What is ownership?
- Rust course
- Beginner
- Lesson 42/115
- 4 min
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 freedThere 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)Practice 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");.
Starter code
fn main() {
// buraya yaz
}Sample 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");
}Expected output
main bitiyor b bırakıldı a bırakıldı
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
Explanation: At the end of the scope the compiler adds the drop call automatically; it is Rust's counterpart to C++'s RAII idea.
2 Cleanup method
impl Drop for Dosya {
fn ____(&mut self) {
println!("kapatıldı");
}
}impl Drop for Dosya {
fn drop(&mut self) {
println!("kapatıldı");
}
}Explanation: The Drop trait's only method is drop, called at the end of the scope.
3 Rule
| Term | Meaning |
|---|---|
One owner |
For every value |
Kapsam sonu |
Automatic drop |
Drop trait |
Custom cleanup |
Reverse order |
Last declared is dropped first |