42 接收者与指针 #
没有改变原始数据
func (c customer) add(v float64) {
c.balance += v
}
改变原始数据
func (c *customer) add(operation float64) {
c.balance += operation
}
43 返回值进行命名 #
getCoordinates(address string) (float32, float32, error)
更为清晰
getCoordinates(address string) (lat, lng float32, err error)
44 返回值非预期 #
忘记赋值err,导致err是nil
func returnAlwaysNil(ctx context.Context) (err error) {
if ctx.Err() != nil {
return err
}
return nil
}
error被覆盖,本来期望返回"new err"
func returnErrCover(ctx context.Context) (err error) {
err = errors.New("new err")
if err := ctx.Err(); err != nil {
return err // return "context canceled" cover "new err"
}
return nil
}
45 接收者nil #
type MultiError struct {
errs []string
}
func (m *MultiError) Add(err error) {
m.errs = append(m.errs, err.Error())
}
func (m *MultiError) Error() string {
return strings.Join(m.errs, ";")
}
type Customer struct {
Age int
Name string
}
func (c Customer) Validate() error {
var m *MultiError
if c.Age < 0 {
m = &MultiError{}
m.Add(errors.New("age is negative"))
}
if c.Name == "" {
if m == nil {
m = &MultiError{}
}
m.Add(errors.New("name is nil"))
}
return m
}
// 调用
func show() {
customer := Customer{Age: 33, Name: "John"}
if err := customer.Validate(); err != nil {
fmt.Printf("customer is invalid: %v", err)
}
}
customer is invalid: <nil>
以上本质是var m *MultiError
m作为*MultiError == nil
但是m却实现了error interface 作为error 不是nil
type error interface {
Error() string
}
修复方案
func (c Customer) Validate() error {
// ...
if m != nil {
return m
}
return nil
}
简化版理解
type mErr struct {
errs []string
}
func (m *mErr) Error() string {
return strings.Join(m.errs, ";")
}
func simpleErr() {
var m *mErr
var e2 error
e2 = m
if err := e2; err != nil {
fmt.Printf("err:%v", err)
}
}
output:
err:<nil>
涉及知识点为interface的值与类型判断
46 文件名作为参数的缺点 #
文件名作为参数的缺点,不利于单元测试.
解决方案为io.Reader作为参数,即接口的优点,抽象代替实际.
47 defer arguments与接收者 #
单词parameter与arguments,通常在英文上下文中为形参与实参的区别.
defer的陷阱
func deferFn2(s int) {
fmt.Println("defer :", s)
}
func deferFn1() {
var s int
defer deferFn2(s)
s = 1
return
}
output:
defer : 0
改法:
func deferFn5() {
var s int
defer func() {
deferFn2(s)
}()
s = 1
return
}
output:
defer : 1
传递通过指针
func deferFn3(s *int) {
fmt.Println("defer :", *s)
}
func deferFn1() {
var s int
defer deferFn3(&s)
s = 1
return
}
output:
defer : 1
闭包传递
func deferFn4() {
i := 0
j := 0
defer func(i int) {
fmt.Println(i, j)
}(i)
i++
j++
}
output:
0 1
即刻打印
func main2() {
s := Struct{id: "foo"}
defer s.print()
s.id = "bar"
}
type Struct struct {
id string
}
func (s Struct) print() {
fmt.Println(s.id)
}
output:
foo
指针类型
func main2() {
s := &Struct2{id: "foo"}
defer s.print()
s.id = "bar"
}
type Struct2 struct {
id string
}
func (s *Struct2) print() {
fmt.Println(s.id)
}
output:
bar