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Rust's Ownership System: 10 Things You Should Knowby@jsdevjournal
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Rust's Ownership System: 10 Things You Should Know

by JSDevJournalSeptember 4th, 2023
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Master Rust's ownership system to write bug-free code that runs lightning fast. This guide explains Rust's 10 ownership rules with real code examples.
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Programming languages like C/C++ give developers a lot of control over memory management. But this comes with a big risk - it's easy to make mistakes that lead to crashes, security holes, or bugs!


Some common issues that C/C++ programmers face include:


  • Using memory after you've freed it. This leads to crashes or weird bugs down the line.
  • Forgetting to free memory when you're done with it. This causes memory leaks over time.
  • Two parts of code accessing the same memory at the same time. This can cause race conditions.


To avoid these problems, Rust uses an ownership system. This adds some rules that the compiler checks to ensure memory safety.


The key idea is that every value in Rust has an owner. The owner is in charge of that value - managing its lifecycle, freeing it, allowing access to it, etc.


By tracking ownership, Rust's compiler can ensure values are valid when you use them, prevent data races, and free memory when needed. All without requiring a garbage collector!

This ownership model powers Rust's safety and speed. By following a few ownership rules, your Rust programs will be protected from entire classes of memory-related problems.


Let's walk through the 10 ownership superpowers that Rust provides:

1. Each value has a variable that’s called its owner

In Rust, every value, like a string or integer, has an owner. The owner is the variable that is bound to that value. For example:


let x = 5; 


Here, x is the owner of the integer value 5. The variable x keeps track of and manages that 5 value.


Think of it like x taking ownership and responsibility over that 5 value. x is now the boss of that value!


This ownership system avoids confusing situations with multiple variables pointing to the same value. With single ownership, it's clear that x is the unique owner of the data 5.

2. When the owner goes out of scope, the value will be dropped

When the owner variable goes out of scope, Rust will call the drop function on the value and clean it up:


{
  let y = 5; // y is the owner of 5
} // y goes out of scope and 5 is dropped


Scope refers to the block that a variable is valid for. In the above example, y only exists within the {} curly braces. Once execution leaves that block, y disappears, and the value 5 is dropped.

This automatic freeing of data avoids memory leaks. As soon as the owner y goes away, Rust cleans up the value. No more worrying about dangling pointers or memory bloat!

3. There can only be one owner at a time

Rust enforces single ownership for each value. This avoids expensive reference counting schemes:


let z = 5; // z owns 5
let x = z; // z's ownership moved to x 
// z no longer owns 5!


In this example, transferring ownership from z to x is cheap. Some languages use reference counting where multiple variables can point to a value, but that has overhead.


With single ownership, Rust just updates an internal owner variable to move ownership from z to x. No costly counter updates.

4. When the owner is copied, the data is moved

Assigning an owner variable to a new variable moves the data:


let s1 = "hello".to_string(); // s1 owns "hello"
                           
let s2 = s1; // s1's ownership moved to s2   
             // s1 can no longer use "hello"


Here, we create the string "hello" and bind it to s1. Then, we assign s1 to a new variable s2.

This transfers ownership from s1 to s2. s1 no longer owns the string! The data itself was not copied, just the ownership moved.


This prevents accidentally making expensive copies. To really copy the data, you must use Rust's clone() method to make the intent clear.

5. Ownership can be borrowed through references

We can create reference variables that borrow ownership:


let s = "hello".to_string(); // s owns "hello"  

let r = &s; // r immutably borrows s
            // s still owns "hello"

println!("{}", r); // prints "hello"


The & operator creates a reference r that borrows ownership from s for this scope.

Think of r as temporarily borrowing the data that s owns. s still retains full ownership over the data. r is just allowed to read the "hello" string.

6. Mutable references have exclusive access

There can only be one mutable reference to data at a time:


let mut s = "hello".to_string();  

let r1 = &mut s; // r1 mutably borrows s

let r2 = &mut s; // error!


This prevents data races at compile time. The mutable reference r1 has exclusive write access to s, so no other references are allowed until r1 is done.


This saves you from subtle concurrency bugs by making simultaneous data access impossible.

7. References must last shorter than their owners

References must have shorter lifetimes than what they are borrowing:


{
  let r;               
  let s = "hello".to_string();
  
  r = &s; // error! r does not live long enough  
          
} // s is dropped here  


Here r goes out of scope before s. So r would be referencing data of s after s is dropped!

Rust prevents use after free bugs by enforcing this rule that references cannot outlive their owners.

8. Structs can be passed via move or borrow

We can transfer or borrow ownership of struct data:


struct User {
  name: String,
  age: u32  
}

let user1 = User {
  name: "John".to_string(),
  age: 27
}; // user1 owns struct   

let user2 = user1; // ownership moved to user2
                  // user1 can no longer use this

let borrow = &user1; // borrow the struct via reference  
                     // user1 still owns data


Structs group related data together, but the ownership rules still apply to their fields.

We can pass struct ownership to functions and threads or immutably borrow them. The same single owner/borrowing rules make struct usage safe.

9. Ownership works the same way on the heap

Ownership applies to heap-allocated data:


let s1 = String::from("hello"); // s1 on stack owns heap data 

let s2 = s1.clone(); // heap data copied to new location   
                     // s1 and s2 own separate data

let r = &s1; // r immutably borrows s1's heap data   
             // s1 still owns heap data


Here s1 is allocated on the stack but contains a String that points to heap-allocated text. The same ownership rules apply even though it's on the heap.


Rust prevents duplicate frees or use after free bugs, even when working with pointers. The ownership system keeps heap allocations safe.

10. Ownership enables safe concurrency

Ownership powers Rust's fearless concurrency:


use std::thread;

let v = vec![1, 2, 3]; 

let handle = thread::spawn(move || {
  println!("Here's a vector: {:?}", v);
});

handle.join().unwrap();


We move ownership of v into the spawned thread by using a move closure. This prevents concurrent access to v from multiple threads.


The ownership system makes concurrency safe and easy in Rust. There's no need for locking because the compiler enforces single ownership.

Conclusion

Rust's ownership system is designed to keep your code safe and fast. By enforcing a few key rules, entire classes of memory bugs are eliminated!


Some key lessons around ownership:


  • Each Rust value has a variable owner responsible for that value.
  • When the owner goes away, the value is cleaned up automatically. No more leaks!
  • Values can only have one owner at a time. This avoids confusion.
  • References can temporarily borrow ownership in a safe way.
  • The compiler checks that references are valid to prevent dangling pointers.
  • Ownership rules prevent data races and enable easy concurrency.


So, while ownership forces you to think about memory management, it's for a good reason - it squashes tons of potential bugs!


The ownership system is a big part of what makes Rust so reliable and fast. Following Rust's ownership rules will keep your code safe even as your programs grow large.

So embrace Rust's compile-time checks as helpful guidance rather than restrictions. Your future self will thank you when your Rust program runs smoothly without crashes or security holes!



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Also published here.