Rust by Example: Ownership

Rust uses a system of ownership to ensure memory safety without garbage collection. Rust enforces this by enforcing these ownership rules:

  1. Each value in Rust has a variable that is its owner.
  2. There can only be one owner at a time.
  3. When the owner goes out of scope, the value is dropped.
Run code Copy code 
#![allow(unused)]
fn main() {

Simple scalar values are copied when assigned to another variable.

 
    let x = 5;
    let y = x;

s1 is moved to s2, and ownership of s1 is invalidated, The destination now controls the value's lifetime.

 
    let s1 = String::from("hello");
    let s2 = s1;

Using a value without transferring ownership, is called borrowing. Borrowing is done using references. References are immutable by default.

 
    let s3 = String::from("hello");
    let len = calculate_length(&s3);

Mutable references, denoted by &mut, allow modification of the borrowed variable's value without transferring ownership.

 
    let mut s4 = String::from("hello");
    let s5 = &mut s4;
    *s5 = String::from("hello, world!");

Passing a value to a function moves or copies it, depending on the type. Here, the value is moved into the function, invalidating the original variable, which causes an error if used afterwards: error: value borrowed here after move.

 
    let s6 = String::from("hello");
    takes_ownership(s6);

Return values of functions can also transfer ownership. Here the return value is moved into s7.

 
    let s7 = gives_ownership();

Returning a value from a function moves or copies it, depending on the type. Here, s8 is moved into the function, which then moves its return value into s9. Using s8 afterward results in the error: error: value borrowed here after move.

 
    let s8 = String::from("hello");
    let s9 = takes_and_gives_back(s8);

Allow a function to use a value without taking ownership by using references. The function takes a reference, leaving the original variable valid, a process called borrowing.

 
    let s10 = String::from("hello");
    let len = calculate_length(&s10);

References are immutable by default (&) and cannot be modified, leading to the error: error: cannot borrow as mutable. Mutable references (&mut) allow value modification without transferring ownership.

 
    let mut s12 = String::from("hello");
    change(&mut s12);
}

fn takes_ownership(s: String) {
    println!("{}", s);
}

fn makes_copy(i: i32) {
    println!("{}", i);
}

fn gives_ownership() -> String {
    let s = String::from("hello");
    return s;
}

fn takes_and_gives_back(s: String) -> String {
    return s;
}

fn calculate_length(s: &String) -> usize {
    return s.len();
}

fn change(s: &mut String) {
    s.push_str(", world");
}

$ rustc ownership.rs
$ ./ownership-borrowing
hello
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