17、nil指针-GOLANG

通向惊恐的nil指针

package main

import "fmt"

func main() {
	var nowhere *int
	fmt.Println(nowhere)
	//fmt.Println(*nowhere) //针对nil指针的解引用会使程序崩溃
}

保护方法

package main

import "fmt"

type person struct {
	age int
}

func (p *person) birthday() {
	p.age++
}
func main() {
	var nobody *person
	fmt.Println(nobody)
	//nobody.birthday()//造成惊恐导致程序崩溃
}

修改

package main

import "fmt"

type person struct {
	age int
}

func (p *person) birthday() {
	if p == nil {
		return
	}
	p.age++
}
func main() {
	var nobody *person
	fmt.Println(nobody)
	nobody.birthday() //通过if判断避免程序崩溃
}

nil函数值

package main

import "fmt"

func main() {
	var fn func(a, b int) int
	fmt.Println(fn == nil)
}

package main

import (
	"fmt"
	"sort"
)

func sortStrings(s []string, less func(i, j int) bool) {
	if less == nil {
		less = func(i, j int) bool {
			return s[i] < s[j]
		}
		sort.Slice(s, less)
	}
}
func main() {
	food := []string{"onion", "carrot", "celery"}
	sortStrings(food, nil)
	fmt.Println(food)
}

nil切片

切片在声明之后没有使用复合字面量或者内置的make函数进行初始化，那么值为nil，不过关键字range，len，append等内置函数可以正常处理为nil的切片

package main

import "fmt"

func main() {
	var soup []string
	fmt.Println(soup == nil)

	for _, value := range soup {
		fmt.Println(value)
	}
	fmt.Println(len(soup))
	soup = append(soup, "onion", "carrot", "celery")
	fmt.Println(soup)
}

不包含任何元素的空切片和值为nil的切片并不相等，但是通常它们可以替换使用

package main

import "fmt"

func mirrpoix(ing []string) []string {
	return append(ing, "onion", "carrot", "celery")
}

func main() {
	soup := mirrpoix(nil)
	fmt.Println(soup)
}

nil的其他选择

package main

import "fmt"

type number struct {
	value int
	valid bool
}

func newNumber(v int) number {
	return number{value: v, valid: true}
}
func (n number) String() string {
	if !n.valid {
		return "not set"
	}
	return fmt.Sprintf("%d", n.value)
}

func main() {
	n := newNumber(42)
	fmt.Println(n)

	e := number{}
	fmt.Println(e)
}