Summary - Smart pointers

Combine Box and Rc to build a tree data structure.

Smart pointers summary

Smart pointers are types that keep data on the heap and carry extra behaviour. Box<T>: single owner, for recursive types and dyn Trait. Rc<T>: shared ownership on a single thread. Arc<T>: shared ownership across threads. Deref lets them be used like the value they hold, and Drop does cleanup at the end of the scope.

The Drop trait

Drop defines code to run when a value is dropped: closing a file, ending a connection, releasing a lock. It isn't called by hand; to drop early, the drop(x) function is used:

struct Kaynak {
    ad: &'static str,
}

impl Drop for Kaynak {
    fn drop(&mut self) {
        println!("kapatıldı: {}", self.ad);
    }
}

This pattern is called RAII: the resource is tied to the lifetime of the value holding it and becomes impossible to forget.

Exercise Drop order

Write the Kaynak type above and its Drop implementation. In main, create _a ("a"); in an inner block create _b ("b") and print iç blok; after the block print son.

Show example solution

struct Kaynak {
    ad: &'static str,
}

impl Drop for Kaynak {
    fn drop(&mut self) {
        println!("kapatıldı: {}", self.ad);
    }
}

fn main() {
    let _a = Kaynak { ad: "a" };
    {
        let _b = Kaynak { ad: "b" };
        println!("iç blok");
    }
    println!("son");
}

Test yourself

In what order are values dropped from a scope?
- In reverse order of declaration
- In order of declaration
- Randomly
- By size
Show answer

In reverse order of declaration — Since a later value may depend on an earlier one, it is dropped first (stack order).

Cleanup method

impl Drop for Kaynak {
    fn drop(____) {
        println!("kapatıldı: {}", self.ad);
    }
}

Show answer

impl Drop for Kaynak {
    fn drop(&mut self) {
        println!("kapatıldı: {}", self.ad);
    }
}

Type and use case

Box<T>

Recursive type
Rc<T>

Sharing a graph node
Arc<T>

Data across threads
impl Drop

Resource cleanup

Next lesson: RefCell<T>

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