go - 零指针嵌入错误

Question

当我尝试打印带有未初始化嵌入错误的指针时，为什么会出现 nil 指针错误:

package main

import (
  "log"
  "errors"
)

type Danger struct {
  error
}

func main() {
  // the nil pointer issue has to do with struct embedding an error value that is nil
  d := &Danger{}
  log.Println(d)

  d = &Danger{errors.New("foobar")}
  log.Println(d)
}

结果

2009/11/10 23:00:00 %!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
2009/11/10 23:00:00 foobar

https://play.golang.org/p/fBuN0XonX9v

这在今天的面试中出现，面试官和被面试者都搞不懂。

Answer 1

spec说:

Given a struct type S and a defined type T, promoted methods are included in the method set of the struct as follows:

• If S contains an embedded field T, the method sets of S and *S both include promoted methods with receiver T. The method set of *S also includes promoted methods with receiver *T.

fmt文档说:

If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

由此，我们可以得出结论，log.Println(d) 将从 error 字段调用提升的 Error 方法。

如果 error 字段为 nil，则调用出现错误。

fmt 文档还说:

If an Error or String method triggers a panic when called by a print routine, the fmt package reformats the error message from the panic, decorating it with an indication that it came through the fmt package.

文本 %!v(PANIC=runtime error: invalid memory address or nil pointer dereference) 是修饰的 panic 值。