Drop - The Rust Programming Language

# 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.