Rust Singleton Design Pattern using Lazy static initialization

Question

In my current project there is a lot inter dependency between different structs, so instead of having Arc<Struct> as members for each of the dependent Structs. I thought of using Singleton Design Pattern to get the instance of that Struct in the impl block. This is a sample code that I've written. I would like to know if this code is safe, efficient and also a good way of modularization of the code.

use std::sync::{Arc, Mutex};
use lazy_static::lazy_static;
use std::thread;

struct A {
    a: Mutex<i32>,
}

lazy_static! {
    static ref A_OBJ: Arc<A> = Arc::new(A::new());
}

impl A {
    fn new() -> A {
        A {
            a: Mutex::new(0),
        }    
    }
    
    pub fn print(&self) {
        let mg = self.a.lock().unwrap();
        println!("{}", mg);
    }
    
    pub fn increment_a(&self) {
        let mut mg = self.a.lock().unwrap();
        *mg += 1;
    }
    
    pub fn get_instance() -> &'static Arc<A> {
        &A_OBJ
    }   
}

fn main() {
    println!("Hello, world!");
    let tjh = thread::spawn(|| {
        println!("inside spawned thread");
        let a_obj = A::get_instance();
        a_obj.print();
        a_obj.increment_a();
        a_obj.print();
    });
    
    let a_obj = A::get_instance();
    a_obj.print();
    a_obj.increment_a();
    a_obj.print();
    let _ = tjh.join();
}

Winston Ewert · Accepted Answer · 2024-01-28 05:48:33Z

Firstly, you don't need use Arc. You can just do:

lazy_static! {
    static ref A_OBJ: A = A::new();
}

pub fn get_instance() -> &'static A {
    &A_OBJ
}

lazy_static ensures that A_OBJ lives for the 'static lifetime. There's no point in doing Arc on top of that.

Is it a good way to modularize code? That's somewhat debatable, but I don't think so. As I see it, a singleton is just a global variable. Sometimes it does make sense to use a global variable, but generally not.

What approach do I take instead?

Don't create unnecessary objects - if it doesn't have state it should just be a function
Don't store dependencies on struct - any dependencies should be passed as function parameters
Create a Context, Application, Server or similar struct to hold the complete state of your application. Any function needing access to the whole system should take a reference to it as a parameter.

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