如何避免Golang中的空指针问题 介绍nil检查与防御性编程技巧

nil

nil

nil

var m map[string]int
m["a"] = 1 // panic: assignment to entry in nil map

func printUser(u *User) {
    if u == nil {
        log.Println("user is nil")
        return
    }
    fmt.Println(u.Name)
}

func addValue(m map[string]int, k string, v int) {
    if m == nil {
        log.Println("map is nil, skipping")
        return
    }
    m[k] = v
}

var u *User
var i interface{} = u

if i == nil {
    fmt.Println("i is nil")
} else {
    fmt.Println("i is not nil") // 会输出这句！
}

i

*User

if i == nil || (reflect.ValueOf(i).Kind() == reflect.Ptr && reflect.ValueOf(i).IsNil()) {
    // 真正的 nil 判断
}

func NewUser(name string) *User {
    if name == "" {
        return &User{Name: "default"} // 而不是 nil
    }
    return &User{Name: name}
}

func FindUser(id int) (*User, bool) {
    // 找不到返回 nil, false
    if user, ok := db[id]; ok {
        return &user, true
    }
    return nil, false
}

if u, ok := FindUser(1); ok {
    fmt.Println(u.Name)
} else {
    fmt.Println("not found")
}

type Config struct {
    Timeout time.Duration
    Tags    []string
    Meta    map[string]string
}

func NewConfig() *Config {
    return &Config{
        Timeout: 30 * time.Second,
        Tags:    make([]string, 0),
        Meta:    make(map[string]string),
    }
}

type Point struct{ X, Y int }

func (p Point) Distance() float64 { ... } // 使用值接收者

func Process(data *Input) error {
    if data == nil {
        return errors.New("input cannot be nil")
    }
    // 或者自动补全
    if data.Timeout == 0 {
        data.Timeout = defaultTimeout
    }
    // ...
}

staticcheck ./...

func TestProcessNilInput(t *testing.T) {
    err := Process(nil)
    if err == nil {
        t.Fatal("expected error for nil input")
    }
}