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本文次要钻研一下golang中的零值

zero value

初始化时没有赋值的变量的默认值如下：

  • false for booleans
  • 0 for numeric types
  • "" for strings
  • nil for pointers, functions, interfaces, slices, channels, and maps

拜访nil实例

package mainimport (    "encoding/json"    "fmt")// https://golang.google.cn/ref/spec#The_zero_valuetype Demo struct {    Name string    Ptr *string}type DemoFunc func() stringtype DemoInterface interface {    Hello() string}func main() {    var demo Demo    fmt.Println("demo.Name=", demo.Name)    // {"Name":"","Ptr":null}    printJson(demo)    var demoPtr *Demo    // null    printJson(demoPtr)    // panic: runtime error: invalid memory address or nil pointer dereference    // [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10a7167]    // fmt.Println(demoPtr.Name)    var demoFunc DemoFunc    // could not marshal object    printJson(demoFunc)    // panic: runtime error: invalid memory address or nil pointer dereference    // fmt.Println(demoFunc())    var demoInterface DemoInterface    // null    printJson(demoInterface)    // panic: runtime error: invalid memory address or nil pointer dereference    // fmt.Println(demoInterface.Hello())    var s []Demo    // []    fmt.Println("[]Demo=", s)    // null    printJson(s)    for _, e := range s {        fmt.Println(e)    }    var sp []*Demo    // []    fmt.Println("[]*Demo=", sp)    // null    printJson(sp)    for _, e := range sp {        fmt.Println(e)    }    var c chan bool    // <nil>    fmt.Println(c)    var m map[string]int    // map[]    fmt.Println("map[string]int=", m)    // 0    fmt.Println(m["abc"])    // null    printJson(m)    for k, e := range m {        fmt.Println(k)        fmt.Println(e)    }}func printJson(data interface{}) {    jsonBytes, err := json.Marshal(data)    if err != nil {        fmt.Println("could not marshal object")        return    }    fmt.Println(string(jsonBytes))}

小结

  • 对于toJson来讲，大部分为null，func类型的会报错
  • 对于map，拜访不存在的key则返回该类型的零值，另外对于零值的slice或map能够间接for range，不会报错
  • 对象指针、func、接口间接拜访其属性或办法，会报panic: runtime error: invalid memory address or nil pointer dereference

doc

  • The_zero_value