Interior mutability
Cell
Interior mutability means having a little bit of mutability on the inside. Remember how in Rust you need to use mut to change a variable? There are also some ways to change them without the word mut. This is because Rust has some ways to let you safely change values inside of a struct that is immutable. Each one of them follows some rules that make sure that changing the values is still safe.
First, let's look at a simple example where we would want this. Imagine a struct called PhoneModel with many fields:
struct PhoneModel { company_name: String, model_name: String, screen_size: f32, memory: usize, date_issued: u32, on_sale: bool, } fn main() { let super_phone_3000 = PhoneModel { company_name: "YY Electronics".to_string(), model_name: "Super Phone 3000".to_string(), screen_size: 7.5, memory: 4_000_000, date_issued: 2020, on_sale: true, }; }
It is better for the fields in PhoneModel to be immutable, because we don't want the data to change. The date_issued and screen_size never change, for example.
But inside is one field called on_sale. A phone model will first be on sale (true), but later the company will stop selling it. Can we make just this one field mutable? Because we don't want to write let mut super_phone_3000. If we do, then every field will become mutable.
Rust has many ways to allow some safe mutability inside of something that is immutable. The most simple way is called Cell. First we use use std::cell::Cell so that we can just write Cell instead of std::cell::Cell every time.
Then we change on_sale: bool to on_sale: Cell<bool>. Now it isn't a bool: it's a Cell that holds a bool.
Cell has a method called .set() where you can change the value. We use .set() to change on_sale: true to on_sale: Cell::new(true).
use std::cell::Cell; struct PhoneModel { company_name: String, model_name: String, screen_size: f32, memory: usize, date_issued: u32, on_sale: Cell<bool>, } fn main() { let super_phone_3000 = PhoneModel { company_name: "YY Electronics".to_string(), model_name: "Super Phone 3000".to_string(), screen_size: 7.5, memory: 4_000_000, date_issued: 2020, on_sale: Cell::new(true), }; // 10 years later, super_phone_3000 is not on sale anymore super_phone_3000.on_sale.set(false); }
Cell works for all types, but works best for simple Copy types because it gives values, not references. Cell has a method called get() for example that only works on Copy types.
Another type you can use is RefCell.
RefCell
A RefCell is another way to change values without needing to declare mut. It means "reference cell", and is like a Cell but uses references instead of copies.
We will create a User struct. So far you can see that it is similar to Cell:
use std::cell::RefCell; #[derive(Debug)] struct User { id: u32, year_registered: u32, username: String, active: RefCell<bool>, // Many other fields } fn main() { let user_1 = User { id: 1, year_registered: 2020, username: "User 1".to_string(), active: RefCell::new(true), }; println!("{:?}", user_1.active); }
This prints RefCell { value: true }.
There are many methods for RefCell. Two of them are .borrow() and .borrow_mut(). With these methods, you can do the same thing you do with & and &mut. The rules are the same:
- Many borrows is fine,
- one mutable borrow is fine,
- but mutable and immutable together is not fine.
So changing the value in a RefCell is very easy:
#![allow(unused)] fn main() { // 🚧 user_1.active.replace(false); println!("{:?}", user_1.active); }
And there are many other methods like replace_with that uses a closure:
#![allow(unused)] fn main() { // 🚧 let date = 2020; user_1 .active .replace_with(|_| if date < 2000 { true } else { false }); println!("{:?}", user_1.active); }
But you have to be careful with a RefCell, because it checks borrows at runtime, not compilation time. Runtime means when the program is actually running (after compilation). So this will compile, even though it is wrong:
use std::cell::RefCell; #[derive(Debug)] struct User { id: u32, year_registered: u32, username: String, active: RefCell<bool>, // Many other fields } fn main() { let user_1 = User { id: 1, year_registered: 2020, username: "User 1".to_string(), active: RefCell::new(true), }; let borrow_one = user_1.active.borrow_mut(); // first mutable borrow - okay let borrow_two = user_1.active.borrow_mut(); // second mutable borrow - not okay }
But if you run it, it will immediately panic.
thread 'main' panicked at 'already borrowed: BorrowMutError', C:\Users\mithr\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\src\libcore\cell.rs:877:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\rust_book.exe` (exit code: 101)
already borrowed: BorrowMutError is the important part. So when you use a RefCell, it is good to compile and run to check.
Mutex
Mutex is another way to change values without declaring mut. Mutex means mutual exclusion, which means "only one at a time". This is why a Mutex is safe, because it only lets one process change it at a time. To do this, it uses .lock(). Lock is like locking a door from the inside. You go into a room, lock the door, and now you can change things inside the room. Nobody else can come in and stop you, because you locked the door.
A Mutex is easier to understand through examples.
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); // A new Mutex<i32>. We don't need to say mut let mut mutex_changer = my_mutex.lock().unwrap(); // mutex_changer is a MutexGuard // It has to be mut because we will change it // Now it has access to the Mutex // Let's print my_mutex to see: println!("{:?}", my_mutex); // This prints "Mutex { data: <locked> }" // So we can't access the data with my_mutex now, // only with mutex_changer println!("{:?}", mutex_changer); // This prints 5. Let's change it to 6. *mutex_changer = 6; // mutex_changer is a MutexGuard<i32> so we use * to change the i32 println!("{:?}", mutex_changer); // Now it says 6 }
But mutex_changer still has a lock after it is done. How do we stop it? A Mutex is unlocked when the MutexGuard goes out of scope. "Go out of scope" means the code block is finished. For example:
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); { let mut mutex_changer = my_mutex.lock().unwrap(); *mutex_changer = 6; } // mutex_changer goes out of scope - now it is gone. It is not locked anymore println!("{:?}", my_mutex); // Now it says: Mutex { data: 6 } }
If you don't want to use a different {} code block, you can use std::mem::drop(mutex_changer). std::mem::drop means "make this go out of scope".
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); let mut mutex_changer = my_mutex.lock().unwrap(); *mutex_changer = 6; std::mem::drop(mutex_changer); // drop mutex_changer - it is gone now // and my_mutex is unlocked println!("{:?}", my_mutex); // Now it says: Mutex { data: 6 } }
You have to be careful with a Mutex because if another variable tries to lock it, it will wait:
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); let mut mutex_changer = my_mutex.lock().unwrap(); // mutex_changer has the lock let mut other_mutex_changer = my_mutex.lock().unwrap(); // other_mutex_changer wants the lock // the program is waiting // and waiting // and will wait forever. println!("This will never print..."); }
One other method is try_lock(). Then it will try once, and if it doesn't get the lock it will give up. Don't do try_lock().unwrap(), because it will panic if it doesn't work. if let or match is better:
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); let mut mutex_changer = my_mutex.lock().unwrap(); let mut other_mutex_changer = my_mutex.try_lock(); // try to get the lock if let Ok(value) = other_mutex_changer { println!("The MutexGuard has: {}", value) } else { println!("Didn't get the lock") } }
Also, you don't need to make a variable to change the Mutex. You can just do this:
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); *my_mutex.lock().unwrap() = 6; println!("{:?}", my_mutex); }
*my_mutex.lock().unwrap() = 6; means "unlock my_mutex and make it 6". There is no variable that holds it so you don't need to call std::mem::drop. You can do it 100 times if you want - it doesn't matter:
use std::sync::Mutex; fn main() { let my_mutex = Mutex::new(5); for _ in 0..100 { *my_mutex.lock().unwrap() += 1; // locks and unlocks 100 times } println!("{:?}", my_mutex); }
RwLock
RwLock means "read write lock". It is like a Mutex but also like a RefCell. You use .write().unwrap() instead of .lock().unwrap() to change it. But you can also use .read().unwrap() to get read access. It is like RefCell because it follows the rules:
- many
.read()variables is okay, - one
.write()variable is okay, - but more than one
.write()or.read()together with.write()is not okay.
The program will run forever if you try to .write() when you can't get access:
use std::sync::RwLock; fn main() { let my_rwlock = RwLock::new(5); let read1 = my_rwlock.read().unwrap(); // one .read() is fine let read2 = my_rwlock.read().unwrap(); // two .read()s is also fine println!("{:?}, {:?}", read1, read2); let write1 = my_rwlock.write().unwrap(); // uh oh, now the program will wait forever }
So we use std::mem::drop, just like in a Mutex.
use std::sync::RwLock; use std::mem::drop; // We will use drop() many times fn main() { let my_rwlock = RwLock::new(5); let read1 = my_rwlock.read().unwrap(); let read2 = my_rwlock.read().unwrap(); println!("{:?}, {:?}", read1, read2); drop(read1); drop(read2); // we dropped both, so we can use .write() now let mut write1 = my_rwlock.write().unwrap(); *write1 = 6; drop(write1); println!("{:?}", my_rwlock); }
And you can use try_read() and try_write() too.
use std::sync::RwLock; fn main() { let my_rwlock = RwLock::new(5); let read1 = my_rwlock.read().unwrap(); let read2 = my_rwlock.read().unwrap(); if let Ok(mut number) = my_rwlock.try_write() { *number += 10; println!("Now the number is {}", number); } else { println!("Couldn't get write access, sorry!") }; }