为什么Go语言允许我调用未实现的方法？

huangapple
go评论119阅读模式
英文:

Why does Go allow me to call methods that are not implemented?

问题

Go似乎没有强制要求结构体遵循接口。为什么以下代码能够编译通过？

  1. package main
  3. type LocalInterface interface {
  4.     SomeMethod(string) error
  5.     SomeOtherMethod(string) error
  6. }
  8. type LocalStruct struct {
  9.     LocalInterface
  10.     myOwnField string
  11. }
  13. func main() {
  14.     var localInterface LocalInterface = &LocalStruct{myOwnField:"test"}
  15.     
  16.     localInterface.SomeMethod("calling some method")
  17. }

看起来这段代码不应该编译通过，因为SomeMethod没有被实现。但是使用go build命令并没有报错。

运行代码会导致运行时错误：

  1. > go run main.go
  2. panic: runtime error: invalid memory address or nil pointer dereference
  3. [signal 0xc0000005 code=0x0 addr=0x20 pc=0x4013b0]
  5. goroutine 1 [running]:
  6. panic(0x460760, 0xc08200a090)
  7.         C:/Go/src/runtime/panic.go:464 +0x3f4
  8. main.(*LocalStruct).SomeMethod(0xc0820064e0, 0x47bf30, 0x13, 0x0, 0x0)
  9.         <autogenerated>:3 +0x70
  10. main.main()
  11.         C:/Users/kdeenanauth/Documents/git/go/src/gitlab.com/kdeenanauth/structTest/main.go:16 +0x98
  12. exit status 2
英文:

Go doesn't seem to enforce the struct adhering to the interface. Why does the following code compile?
<!-- language: go -->

  1. package main
  3. type LocalInterface interface {
  4.     SomeMethod(string) error
  5.     SomeOtherMethod(string) error
  6. }
  8. type LocalStruct struct {
  9.     LocalInterface
  10.     myOwnField string
  11. }
  13. func main() {
  14.     var localInterface LocalInterface = &amp;LocalStruct{myOwnField:&quot;test&quot;}
  15.     
  16.     localInterface.SomeMethod(&quot;calling some method&quot;)
  17. }

It seems like this should not compile since SomeMethod is not implemented.go build results in no issues.

Running it results in the runtime error:

  1. &gt; go run main.go
  2. panic: runtime error: invalid memory address or nil pointer dereference
  3. [signal 0xc0000005 code=0x0 addr=0x20 pc=0x4013b0]
  5. goroutine 1 [running]:
  6. panic(0x460760, 0xc08200a090)
  7.         C:/Go/src/runtime/panic.go:464 +0x3f4
  8. main.(*LocalStruct).SomeMethod(0xc0820064e0, 0x47bf30, 0x13, 0x0, 0x0)
  9.         &lt;autogenerated&gt;:3 +0x70
  10. main.main()
  11.         C:/Users/kdeenanauth/Documents/git/go/src/gitlab.com/kdeenanauth/structTest/main.go:16 +0x98
  12. exit status 2

答案1

得分: 7

当一个类型被嵌入（例如，在你的例子中，LocalInterface 被嵌入在 LocalStruct 中），Go 会创建一个嵌入类型的字段，并将其方法提升到包含它的类型中。

所以以下声明：

  1. type LocalStruct struct {
  2.     LocalInterface
  3.     myOwnField string
  4. }

等同于

  1. type LocalStruct struct {
  2.     LocalInterface LocalInterface
  3.     myOwnField string
  4. }
  6. func (ls *LocalStruct) SomeMethod(s string) error {
  7.     return ls.LocalInterface.SomeMethod(s)
  8. }

你的程序因为 LocalInterface 字段是 nil 而导致空指针解引用的恐慌。

以下程序“修复”了这个恐慌（http://play.golang.org/p/Oc3Mfn6LaL）：

  1. package main
  3. type LocalInterface interface {
  4.     SomeMethod(string) error
  5. }
  7. type LocalStruct struct {
  8.     LocalInterface
  9.     myOwnField string
  10. }
  12. type A int
  14. func (a A) SomeMethod(s string) error {
  15.     println(s)
  16.     return nil
  17. }
  19. func main() {
  20.     var localInterface LocalInterface = &LocalStruct{
  21.         LocalInterface: A(10),
  22.         myOwnField:     "test",
  23.     }
  25.     localInterface.SomeMethod("calling some method")
  26. }
英文:

When a type is embedded (in your example LocalInterface is embedded inside LocalStruct), Go creates a field of the embedded type and promotes its methods to the enclosing type.

So the following declaration

  1. type LocalStruct struct {
  2.     LocalInterface
  3.     myOwnField string
  4. }

is equivalent to

  1. type LocalStruct struct {
  2.     LocalInterface LocalInterface
  3.     myOwnField string
  4. }
  6. func (ls *LocalStruct) SomeMethod(s string) error {
  7.     return ls.LocalInterface.SomeMethod(s)
  8. }

Your program panics with nil pointer dereference because LocalInterface field is nil.

The following program "fixes" the panic (http://play.golang.org/p/Oc3Mfn6LaL):

  1. package main
  3. type LocalInterface interface {
  4. 	SomeMethod(string) error
  5. }
  7. type LocalStruct struct {
  8. 	 LocalInterface
  9. 	myOwnField string
  10. }
  12. type A int
  14. func (a A) SomeMethod(s string) error {
  15. 	println(s)
  16. 	return nil
  17. }
  19. func main() {
  20. 	var localInterface LocalInterface = &amp;LocalStruct{
  21. 		LocalInterface: A(10),
  22. 		myOwnField:     &quot;test&quot;,
  23. 	}
  25. 	localInterface.SomeMethod(&quot;calling some method&quot;)
  26. }

答案2

得分: 1

进一步调查后，我发现避免嵌入可以得到适当的错误处理。在我的情况下，这将是首选方法：

  1. package main
  3. type LocalInterface interface {
  4.     SomeMethod(string) error
  5.     SomeOtherMethod(string) error
  6. }
  8. type LocalStruct struct {
  9.     myOwnField string
  10. }
  12. func main() {
  13.     var localInterface LocalInterface = &LocalStruct{myOwnField:"test"}
  15.     localInterface.SomeMethod("calling some method")
  16. }

结果为：

  1. .\main.go:13: 无法将 LocalStruct 字面量（类型 *LocalStruct）分配给 LocalInterface 类型：
  2.         *LocalStruct 未实现 LocalInterface（缺少 SomeMethod 方法）
英文:

Upon further investigation I found that avoiding embedding gets the proper error handling treatment. That would be preferred in my case:
<!-- language: go -->

  1. package main
  3. type LocalInterface interface {
  4.     SomeMethod(string) error
  5.     SomeOtherMethod(string) error
  6. }
  8. type LocalStruct struct {
  9.     myOwnField string
  10. }
  12. func main() {
  13.     var localInterface LocalInterface = &amp;LocalStruct{myOwnField:&quot;test&quot;}
  15.     localInterface.SomeMethod(&quot;calling some method&quot;)
  16. }

Results in:
>.\main.go:13: cannot use LocalStruct literal (type *LocalStruct) as type LocalInterface in assignment:
*LocalStruct does not implement LocalInterface (missing SomeMethod method)

huangapple
  • 本文由 发表于 2016年4月21日 11:10:24
  • 转载请务必保留本文链接：https://go.coder-hub.com/36758900.html
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