Go类型断言转换

Question

我可以像这样将 int 转换为 float64:

var a int = 10
var b float64 = float64(a)

关于类型断言，Effective Go 指出:“类型必须是接口(interface)持有的具体类型，或者是值可以转换为的第二个接口(interface)类型。”

考虑到这一点，为什么以下操作会失败:

func foo(a interface{}) {
    fmt.Println(a.(float64))
}

func main() {
    var a int = 10
    foo(a)
}

这会导致 panic :接口(interface)转换:接口(interface)是 int，不是 float64。

请注意 Go 规范说:

'对于接口(interface)类型的表达式x和类型T，初级表达式

x.(T)

断言 x 不是 nil 并且存储在 x 中的值是 T 类型。'

这确实与 Effective Go 声明相矛盾，但似乎更符合我所看到的。

Answer 1

This sentence在 Effective Go 中似乎确实令人困惑。看来作者当时在想结构体。

The chapter on assertions in the specification更清楚了:

For an expression x of interface type and a type T, the primary expression

x.(T) asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

More precisely, if T is not an interface type, x.(T) asserts that the dynamic type of x is identical to the type T. In this case, T must implement the (interface) type of x; otherwise the type assertion is invalid since it is not possible for x to store a value of type T. If T is an interface type, x.(T) asserts that the dynamic type of x implements the interface T.

事实you can convert your int to a float (反之亦然)并不意味着您可以断言它们是同一类型。