Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.
Singleton has almost the same pros and cons as global variables. Although they’re super-handy, they break the modularity of your code.
You can’t just use a class that depends on a Singleton in some other context, without carrying over the Singleton to the other context. Most of the time, this limitation comes up during the creation of unit tests.
By definition, Singleton is a global mutable object. In Rust this is a static mut item. Thus, to avoid all sorts of concurrency issues, the function or block that is either reading or writing to a mutable static variable should be marked as an unsafe block.
For this reason, the Singleton pattern can be percieved as unsafe. However, the pattern is still widely used in practice. A good read-world example of Singleton is a log crate that introduces log!, debug! and other logging macros, which you can use throughout your code after setting up a concrete logger instance, such as env_logger. As we can see, env_logger uses log::set_boxed_logger under the hood, which has an unsafe block to set up a global logger object.
In order to provide safe and usable access to a singleton object, introduce an API hiding unsafe blocks under the hood.
See the thread about a mutable Singleton on Stackoverflow for more information.
Starting with Rust 1.63, Mutex::new is const, you can use global static Mutex locks without needing lazy initialization. See the Singleton using Mutex example below.
Safe Singleton
A pure safe way to implement Singleton in Rust is using no global variables at all and passing everything around through function arguments. The oldest living variable is an object created at the start of the main().
local.rs
//! A pure safe way to implement Singleton in Rust is using no static variables
//! and passing everything around through function arguments.
//! The oldest living variable is an object created at the start of the `main()`.
fn change(global_state: &mut u32) {
*global_state += 1;
}
fn main() {
let mut global_state = 0u32;
change(&mut global_state);
println!("Final state: {}", global_state);
}
Output
Final state: 1
Lazy Singleton
This is a singleton implementation via lazy_static!, which allows declaring a static variable with lazy initialization at first access. It is actually implemented via unsafe with static mut manipulation, however, it keeps your code clear of unsafe blocks.
lazy.rs
//! Taken from: https://stackoverflow.com/questions/27791532/how-do-i-create-a-global-mutable-singleton
//!
//! Rust doesn't really allow a singleton pattern without `unsafe` because it
//! doesn't have a safe mutable global state.
//!
//! `lazy-static` allows declaring a static variable with lazy initialization
//! at first access. It is actually implemented via `unsafe` with `static mut`
//! manipulation, however, it keeps your code clear of `unsafe` blocks.
//!
//! `Mutex` provides safe access to a single object.
use lazy_static::lazy_static;
use std::sync::Mutex;
lazy_static! {
static ref ARRAY: Mutex<Vec<u8>> = Mutex::new(vec![]);
}
fn do_a_call() {
ARRAY.lock().unwrap().push(1);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
println!("Called {}", ARRAY.lock().unwrap().len());
}
Output
Called 3
Singleton using Mutex
Starting with Rust 1.63, it can be easier to work with global mutable singletons, although it’s still preferable to avoid global variables in mostcases.
Now that Mutex::new is const, you can use global static Mutex locks without needing lazy initialization.
mutex.rs
//! ructc 1.63
//! https://stackoverflow.com/questions/27791532/how-do-i-create-a-global-mutable-singleton
//!
//! Starting with Rust 1.63, it can be easier to work with global mutable
//! singletons, although it's still preferable to avoid global variables in most
//! cases.
//!
//! Now that `Mutex::new` is `const`, you can use global static `Mutex` locks
//! without needing lazy initialization.
use std::sync::Mutex;
static ARRAY: Mutex<Vec<i32>> = Mutex::new(Vec::new());
fn do_a_call() {
ARRAY.lock().unwrap().push(1);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
let array = ARRAY.lock().unwrap();
println!("Called {} times: {:?}", array.len(), array);
drop(array);
*ARRAY.lock().unwrap() = vec![3, 4, 5];
println!("New singleton object: {:?}", ARRAY.lock().unwrap());
}
