go - 为什么 Go 允许我调用未实现的方法？

Question

Go 似乎并没有强制遵循接口(interface)的结构。为什么下面的代码可以编译？

package main

type LocalInterface interface {
    SomeMethod(string) error
    SomeOtherMethod(string) error
}

type LocalStruct struct {
    LocalInterface
    myOwnField string
}

func main() {
    var localInterface LocalInterface = &LocalStruct{myOwnField:"test"}

    localInterface.SomeMethod("calling some method")
}

这似乎不应该编译，因为 SomeMethod 没有实现。go build 不会产生任何问题。

运行它会导致运行时错误:

> go run main.go
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x20 pc=0x4013b0]

goroutine 1 [running]:
panic(0x460760, 0xc08200a090)
        C:/Go/src/runtime/panic.go:464 +0x3f4
main.(*LocalStruct).SomeMethod(0xc0820064e0, 0x47bf30, 0x13, 0x0, 0x0)
        <autogenerated>:3 +0x70
main.main()
        C:/Users/kdeenanauth/Documents/git/go/src/gitlab.com/kdeenanauth/structTest/main.go:16 +0x98
exit status 2

Answer 1

当嵌入类型时(在您的示例中，LocalInterface 嵌入在 LocalStruct 中)，Go 创建嵌入类型的字段并将其方法提升为封闭类型。

所以下面的声明

type LocalStruct struct {
    LocalInterface
    myOwnField string
}

相当于

type LocalStruct struct {
    LocalInterface LocalInterface
    myOwnField string
}

func (ls *LocalStruct) SomeMethod(s string) error {
    return ls.LocalInterface.SomeMethod(s)
}

因为 LocalInterface 字段为 nil，您的程序因 nil 指针取消引用而发生困惑。

以下程序“修复”了 panic (http://play.golang.org/p/Oc3Mfn6LaL):

package main

type LocalInterface interface {
    SomeMethod(string) error
}

type LocalStruct struct {
     LocalInterface
    myOwnField string
}

type A int

func (a A) SomeMethod(s string) error {
    println(s)
    return nil
}

func main() {
    var localInterface LocalInterface = &LocalStruct{
        LocalInterface: A(10),
        myOwnField:     "test",
    }

    localInterface.SomeMethod("calling some method")
}