Drop

Drop trait 只有一个方法：drop，它会在对象离开作用域时自动调用。Drop trait 的主要用途是释放实现该 trait 的实例所拥有的资源。

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

下面的例子在 drop 函数中添加了一个控制台打印，用于宣告它被调用的时机。

struct Droppable {
    name: &'static str,
}

// 这个简单的 `drop` 实现添加了一个控制台打印
impl Drop for Droppable {
    fn drop(&mut self) {
        println!("> 正在释放 {}", self.name);
    }
}

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

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

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

            println!("正在退出块 B");
        }
        println!("刚刚退出了块 B");

        println!("正在退出块 A");
    }
    println!("刚刚退出了块 A");

    // 可以使用 `drop` 函数手动释放变量
    drop(_a);
    // TODO ^ 试试注释掉这一行

    println!("main 函数结束");

    // `_a` 在这里**不会**被再次 `drop`，因为它已经
    // 被（手动）`drop` 过了
}

For a more practical example, here's how the Drop trait can be used to automatically clean up temporary files when they're no longer needed:

use std::fs::File;
use std::path::PathBuf;

struct TempFile {
    file: File,
    path: PathBuf,
}

impl TempFile {
    fn new(path: PathBuf) -> std::io::Result<Self> {
        // Note: File::create() will overwrite existing files
        let file = File::create(&path)?;
        
        Ok(Self { file, path })
    }
}

// When TempFile is dropped:
// 1. First, the File will be automatically closed (Drop for File)
// 2. Then our drop implementation will remove the file
impl Drop for TempFile {
    fn drop(&mut self) {
        // Note: File is already closed at this point
        if let Err(e) = std::fs::remove_file(&self.path) {
            eprintln!("Failed to remove temporary file: {}", e);
        }
        println!("> Dropped temporary file: {:?}", self.path);
    }
}

fn main() -> std::io::Result<()> {
    // Create a new scope to demonstrate drop behavior
    {
        let temp = TempFile::new("test.txt".into())?;
        println!("Temporary file created");
        // File will be automatically cleaned up when temp goes out of scope
    }
    println!("End of scope - file should be cleaned up");

    // We can also manually drop if needed
    let temp2 = TempFile::new("another_test.txt".into())?;
    drop(temp2); // Explicitly drop the file
    println!("Manually dropped file");
    
    Ok(())
}