The Drop Trait

Values which implement Drop can specify code to run when they go out of scope:

struct Droppable {
    name: &'static str,
}
impl Drop for Droppable {
    fn drop(&mut self) {
        println!("Dropping {}", self.name);
    }
}
fn main() {
    let a = Droppable { name: "a" };
    {
        let b = Droppable { name: "b" };
        {
            let c = Droppable { name: "c" };
            let d = Droppable { name: "d" };
            println!("Exiting block B");
        }
        println!("Exiting block A");
    }
    drop(a);
    println!("Exiting main");
}

Discussion points:

• Why does not Drop::drop take self?
  • Short-answer: If it did, std::mem::drop would be called at the end of the block, resulting in another call to Drop::drop, and a stack overflow!
• Try replacing drop(a) with a.drop().