在结构中打印nil错误时出现恐慌

Question

The following code will panic when printing a nil value of an error type field in a struct. Why?

package main

import (
    "fmt"
)

type ta struct {
    int
}

type tb struct {
    int
    *ta
}

type tc struct {
    int
    error
}

type td struct {
    int
    error interface{}
}

func main() {
    fmt.Println(fmt.Sprintf("%v", ta{1}))
    fmt.Println(fmt.Sprintf("%v", tb{1, nil}))
    fmt.Println(fmt.Sprintf("%v", tc{1, nil}))
    fmt.Println(fmt.Sprintf("%v", td{1, nil}))
}

▸ go run main.go
{1}
{1 <nil>}
%!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
{1 <nil>}

https://play.golang.org/p/SrzUsJ3pyRC

Follow up question: if I have such a value tc{1, nil}, how to print it without causing panic?

Answer 1

In tc, you're embedding a value of type error. Sprintf sees that tc implements error (because of embedding), and tries to call Error() on it, which triggers the error because the value is nil.

If you want to print it without causing a panic, don't embed error (put it in a named field), or make sure the embedded error value is not nil.