What is ownership?

Learn what the ownership rules do for you.

What is ownership?

Ownership is Rust's memory management model and rests on three rules:

Every value has an owner (a variable).
There can be only one owner at a time.
When the owner goes out of scope, the value is dropped automatically (drop).

{
    let s = String::from("merhaba");  // s is the owner
    println!("{s}");
}   // scope ends: s is dropped, memory freed

There is no garbage collector and no manual free; the compiler inserts the drop point itself.

The Drop trait

By implementing Drop for a type, you can see what happens when it is dropped; cleanups such as closing files and releasing locks become automatic this way:

struct Kaynak(&'static str);

impl Drop for Kaynak {
    fn drop(&mut self) {
        println!("{} bırakıldı", self.0);
    }
}

let _a = Kaynak("a");
let _b = Kaynak("b");
// end of scope: b is dropped first, then a (reverse order)

Exercise Drop order

Write the Kaynak type above and its Drop implementation. In main, declare let _a = Kaynak("a"); and let _b = Kaynak("b");, and print println!("main bitiyor");.

Show example solution

struct Kaynak(&'static str);

impl Drop for Kaynak {
    fn drop(&mut self) {
        println!("{} bırakıldı", self.0);
    }
}

fn main() {
    let _a = Kaynak("a");
    let _b = Kaynak("b");
    println!("main bitiyor");
}

Test yourself

When is a <code>String</code>'s memory freed?
- When the program ends
- When the GC runs
- When free is called
- When its owner goes out of scope
Show answer

When its owner goes out of scope — At the end of the scope the compiler adds the drop call automatically; it is Rust's counterpart to C++'s RAII idea.

Cleanup method

impl Drop for Dosya {
    fn ____(&mut self) {
        println!("kapatıldı");
    }
}

Show answer

impl Drop for Dosya {
    fn drop(&mut self) {
        println!("kapatıldı");
    }
}

Rule

One owner

For every value
Kapsam sonu

Automatic drop
Drop trait

Custom cleanup
Reverse order

Last declared is dropped first

Next lesson: Move semantics

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