Drop

Drop 特质只有一个方法：drop，当对象超出作用域时会自动调用。 Drop 特质的主要用途是释放实现者实例拥有的资源。

Box、Vec、String、File 和 Process 是一些实现了 Drop 特质以释放资源的类型示例。 Drop 特质也可以为任何自定义数据类型手动实现。

以下示例在 drop 函数中添加了一个控制台打印，以声明何时调用它。

struct Droppable {
    name: &'static str,
}

// This trivial implementation of `drop` adds a print to console.
impl Drop for Droppable {
    fn drop(&mut self) {
        println!("> Dropping {}", self.name);
    }
}

fn main() {
    let _a = Droppable { name: "a" };

    // block A
    {
        let _b = Droppable { name: "b" };

        // block B
        {
            let _c = Droppable { name: "c" };
            let _d = Droppable { name: "d" };

            println!("Exiting block B");
        }
        println!("Just exited block B");

        println!("Exiting block A");
    }
    println!("Just exited block A");

    // Variable can be manually dropped using the `drop` function
    drop(_a);
    // TODO ^ Try commenting this line

    println!("end of the main function");

    // `_a` *won't* be `drop`ed again here, because it already has been
    // (manually) `drop`ed
}