Drop

Now that we’ve discussed traits, let’s talk about a particular trait provided by the Rust standard library, Drop. The Drop trait provides a way to run some code when a value goes out of scope. For example:

struct HasDrop;

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

fn main() {
    let x = HasDrop;

    // Do stuff.

} // `x` goes out of scope here.

When x goes out of scope at the end of main(), the code for Drop will run. Drop has one method, which is also called drop(). It takes a mutable reference to self.

That’s it! The mechanics of Drop are very simple, but there are some subtleties. For example, values are dropped in the opposite order they are declared. Here’s another example:

struct Firework {
    strength: i32,
}

impl Drop for Firework {
    fn drop(&mut self) {
        println!("BOOM times {}!!!", self.strength);
    }
}

fn main() {
    let firecracker = Firework { strength: 1 };
    let tnt = Firework { strength: 100 };
}

This will output:

BOOM times 100!!!
BOOM times 1!!!

The tnt goes off before the firecracker does, because it was declared afterwards. Last in, first out.

So what is Drop good for? Generally, Drop is used to clean up any resources associated with a struct. For example, the Arc<T> type is a reference-counted type. When Drop is called, it will decrement the reference count, and if the total number of references is zero, will clean up the underlying value.