2017-03-04 20:18
浏览 833


I have a nested struct.

type ConfigOne struct {
    // Daemon section from config file.
    Daemon daemon
type daemon struct {
    Loglevel int
    Logfile string

And I have a String() string method on that type, which I am trying to return the nested struct elements as

func (c ConfigOne)String()  string{
    return fmt.Sprintf("%+v
", c)

When I am trying to print it as

c := &modules.ConfigOne{}
c.Daemon.Loglevel = 1
c.Daemon.Logfile = "/tmp/test.log"

I am getting the error

runtime: goroutine stack exceeds 1000000000-byte limit fatal error: stack overflow

runtime stack: runtime.throw(0x6ea3b7, 0xe) ...

After going through the error, I could see repeated lines similar to the below one

modules/structs.go:31 +0x77 fp=0xc440100398 sp=0xc440100328 go-consume/modules.(*ConfigOne).String(0xc42abcb4e0, 0x70bc08, 0xc42abd6300) :1 +0x64 fp=0xc4401003d8 sp=0xc440100398 fmt.(*pp).handleMethods(0xc42abd6300, 0xc400000076, 0x410301)

and finally, before dying,

modules/structs.go:31 +0xc0 fp=0xc440103d18 sp=0xc440103ca8 ...additional frames elided...

goroutine 17 [syscall, locked to thread]: runtime.goexit()

which I believe is caused by going into some infinite recursion.

I tried my luck to find the cause and reached here, which I believe is the same issue. However I couldn't understand the explanation in that thread.

If I try to print the individual nested struct as

func (c ConfigOne)String() string{
    //return fmt.Sprintf("%+v
", c.Daemon.Loglevel)
    return fmt.Sprintf("%+v
", c.Daemon)

it is working fine, and log shows the fields as

2017/03/05 01:28:25 go-consume.go:38: INFO: {Loglevel:1 Logfile:/tmp/test.log}

Can someone kindly explain how the former String() method is resulting in an infinite recursion and a stackoverflow, and what is the best way to overcome this?

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2条回答 默认 最新

  • dscuu86620 2017-03-04 20:45

    The %v and %+v formats use the value of String() if the type implements it. Therefore, using %+v on a type within the String() function for that type causes infinite recursion. Instead of using %+v in the String() function, you'll have to construct your own string, showing the contents of the structure in whatever way you see fit.

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  • sydiesel 2021-03-24 18:07

    You can use the following wording

    package main
    import (
    type AAA struct {
    	Name string
    	Age  int
    func (a *AAA) String() string {
    	return fmt.Sprintf("%+v", *a)
    func main() {
    	as := []*AAA{
    			Name: "111",
    			Age:  10,
    			Name: "222",
    			Age:  20,
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