `T: BorrowFrom<U>`, then `&T` can be borrowed from `&U`. A given
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//! type can be borrowed as multiple different types. In particular, `Vec<T>:
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//! BorrowFrom<Vec<T>>` and `[T]: BorrowFrom<Vec<T>>`.
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//!
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//! # The `ToOwned` trait
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//!
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//! Some types make it possible to go from borrowed to owned, usually by
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//! implementing the `Clone` trait. But `Clone` works only for going from `&T`
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//! to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
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//! from any borrow of a given type.
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//!
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//! # The `Cow` (clone-on-write) type
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//!
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//! The type `Cow` is a smart pointer providing clone-on-write functionality: it
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//! can enclose and provide immutable access to borrowed data, and clone the
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//! data lazily when mutation or ownership is required. The type is designed to
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//! work with general borrowed data via the `BorrowFrom` trait.
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//!
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//! `Cow` implements both `Deref` and `DerefMut`, which means that you can call
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//! methods directly on the data it encloses. The first time a mutable reference
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//! is required, the data will be cloned (via `to_owned`) if it is not
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//! already owned.
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#![unstable = "recently added as part of collections reform"]
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use clone::Clone;
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use kinds::Sized;
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use ops::Deref;
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use self::Cow::*;
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/// A trait for borrowing data.
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pub trait BorrowFrom<Sized? Owned> for Sized? {
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/// Immutably borrow from an owned value.
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fn borrow_from(owned: &Owned) -> &Self;
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}
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/// A trait for mutably borrowing data.
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pub trait BorrowFromMut<Sized? Owned> for Sized? : BorrowFrom<Owned> {
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/// Mutably borrow from an owned value.
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fn borrow_from_mut(owned: &mut Owned) -> &mut Self;
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}
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impl<Sized? T> BorrowFrom<T> for T {
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fn borrow_from(owned: &T) -> &T { owned }
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}
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impl<Sized? T> BorrowFromMut<T> for T {
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fn borrow_from_mut(owned: &mut T) -> &mut T { owned }
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}
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impl BorrowFrom<&'static str> for str {
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fn borrow_from<'a>(owned: &'a &'static str) -> &'a str { &**owned }
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}
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/// A generalization of Clone to borrowed data.
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pub trait ToOwned<Owned> for Sized?: BorrowFrom<Owned> {
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/// Create owned data from borrowed data, usually by copying.
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fn to_owned(&self) -> Owned;
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}
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impl<T> ToOwned<T> for T where T: Clone {
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fn to_owned(&self) -> T { self.clone() }
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}
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/// A clone-on-write smart pointer.
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pub enum Cow<'a, T, B: 'a> where B: ToOwned<T> {
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/// Borrowed data.
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Borrowed(&'a B),
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/// Owned data.
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Owned(T)
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}
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impl<'a, T, B> Cow<'a, T, B> where B: ToOwned<T> {
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/// Acquire a mutable reference to the owned form of the data.
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///
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/// Copies the data if it is not already owned.
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pub fn to_mut(&mut self) -> &mut T {
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match *self {
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Borrowed(borrowed) => {
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*self = Owned(borrowed.to_owned());
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self.to_mut()
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}
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Owned(ref mut owned) => owned
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}
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}
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/// Extract the owned data.
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///
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/// Copies the data if it is not already owned.
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pub fn into_owned(self) -> T {
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match self {
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Borrowed(borrowed) => borrowed.to_owned(),
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Owned(owned) => owned
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}
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}
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}
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impl<'a, T, B> Deref<B> for Cow<'a, T, B> where B: ToOwned<T> {
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fn deref(&self) -> &B {
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match *self {
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Borrowed(borrowed) => borrowed,
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Owned(ref owned) => BorrowFrom::borrow_from(owned)
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}
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}
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}
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