2.7 KiB
Collections are smart pointers
Description
Use the Deref trait to treat collections like smart pointers, offering owning
and borrowed views of data.
Example
use std::ops::Deref;
struct Vec<T> {
data: T,
//..
}
impl<T> Deref for Vec<T> {
type Target = [T];
fn deref(&self) -> &[T] {
//..
}
}
A Vec<T> is an owning collection of Ts, a slice (&[T]) is a borrowed
collection of Ts. Implementing Deref for Vec allows implicit dereferencing
from &Vec<T> to &[T] and includes the relationship in auto-derefencing
searches. Most methods you might expect to be implemented for Vecs are instead
implemented for slices.
See also String and &str.
Motivation
Ownership and borrowing are key aspects of the Rust language. Data structures must account for these semantics properly in order to give a good user experience. When implementing a data structure which owns its data, offering a borrowed view of that data allows for more flexible APIs.
Advantages
Most methods can be implemented only for the borrowed view, they are then implicitly available for the owning view.
Gives clients a choice between borrowing or taking ownership of data.
Disadvantages
Methods and traits only available via dereferencing are not taken into account
when bounds checking, so generic programming with data structures using this
pattern can get complex (see the Borrow and AsRef traits, etc.).
Discussion
Smart pointers and collections are analogous: a smart pointer points to a single object, whereas a collection points to many objects. From the point of view of the type system there is little difference between the two. A collection owns its data if the only way to access each datum is via the collection and the collection is responsible for deleting the data (even in cases of shared ownership, some kind of borrowed view may be appropriate). If a collection owns its data, it is usually useful to provide a view of the data as borrowed so that it can be multiply referenced.
Most smart pointers (e.g., Foo<T>) implement Deref<Target=T>. However,
collections will usually dereference to a custom type. [T] and str have some
language support, but in the general case, this is not necessary. Foo<T> can
implement Deref<Target=Bar<T>> where Bar is a dynamically sized type and
&Bar<T> is a borrowed view of the data in Foo<T>.
Commonly, ordered collections will implement Index for Ranges to provide
slicing syntax. The target will be the borrowed view.