golang (*interface{})(nil) 是否为 nil？

Question

代码片段如下:

package main

import (
    "fmt"
    "reflect"
)

func main() {
    a := (*interface{})(nil)
    fmt.Println(reflect.TypeOf(a), reflect.ValueOf(a))
    var b interface{} = (*interface{})(nil)
    fmt.Println(reflect.TypeOf(b), reflect.ValueOf(b))
    fmt.Println(a == nil, b == nil)
}

输出如下:

*interface {} <nil>
*interface {} <nil>
true false

所以 var interface{} 与 := 不同，为什么？

Answer 1

根据 golang faq

Under the covers, interfaces are implemented as two elements, a type and a value. The value, called the interface's dynamic value, is an arbitrary concrete value and the type is that of the value. For the int value 3, an interface value contains, schematically, (int, 3).

An interface value is nil only if the inner value and type are both unset, (nil, nil). In particular, a nil interface will always hold a nil type. If we store a nil pointer of type *int inside an interface value, the inner type will be *int regardless of the value of the pointer: (*int, nil). Such an interface value will therefore be non-nil even when the pointer inside is nil.

a := (*interface{})(nil) 等于 var a *interface{} = nil。

但是 var b interface{} = (*interface{})(nil) ，意味着 b 是 interface{} 类型，并且interface{} 变量只有 nil 当它的类型和值都是 nil 时，显然类型 *interface{} 是不是 nil。