22. Smart pointers

smartp.md

22. Smart pointers

Box

This is just a unique pointer, cannot be cloned, single threaded.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let mut b = Box::new(Person{name: "Max".into(), age: 54});
    b.age = 55;
    println!("Name: {}, Age: {}", b.name, b.age);
    println!("Byte size: {}", std::mem::size_of::<Box<Person>>());
}

std::rc::Rc

This is a reference counted pointer, can be cloned, if there is only one reference, one can get mutable access. Single threaded.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let mut b = std::rc::Rc::new(Person{name: "Max".into(), age: 54});
    {
        let c = b.clone();
        println!("Name: {}, Age: {}", c.name, c.age);
    }
    {
        let m = std::rc::Rc::get_mut(&mut b);
        if let Some(p) = m {
            p.age = 55;
        } else {
            println!("Cannot get mutable reference!");
        }
    }
    println!("Name: {}, Age: {}", b.name, b.age);
    println!("Byte size: {}", std::mem::size_of::<std::rc::Rc<Person>>());
}

std::sync::Arc

Multi-threaded reference counted smart pointer using atomics for the reference count.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let mut b = std::sync::Arc::new(Person{name: "Max".into(), age: 54});
    // let c = b.clone();
    {
        let m = std::sync::Arc::get_mut(&mut b);
        if let Some(p) = m {
            p.age = 55;
        } else {
            println!("Cannot get mutable reference!");
        }
    }
    println!("Name: {}, Age: {}", b.name, b.age);
    // println!("Name: {}, Age: {}", c.name, c.age);
    println!("Byte size: {}", std::mem::size_of::<std::sync::Mutex::<Person>>());
}

std::sync::Mutex

Mutex using the type system to ensure proper usage.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let b = std::sync::Mutex::new(Person{name: "Max".into(), age: 54});
    {
        let g = b.lock();
        if let Ok(mut p) = g {
            p.age = 55;
        } else {
            println!("Cannot get mutex!");
        }
    }
    {
        let g = b.lock().unwrap();
        println!("Name: {}, Age: {}", g.name, g.age);
    }
    println!("Byte size: {}", std::mem::size_of::<std::sync::Mutex::<Person>>());
}

std::sync::RwLock

Read/Write lock using the type system to ensure proper usage.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let b = std::sync::RwLock::new(Person{name: "Max".into(), age: 54});
    {
        let g = b.write();
        if let Ok(mut p) = g {
            p.age = 55;
        } else {
            println!("Cannot get mutex!");
        }
    }
    {
        let g = b.read().unwrap();
        println!("Name: {}, Age: {}", g.name, g.age);
    }
    println!("Byte size: {}", std::mem::size_of::<std::sync::RwLock::<Person>>());
}

std::cell::RefCell

Single threaded, run-time tracking immutable references and allows one mutable reference at a time.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let b = std::cell::RefCell::new(Person{name: "Max".into(), age: 54});
    {
        let c = b.borrow();
        println!("Name: {}, Age: {}", c.name, c.age);
    }
    {
        b.borrow_mut().age = 55;
    }
    println!("Name: {}, Age: {}", b.borrow().name, b.borrow().age);
    println!("Byte size: {}", std::mem::size_of::<std::cell::RefCell<Person>>());
}

std::cell::Cell

Single threaded, allows mutation just be moving the value in and out.

#[derive(Debug)]
struct Person {
    pub name: String,
    pub age: u32,
}

fn main() {
    let b = std::cell::Cell::new(Person{name: "Max".into(), age: 54});
    b.replace(Person{name: "Max2".into(), age: 55});
    let c = b.into_inner();
    // println!("Name: {}, Age: {}", b.name, b.age);    // not allowed
    println!("Name: {}, Age: {}", c.name, c.age);
    println!("Byte size: {}", std::mem::size_of::<std::cell::Cell<Person>>());
}

References

• Rust book: https://doc.rust-lang.org/book/ch15-00-smart-pointers.html
• Box: https://doc.rust-lang.org/std/boxed/index.html
• std::rc: https://doc.rust-lang.org/std/rc/index.html
• std::rc::Rc: https://doc.rust-lang.org/std/rc/struct.Rc.html
• std::sync::Arc: https://doc.rust-lang.org/std/sync/struct.Arc.html
• std::sync::Mutex: https://doc.rust-lang.org/std/sync/struct.Mutex.html
• std::sync::RwLock: https://doc.rust-lang.org/std/sync/struct.RwLock.html
• std::cell: https://doc.rust-lang.org/std/cell/index.html
• std::cell::RefCell: https://doc.rust-lang.org/std/cell/struct.RefCell.html
• std::cell::Cell: https://doc.rust-lang.org/std/cell/struct.Cell.html
• Video: https://youtu.be/f2rlKJ-tX3U
• Overview: https://gist.github.com/max-itzpapalotl/18f7675a60f6f9603250367bcb63992e