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## Smart Pointers
#### @wusyong, 吳昱緯
----
### Pointers
- General concept for a variable that contains an address in memory
- Address refers to, or “points at,” some other data
- References are indicated by the & symbol and borrow the value they point to
----
----
### Smart Pointers
- Data structures that not only act like a pointer but also have additional metadata and capabilities
- Dynamic allocation
- Modify immutable data
- Own the data they point to
- Usually implemented by struct
- ~~No smart enough~~
----
----
### Smart Pointers
- Dereference: Deref trait
- Free memory: Drop trait
Smart Pointers = Struct + Deref + Drop
---
## Deref: Treating Smart Pointers Like Regular References
- Customize dereference operator `*`
- DerefMut: dereference of &mut
- Deref coercions: Implicit dereference
----
### Deref trait
```rust
pub trait Deref {
/// The resulting type after dereferencing.
type Target: T;
/// Dereferences the value.
fn deref(&self) -> &Self::Target;
}
```
----
### Following the Pointer to the Value with the Dereference Operator
```rust
fn main() {
let x = 5;
let y = &x;
assert_eq!(5, x);
assert_eq!(5, *y);
}
error[E0277]: the trait bound `{integer}: std::cmp::PartialEq<&{integer}>` is
not satisfied
--> src/main.rs:6:5
|
6 | assert_eq!(5, y);
| ^^^^^^^^^^^^^^^^^ can't compare `{integer}` with `&{integer}`
|
= help: the trait `std::cmp::PartialEq<&{integer}>` is not implemented for
`{integer}`
```
----
### Defining Our Own Smart Pointer
```rust
struct MyBox<T>(T);
impl<T> MyBox<T> {
fn new(x: T) -> MyBox<T> {
MyBox(x)
}
}
fn main() {
let x = 5;
let y = MyBox::new(x);
assert_eq!(5, x);
assert_eq!(5, *y);
}
error[E0614]: type `MyBox<{integer}>` cannot be dereferenced
--> src/main.rs:14:19
|
14 | assert_eq!(5, *y);
|
```
----
### Treating a Type Like a Reference by Implementing the Deref Trait
```rust
use std::ops::Deref;
impl<T> Deref for MyBox<T> {
type Target = T;
fn deref(&self) -> &T {
&self.0
}
}
// *(y.deref()) == *y
```
----
### Implicit Deref Coercions with Functions and Methods
```rust
fn hello(name: &str) {
println!("Hello, {}!", name);
}
fn main() {
let m = MyBox::new(String::from("Rust"));
hello(&m); // hello(&(*m)[..]);
}
```
----
### How Deref Coercion Interacts with Mutability
* From &T to &U when T: Deref<Target=U>
* From &mut T to &mut U when T: DerefMut<Target=U>
* From &mut T to &U when T: Deref<Target=U>
---
## Drop: Running Code on Cleanup
- Destructor
- RAII
----
### Drop trait
```rust
struct CustomSmartPointer {
data: String,
}
impl Drop for CustomSmartPointer {
fn drop(&mut self) {
println!("Dropping CustomSmartPointer with data `{}`!", self.data);
}
}
fn main() {
let c = CustomSmartPointer { data: String::from("my stuff") };
let d = CustomSmartPointer { data: String::from("other stuff") };
println!("CustomSmartPointers created.");
}
CustomSmartPointers created.
Dropping CustomSmartPointer with data `other stuff`!
Dropping CustomSmartPointer with data `my stuff`!
```
----
### Dropping a Value Early with std::mem::drop
Explicit destructor calls are not allowed
```rust
fn main() {
let c = CustomSmartPointer { data: String::from("some data") };
println!("CustomSmartPointer created.");
drop(c); // c.drop() will have double free!
println!("CustomSmartPointer dropped before the end of main.");
}
CustomSmartPointer created.
Dropping CustomSmartPointer with data `some data`!
CustomSmartPointer dropped before the end of main.
```
---
## Box 📦
- Point to Data on the Heap
- Similar to C++ std::unique_ptr
- pointer size on stack is `1 usize`
- keep content ownership
----
----
### Using a `Box<T>` to Store Data on the Heap
```rust
fn main() {
let b = Box::new(5);
println!("b = {}", b);
}
```
----
### Enabling Recursive Types with Boxes
Let's build a cons list. A cons list is a data structure that comes from the Lisp programming language and its dialects. In Lisp, the cons function (short for “construct function”) constructs a new pair from its two arguments, which usually are a single value and another pair. These pairs containing pairs form a list.
----
### Computing the Size of a Non-Recursive Type
```rust
enum List {
Cons(i32, List),
Nil,
}
use crate::List::{Cons, Nil};
fn main() {
let list = Cons(1, Cons(2, Cons(3, Nil)));
}
error[E0072]: recursive type `List` has infinite size
--> src/main.rs:1:1
|
1 | enum List {
| ^^^^^^^^^ recursive type has infinite size
2 | Cons(i32, List),
| ----- recursive without indirection
|
= help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to
make `List` representable
```
----
### Computing the Size of a Non-Recursive Type
----
### Using `Box<T>` to Get a Recursive Type with a Known Size
```rust
enum List {
Cons(i32, Box<List>),
Nil,
}
use crate::List::{Cons, Nil};
fn main() {
let list = Cons(1,
Box::new(Cons(2,
Box::new(Cons(3,
Box::new(Nil))))));
}
```
----
### Using `Box<T>` to Get a Recursive Type with a Known Size
---
## Rc ➡️ 📦 ⬅️
- Reference counting
- Similar to C++ std::shared_ptr
- Single thread
- Use Weak to prevent memory leak (This Chapter)
- USe Arc atomic reference counting (Next Chapter)
----
### Using `Rc<T>` to Share Data
----
### Using `Rc<T>` to Share Data
Prefer Rc::clone(&a) than a.clone()
```rust
enum List {
Cons(i32, Rc<List>),
Nil,
}
use crate::List::{Cons, Nil};
use std::rc::Rc;
fn main() {
let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
let b = Cons(3, Rc::clone(&a));
let c = Cons(4, Rc::clone(&a));
}
```
----
### Cloning an `Rc<T>` Increases the Reference Count
```rust
fn main() {
let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
println!("count after creating a = {}", Rc::strong_count(&a));
let b = Cons(3, Rc::clone(&a));
println!("count after creating b = {}", Rc::strong_count(&a));
{
let c = Cons(4, Rc::clone(&a));
println!("count after creating c = {}", Rc::strong_count(&a));
}
println!("count after c goes out of scope = {}", Rc::strong_count(&a));
}
count after creating a = 1
count after creating b = 2
count after creating c = 3
count after c goes out of scope = 2
```
---
## Cell and RefCell 📦 💵 🎩 🐇
- Interior mutability
- Cell: Ownership transfer
- RefCell: Enforcing Borrowing Rules at Runtime
- Not on heap
----
### Interior mutability
Uses unsafe code(`UnsafeCell<T>`) inside a data structure to bend Rust’s usual rules that govern mutation and borrowing.
----
----
### Cell struct
```rust
pub struct Cell<T: ?Sized> {
value: UnsafeCell<T>,
}
use std::cell::Cell;
let c = Cell::new(5);
c.set(10); // discard 5 and value set to 10
c.get(); // 10 (a copy but not a reference)
c.replace(7) // 10 (returns the replaced value)
```
----
### RefCell struct
```rust
pub struct RefCell<T: ?Sized> {
borrow: Cell<BorrowFlag>,
value: UnsafeCell<T>,
}
```
----
### Enforcing Borrowing Rules at Runtime with `RefCell<T>`
* Recap borrow rules
* At any given time, you can have either (but not both of) one mutable reference or any number of immutable references.
* References must always be valid.
* When you’re sure your code follows the borrowing rules but the compiler is unable to understand and guarantee that
----
### A Mutable Borrow to an Immutable Value
```rust
fn main() {
let x = 5;
let y = &mut x;
}
error[E0596]: cannot borrow immutable local variable `x` as mutable
--> src/main.rs:3:18
|
2 | let x = 5;
| - consider changing this to `mut x`
3 | let y = &mut x;
| ^ cannot borrow mutably
```
----
### A Use Case for Interior Mutability: Mock Objects
```rust
pub trait Messenger {
fn send(&self, msg: &str);
}
pub struct LimitTracker<'a, T: Messenger> {
messenger: &'a T,
value: usize,
max: usize,
}
impl<'a, T> LimitTracker<'a, T>
where T: Messenger {
pub fn new(messenger: &T, max: usize) -> LimitTracker<T> {
LimitTracker {
messenger,
value: 0,
max,
}
}
pub fn set_value(&mut self, value: usize) {
self.value = value;
let percentage_of_max = self.value as f64 / self.max as f64;
if percentage_of_max >= 1.0 {
self.messenger.send("Error: You are over your quota!");
} else if percentage_of_max >= 0.9 {
self.messenger.send("Urgent warning: You've used up over 90% of your quota!");
} else if percentage_of_max >= 0.75 {
self.messenger.send("Warning: You've used up over 75% of your quota!");
}
}
}
```
----
### Mocking test
```rust
#[cfg(test)]
mod tests {
use super::*;
struct MockMessenger {
sent_messages: Vec<String>,
}
impl MockMessenger {
fn new() -> MockMessenger {
MockMessenger { sent_messages: vec![] }
}
}
impl Messenger for MockMessenger {
fn send(&self, message: &str) {
self.sent_messages.push(String::from(message));
}
}
#[test]
fn it_sends_an_over_75_percent_warning_message() {
let mock_messenger = MockMessenger::new();
let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);
limit_tracker.set_value(80);
assert_eq!(mock_messenger.sent_messages.len(), 1);
}
}
error[E0596]: cannot borrow immutable field `self.sent_messages` as mutable
--> src/lib.rs:52:13
|
51 | fn send(&self, message: &str) {
| ----- use `&mut self` here to make mutable
52 | self.sent_messages.push(String::from(message));
| ^^^^^^^^^^^^^^^^^^ cannot mutably borrow immutable field
```
----
### RefCell to the rescue
```rust
#[cfg(test)]
mod tests {
use super::*;
use std::cell::RefCell;
struct MockMessenger {
sent_messages: RefCell<Vec<String>>,
}
impl MockMessenger {
fn new() -> MockMessenger {
MockMessenger { sent_messages: RefCell::new(vec![]) }
}
}
impl Messenger for MockMessenger {
fn send(&self, message: &str) {
self.sent_messages.borrow_mut().push(String::from(message));
}
}
#[test]
fn it_sends_an_over_75_percent_warning_message() {
// --snip--
assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
}
}
```
----
### Keeping Track of Borrows at Runtime with `RefCell<T>`
- borrow
- &T
- Return `Ref<T>`
- RefCell::borrow_mut
- &mut T
- Return `RefMut<T>`
----
### Having Multiple Owners of Mutable Data by Combining `Rc<T>` and `RefCell<T>`
```rust
#[derive(Debug)]
enum List {
Cons(Rc<RefCell<i32>>, Rc<List>),
Nil,
}
use crate::List::{Cons, Nil};
use std::rc::Rc;
use std::cell::RefCell;
fn main() {
let value = Rc::new(RefCell::new(5));
let a = Rc::new(Cons(Rc::clone(&value), Rc::new(Nil)));
let b = Cons(Rc::new(RefCell::new(6)), Rc::clone(&a));
let c = Cons(Rc::new(RefCell::new(10)), Rc::clone(&a));
*value.borrow_mut() += 10;
println!("a after = {:?}", a);
println!("b after = {:?}", b);
println!("c after = {:?}", c);
}
a after = Cons(RefCell { value: 15 }, Nil)
b after = Cons(RefCell { value: 6 }, Cons(RefCell { value: 15 }, Nil))
c after = Cons(RefCell { value: 10 }, Cons(RefCell { value: 15 }, Nil))
```
---
## Reference Cycles Can Leak Memory
```rust
use std::rc::Rc;
use std::cell::RefCell;
use crate::List::{Cons, Nil};
#[derive(Debug)]
enum List {
Cons(i32, RefCell<Rc<List>>),
Nil,
}
impl List {
fn tail(&self) -> Option<&RefCell<Rc<List>>> {
match self {
Cons(_, item) => Some(item),
Nil => None,
}
}
}
```
----
### Creating a reference cycle of two List values pointing to each other
```rust
fn main() {
let a = Rc::new(Cons(5, RefCell::new(Rc::new(Nil))));
println!("a initial rc count = {}", Rc::strong_count(&a));
println!("a next item = {:?}", a.tail());
let b = Rc::new(Cons(10, RefCell::new(Rc::clone(&a))));
println!("a rc count after b creation = {}", Rc::strong_count(&a));
println!("b initial rc count = {}", Rc::strong_count(&b));
println!("b next item = {:?}", b.tail());
if let Some(link) = a.tail() {
*link.borrow_mut() = Rc::clone(&b);
}
println!("b rc count after changing a = {}", Rc::strong_count(&b));
println!("a rc count after changing a = {}", Rc::strong_count(&a));
// Uncomment the next line to see that we have a cycle;
// it will overflow the stack
// println!("a next item = {:?}", a.tail());
}
a initial rc count = 1
a next item = Some(RefCell { value: Nil })
a rc count after b creation = 2
b initial rc count = 1
b next item = Some(RefCell { value: Cons(5, RefCell { value: Nil }) })
b rc count after changing a = 2
a rc count after changing a = 2
```
----
----
### Preventing Reference Cycles: Turning an `Rc<T>` into a `Weak<T>`
- Weak reference
- Rc::downgrade / Rc::upgrade
- `Rc<T>` instance is only cleaned up if its strong_count is 0
- weak_count doesn’t need to be 0 for the `Rc<T>` instance to be cleaned up.
----
### Creating a Tree Data Structure: a Node with Child Nodes
```rust
use std::rc::Rc;
use std::cell::RefCell;
#[derive(Debug)]
struct Node {
value: i32,
children: RefCell<Vec<Rc<Node>>>,
}
fn main() {
let leaf = Rc::new(Node {
value: 3,
children: RefCell::new(vec![]),
});
let branch = Rc::new(Node {
value: 5,
children: RefCell::new(vec![Rc::clone(&leaf)]),
});
}
```
----
### Adding a Reference from a Child to Its Parent
```rust
use std::rc::{Rc, Weak};
use std::cell::RefCell;
#[derive(Debug)]
struct Node {
value: i32,
parent: RefCell<Weak<Node>>,
children: RefCell<Vec<Rc<Node>>>,
}
fn main() {
let leaf = Rc::new(Node {
value: 3,
parent: RefCell::new(Weak::new()),
children: RefCell::new(vec![]),
});
println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());
// leaf parent = None
let branch = Rc::new(Node {
value: 5,
parent: RefCell::new(Weak::new()),
children: RefCell::new(vec![Rc::clone(&leaf)]),
});
*leaf.parent.borrow_mut() = Rc::downgrade(&branch);
println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());
//leaf parent = Some(Node { value: 5, parent: RefCell { value: (Weak) },
//children: RefCell { value: [Node { value: 3, parent: RefCell { value: (Weak) },
//children: RefCell { value: [] } }] } })
}
```
----
### Visualizing Changes to strong_count and weak_count
```rust
fn main() {
let leaf = Rc::new(Node {
value: 3,
parent: RefCell::new(Weak::new()),
children: RefCell::new(vec![]),
});
println!(
"leaf strong = {}, weak = {}",
Rc::strong_count(&leaf),
Rc::weak_count(&leaf),
); // 1, 0
{
let branch = Rc::new(Node {
value: 5,
parent: RefCell::new(Weak::new()),
children: RefCell::new(vec![Rc::clone(&leaf)]),
});
*leaf.parent.borrow_mut() = Rc::downgrade(&branch);
println!(
"branch strong = {}, weak = {}",
Rc::strong_count(&branch),
Rc::weak_count(&branch),
); // 1, 1
println!(
"leaf strong = {}, weak = {}",
Rc::strong_count(&leaf),
Rc::weak_count(&leaf),
); // 2, 0
}
println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());
println!(
"leaf strong = {}, weak = {}",
Rc::strong_count(&leaf),
Rc::weak_count(&leaf),
); // 1, 0
}
```
---
## Summary
- Smart Pointers
- Box
- Rc
- RefCell
- Weak
----
### Cow
- ~~靠！~~
- Clone on write
```rust
pub enum Cow<'a, B: ?Sized + 'a>
where B: ToOwned
{
/// Borrowed data.
Borrowed(&'a B),
/// Owned data.
Owned(<B as ToOwned>::Owned),
}
```
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