为什么Golang在使用不同的输出功能时显示不同的值

Question

I try to check when Golang return a local value as nil, then I use this code.

package main

import (
    "fmt"
)

type S struct{}

func InitEfacePointer() interface{} {
    var s *S
    println(s)
    return s
}

func main() {
    s := InitEfacePointer()
    fmt.Println(s)
    //println(s)
}

The output is 0x0

But when I just use println to output value.

package main

type S struct{}

func InitEfacePointer() interface{} {
    var s *S
    println(s)
    return s
}

func main() {
    s := InitEfacePointer()
    println(s)
}

The output changed to 0x0 (0x93d40,0x0)

Could anyone explain the mechanism of this behavior? Thanks!

Answer 1

First of all, fmt.Println and builtin println are very different matteres: they are different both in implentation and in purpose. fmt.Println deals with many complex cases using reflect while println only deal with some base cases and is only for "bootstrap or debug" (as the spec says).

In this specific case, you are printing an interface{} you returned from InitEfacePointer(). The fmt.Println looks into the interface using reflect and gets the underlying data: a nil pointer to a string and it then prints it out: 0x0. On the other hand, builtin println takes the interface and by magic of the compiler (or not), it recognize it is an interface. As this post, golang interface are auctually two pointers, one to information about the type stored and one to the underlying data. So the builtin printlngoes into the interface details, 0x93d40 being the "type info" and 0x0 being the underlying data. (The first 0x0 is from the function call).

Furthermore, testing whether the returned interface is nil seems like a common mistake. Read here: https://golang.org/doc/faq#nil_error