编辑代码

use std::mem;

#[allow(dead_code)]
#[derive(Debug, Clone, Copy)]
struct Point {
    x: f64,
    y: f64,
}

#[allow(dead_code)]
struct Rectangle {
    p1: Point,
    p2: Point,
}

fn origin() -> Point {
    Point { x: 0.0, y: 0.0 }
}

fn boxed_origin() -> Box<Point> {
    // 在堆上分配这个点（point），并返回一个指向它的指针
    Box::new(Point { x: 0.0, y: 0.0 })
}

fn main() {
    // （所有的类型标注都不是必需的）
    // 栈分配的变量
    let point: Point = origin();
    let rectangle: Rectangle = Rectangle {
        p1: origin(),
        p2: Point { x: 3.0, y: 4.0 }
    };

    // 堆分配的 rectangle（矩形）
    let boxed_rectangle: Box<Rectangle> = Box::new(Rectangle {
        p1: origin(),
        p2: origin()
    });

    // 函数的输出可以装箱
    let boxed_point: Box<Point> = Box::new(origin());

    // 两层装箱
    let box_in_a_box: Box<Box<Point>> = Box::new(boxed_origin());

    println!("Point occupies {} bytes in the stack",
             mem::size_of_val(&point));
    println!("Rectangle occupies {} bytes in the stack",
             mem::size_of_val(&rectangle));

    // box 的宽度就是指针宽度
    println!("Boxed point occupies {} bytes in the stack",
             mem::size_of_val(&boxed_point));
    println!("Boxed rectangle occupies {} bytes in the stack",
             mem::size_of_val(&boxed_rectangle));
    println!("Boxed box occupies {} bytes in the stack",
             mem::size_of_val(&box_in_a_box));

    // 将包含在 `boxed_point` 中的数据复制到 `unboxed_point`
    let unboxed_point: Point = *boxed_point;
    println!("Unboxed point occupies {} bytes in the stack",
             mem::size_of_val(&unboxed_point));
    
    let s1 = "Hello Rust";
    println!("&str {} bytes in the stack",mem::size_of_val(s1));

    // 用Box将这个&str装起来后这个box的宽度依然是8个字节,即它存的是一个内存地址，内存地址固定占用8个字节
    let s2 = Box::new("Hello Rust");
    println!("Box &str {}bytes in the stack",mem::size_of_val(&s2));
}