Rust pointers

Documentation

custom smart pointers

https://doc.rust-lang.org/stable/book/ch15-02-deref.html

Pointers/References

let foo = String::new("hi");
&foo   // `&` get reference to foo

let ref = &foo;
*ref   // `*` de-reference to get foo instance

Smart Pointers

The standard library implements some reusable smart-pointers to handle edge-cases.
Libraries may implment their own smart-pointers (and so can you).

Pointer Relevant Traits

• Deref alters how the de-reference operator (*) behaves
• DerefMut alters how the de-reference operator (*) behaves for mutable references
• Drop alters the object's destructor, normally run when the object falls out of scope.

Box<T>

Box types let you represent recursive types, where the total size is unknown at compile time,
by replacing your type with a pointer to it.

The docs use the example of a recursive enum.
An Enum's size is determined by it's largest possible value.
If an enum refers to an instance of itself the compiler is unable to determine it's size.

The box type allocates your object to the heap, and returns a pointer to it.
Pointers have a known size, so you can assign that instead.
Also, it implements the Deref trait, which allows you to use box as if it were a reference to the object you assigned to it.

#[derive(Debug)]
enum Path {
    Dir(String, Box<Path>),
    File(String),
}

impl Path {
    fn file(file: &str) -> Box<Path> {
        Box::new(
            Path::File(file.to_string())
        )
    }

    fn dir(dir: &str, path: Box<Path>) -> Box<Path> {
        Box::new(
            Path::Dir(dir.to_string(), path)
        )
    }
}

fn main() {
    let mypath = Path::dir("/var", Path::dir("/tmp", Path::file("/foo.txt")));
    dbg!(mypath);
}

Rc<T>

The Rc (Reference Counting) smart-pointer lets you have an object with multiple owners.
This is useful for graphs, where a node may have multiple parents.

This behaves similarly to Box in that it returns a pointer with an overridden dereference operator, but it also ensures that as long as one reference still exists to an object, it won't get drop() called on it.

NOTE:

the rust docs make a strong argument that because of ownership semantics,
a parent should own children, and not the other way around.

use std::rc::Rc;

#[derive(Debug)]
enum Node {
    Children(Vec<Rc<Node>>),
    Leaf,
}

fn main() {
    let b1 = Node::Leaf;
    let b2 = Node::Leaf;
    let a = Node::Children(vec![
        Rc::clone(&b1),
        Rc::clone(&b2),
    ]);
    dbg!(&b1);
    dbg!(&b2);
}

Weak<T>

Weak<T> is a companion to Rc<T>.
It allows you to downgrade an Rc reference so that the Weak reference does not impact the deletion of the object.
The example given in the docs is that a node could have a weak reference to it's parent.

See https://doc.rust-lang.org/stable/book/ch15-06-reference-cycles.html

Arc<T>

Behaves like Rc<T>, but is threadsafe.

RefCell<T>

This wraps an object with a single parent, but allows a mutable reference while immutable references exist.
See https://doc.rust-lang.org/stable/book/ch15-05-interior-mutability.html