Rc: for mutiple ownership, just like what python have done

When multiple ownership is needed, Rc(Reference Counting) can be used:

  • Rc keeps track of the number of the references which means the number of owners of the value wrapped inside an Rc.
  • Reference count of an Rc increases by 1 whenever an Rc is cloned
  • and decreases by 1 whenever one cloned Rc is dropped out of the scope.
  • When an Rc’s reference count becomes zero (which means there are no remaining owners), both the Rc and the value are all dropped.
  • Cloning an Rc never performs a deep copy.
  • Cloning creates just another pointer to the wrapped value, and increments the count.

Rc Example: Just like what Python have done

 
use std::rc::Rc;

fn main() {
    let rc_examples = "Rc examples".to_string();
    {
        println!("--- rc_a is created ---");
        
        let rc_a: Rc<String> = Rc::new(rc_examples);
        println!("Reference Count of rc_a: {}", Rc::strong_count(&rc_a));
        
        {
            println!("--- rc_a is cloned to rc_b ---");
            
            let rc_b: Rc<String> = Rc::clone(&rc_a);
            println!("Reference Count of rc_b: {}", Rc::strong_count(&rc_b));
            println!("Reference Count of rc_a: {}", Rc::strong_count(&rc_a));
            
            // Two `Rc`s are equal if their inner values are equal
            println!("rc_a and rc_b are equal: {}", rc_a.eq(&rc_b));
            
            // We can use methods of a value directly
            println!("Length of the value inside rc_a: {}", rc_a.len());
            println!("Value of rc_b: {}", rc_b);
            
            println!("--- rc_b is dropped out of scope ---");
        }
        
        println!("Reference Count of rc_a: {}", Rc::strong_count(&rc_a));
        
        println!("--- rc_a is dropped out of scope ---");
    }
    
    // Error! `rc_examples` already moved into `rc_a`
    // And when `rc_a` is dropped, `rc_examples` is dropped together
    // println!("rc_examples: {}", rc_examples);
    // TODO ^ Try uncommenting this line
}

See also:

std::rc and std::sync::arc.