RAII 对比

构造器-析构器

C++

#include <cstdio>

struct Locker {
    Locker() {
        puts("lock");
    }
    ~Locker() {
        puts("unlock");
    }
};

int main() {
    Locker locker;
}

Rust

struct Locker {}

impl Locker {
    pub fn new() -> Self {
        println!("lock");
        Locker {}
    }
}

impl Drop for Locker {
    fn drop(&mut self) {
        println!("unlock")
    }
}


pub fn main() {
    let locker = Locker::new();
}

控制块

Java

class Locker implements AutoCloseable {
    public Locker() {
        System.out.println("lock");
    }

    @Override
    public void close() {
        System.out.println("unlock");
    }

    public static void main(String[] args) {
        try (Locker locker = new Locker()) {
            
        }
    }
}

C#

using System;

class Locker : IDisposable
{
    public Locker()
    {
        Console.WriteLine("lock");
    }

    public void Dispose()
    {
        Console.WriteLine("unlock");
    }
    
    static void Main(string[] args)
    {
        using var locker = new Locker();
    }
}

Python

class Locker:
    def __enter__(self):
        print("lock")

    def __exit__(self, *args):
        print("unlock")

with Locker():
    pass

defer 语句

Go

import "fmt"

func main() {
	defer fmt.Println("unlock")
	fmt.Println("lock")
}

Swift

defer {
    print("unlock")
}
print("lock")

思考题

这样写实现 RAII 有什么问题：

Java

var r1 = open("r1");
var r2 = open("r2");
try {
    // ...
} finally {
    r1.close();
    r2.close();
}

C#

var r1 = open("r1");
var r2 = open("r2");
try {
    // ...
} finally {
    r1.close();
    r2.close();
}

Python

r1 = open("r1")
r2 = open("r2")
try:
    # ...
finally:
    r1.close()
    r2.close()

正确的写法应该是：

Java

try (var r1 = open("r1"); var r2 = open("r2")) {
    // ...
}

C#

using var r1 = open("r1");
using var r2 = open("r2");
// ...

Python

with open("r1") as r1, open("r2") as r2:
    # ...

这种写法语前面的写法不同，而是等价于下面的写法：

Java

var r1 = open("r1");
try {
    var r2 = open("r2");
    try {
        // ...
    } finally {
        r2.close();
    }
} finally {
    r1.close();
}

C#

var r1 = open("r1");
try {
    var r2 = open("r2");
    try {
        // ...
    } finally {
        r2.close();
    }
} finally {
    r1.close();
}

Python

r1 = open("r1")
try:
    r2 = open("r2")
    try:
        # ...
    finally:
        r2.close()
finally:
    r1.close()

因此这个语法糖相当于是把嵌套结构变成并列结构了，大大简化了代码编写。