go - 不可能型开关盒

Question

此程序无法编译:

package main

type Validator struct {
}

// Error implements error interface
func (v *Validator) Error() string {
    return ""
}

func test() error {
    return &Validator{}
}

func main() {
    switch test().(type) {
    case nil:
        println("No error")
    case Validator:
        println("Validation error")
        return
    default:
        println("Unknown error")
        return
    }
}

错误是:

prog.go:19: impossible type switch case: test() (type error) cannot have dynamic type Validator (missing Error method)

但是 Validator 结构体有方法 Error。

Answer 1

你有两种不同的类型，Validator和指针类型*Validator，这两种类型有不同的方法集。

你只为指针定义了一个Error()方法，而Validator没有这个方法。

你可以做的是以下改变:

// Error implements error interface
func (v Validator) Error() string {
    return ""
}

...

case *Validator, Validator: // You are actually getting a *Validator

这实现了 Validator 和 *Validator 的 Error()。作为 Go specification 说:

The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T)