GoLang nil与interface的空指针深入分析

func main() {
    var p *struct{}
    fmt.Println(unsafe.Sizeof(p), p == nil) // 8
    var s []int
    fmt.Println(unsafe.Sizeof(s), s == nil) // 24
    var m map[int]bool
    fmt.Println(unsafe.Sizeof(m), m == nil) // 8
    var c chan string
    fmt.Println(unsafe.Sizeof(c), c == nil) // 8
    var f func()
    fmt.Println(unsafe.Sizeof(f), f == nil) // 8
    var i interface{}
    fmt.Println(unsafe.Sizeof(i), i == nil) // 16
}

func inFun(v interface{}) {
    fmt.Println("fun-interface:", v == nil)
}
func main() {
    var a interface{}
    var b []string
    var c string
    fmt.Println(a == nil)
    inFun(a)    // true
    fmt.Println(b == nil)
    inFun(b)    // false
    //fmt.Println(c == nil) // can not compare with nil
    inFun(c)    // false
}
// true
// fun-interface: true
// true                
// fun-interface: false
// fun-interface: false

func nilCheck(v interface{}) {
    defer func() {
        if err := recover(); err != nil {
            fmt.Println("panic:", err)
        }
    }()
    if v == nil {
        fmt.Println("nilCheck: interface is nil")
        return
    }
    vi := reflect.ValueOf(v)
    fmt.Println("nilCheck:", vi.IsNil())
}
func main() {
    var a interface{}
    var b []string
    var c string
    nilCheck(a)
    nilCheck(b)
    nilCheck(c)
}
// nilCheck: interface is nil
// nilCheck: true                                             
// panic: reflect: call of reflect.Value.IsNil on string Value

func (v Value) IsNil() bool {
    k := v.kind()
    switch k {
    case Chan, Func, Map, Pointer, UnsafePointer:
        if v.flag&flagMethod != 0 {
            return false
        }
        ptr := v.ptr
        if v.flag&flagIndir != 0 {
            ptr = *(*unsafe.Pointer)(ptr)
        }
        return ptr == nil
    case Interface, Slice:
        // Both interface and slice are nil if first word is 0.
        // Both are always bigger than a word; assume flagIndir.
        return *(*unsafe.Pointer)(v.ptr) == nil
    }
    panic(&ValueError{"reflect.Value.IsNil", v.kind()})
}