defer 关键字在 Go 中的工作原理

Question

存在一个问题，提示用户输入票价年龄，但当if输入的年龄高于 13 岁时，报表仅打印 9.99：

尝试过:=，并且fmt.Println(ticketPrice == 19.99)。
如果有人有其他建议，请告诉我。

package main

import "fmt"

func main() {

    var age int

    var ticketprice float64

    defer fmt.Println("Your age is:", getAge(age)) // calls getAge

    defer fmt.Println("Your ticket price is:", printTicket(age,

        ticketprice)) // calls printTicket

}

func printTicket(age int, ticketPrice float64) float64 {

    if age <= 13 {

        ticketPrice = 9.99 // only calls 9.99, not anything else

    } else if age > 13 && age < 65 {

        ticketPrice = 19.99 // not being called?

    } else if age >= 65 {

        ticketPrice = 12.99 // not being called?

    }

    return ticketPrice // returns ticket price

} // Whole function is not being used for some reason. Only returns 9.99, nothing else

func getAge(age int) int {

    fmt.Println("What is your age?")

    fmt.Scan(&age)

    for age < 0 || age > 100 {

        fmt.Println("That cannot be, please enter your age again")

        fmt.Scan(&age)

    }

    return age

}

func getName(name string) string {

    fmt.Println("What is your name?")

    fmt.Scan(&name)

    return name

}

Answer 1

请参阅Defer_statements：每次执行“defer”语句时，调用的函数值和参数都会照常评估并重新保存，但不会调用实际函数。相反，延迟函数会在周围函数返回之前立即调用，调用顺序与延迟函数相反。也就是说，如果周围函数通过显式 return 语句返回，则延迟函数将在该 return 语句设置任何结果参数之后但在函数返回到其调用者之前执行。如果延迟函数值求值为 nil，则在调用该函数时（而不是执行“defer”语句时）会发生执行混乱。为了理解评估顺序，让我们尝试一下：defer having()(fun("with Go."))让我们运行它并阅读评估顺序的代码注释：package mainimport "fmt"func main() {    defer having()(fun("with Go."))    fmt.Print("some ") // evaluation order: 3}func having() func(string) {    fmt.Print("Go ") // evaluation order: 1    return funWithGo}func fun(msg string) string {    fmt.Print("have ") // evaluation order: 2    return msg}func funWithGo(msg string) {    fmt.Println("fun", msg) // evaluation order: 4}输出：Go have some fun with Go.这样就更漂亮了，同样的结果，只需将所有函数替换为匿名函数即可：package mainimport "fmt"func main() {    defer func() func(string) {        fmt.Print("Go ") // evaluation order: 1        return func(msg string) {            fmt.Println("fun", msg) // evaluation order: 4        }    }()(func(msg string) string {        fmt.Print("have ") // evaluation order: 2        return msg    }("with Go."))    fmt.Print("some ") // evaluation order: 3}我希望这可以帮助您了解其defer工作原理：代码的工作版本，内部有足够的文档（注意首先评估的参数和调用将推迟到以相反顺序返回的函数）：deferfmt.Printlnpackage mainimport "fmt"func printTicket(age int) float64 {    fmt.Println("...order is 2...")    switch {    case age <= 13:        return 9.99    case age > 13 && age < 65:        return 19.99    default:        return 12.99    }}func main() {    age := 999    defer fmt.Println("...order is 4...Your age is:", getAge(&age))    defer fmt.Println("...order is 3...Your ticket price is:", printTicket(age))}func getAge(age *int) int {    fmt.Println("...order is 1...")    fmt.Print("Enter age=")    fmt.Scanln(age)    return *age}输出：...order is 1...Enter age=999...order is 2......order is 3...Your ticket price is: 12.99...order is 4...Your age is: 999