Drop 特征

用于实现 Drop 的值可以指定在超出范围时运行的代码：

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");
}
  • Note that std::mem::drop is not the same as std::ops::Drop::drop.
  • Values are automatically dropped when they go out of scope.
  • When a value is dropped, if it implements std::ops::Drop then its Drop::drop implementation will be called.
  • All its fields will then be dropped too, whether or not it implements Drop.
  • std::mem::drop is just an empty function that takes any value. The significance is that it takes ownership of the value, so at the end of its scope it gets dropped. This makes it a convenient way to explicitly drop values earlier than they would otherwise go out of scope.
    • This can be useful for objects that do some work on drop: releasing locks, closing files, etc.

讨论点：

  • 为什么 Drop::drop 不使用 self？
    • 简答：如果这样的话，系统会在代码块结尾 调用 std::mem::drop，进而引发再一次调用 Drop::drop，并引发堆栈 溢出！
  • 尝试用 a.drop() 替换 drop(a)。