golang的defer

#############################

package main

import (
    "fmt"
    "time"
)

// 规则一 当defer被声明时，其参数就会被实时解析
// 规则二 多个defer执行顺序：defer执行顺序为先进后出
// 规则三 defer可以读取有名返回值
// 规则四 执行时机：函数return之后，函数返回给调用方之前之前
func DeferTimeX() {
    startAt := time.Now()
    defer fmt.Println(time.Since(startAt))
    time.Sleep(10 * time.Second)
}

func DeferTimeY() {
    startAt := time.Now()
    defer func() {
        fmt.Println(time.Since(startAt))
    }()
    time.Sleep(3 * time.Second)
}

func DeferOrder() {
    defer fmt.Println("defer runs")
    fmt.Println("block ends")
    for i := 0; i < 10; i++ {
        defer fmt.Println(i)
    }
    fmt.Println("main ends")
}

func DeferPara() {
    i := 2
    defer fmt.Println("************* ", i) //输出2，因为i此时就是2
    i = i * 2
    defer fmt.Println("$$$$$$$$$$$$$ ", i) //输出4，因为i此时就是4
    i = i * 2
    fmt.Println("=================== ", i) //输出8

}

// 没有修改返回值
func DeferFuncX() int {
    var i int
    fmt.Println("DeferFuncX ..........")
    fmt.Println("init at :", i)
    defer func() {
        fmt.Println("starting at :", i) //i此时就是16
        i++
        fmt.Println("end at :", i) //输出17
    }()
    return 16
}

// 修改了返回值
func DeferFuncY() (i int) {
    fmt.Println("DeferFuncY ..........")
    fmt.Println("init at :", i)
    defer func() {
        fmt.Println("starting at :", i) //i此时就是16
        i++
        fmt.Println("end at :", i) //输出17
    }()
    return 16
}

func main() {
    DeferTimeX()
    DeferTimeY()
    DeferOrder()
    DeferPara()
    fmt.Println(DeferFuncX())
    fmt.Println(DeferFuncY())

}

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