纠正有关格式错误参数的“ go vet”警告

Question

I have a struct that contains a pointer to another struct. That is,

type InnerStruct struct {
    whatever bool
}

type OuterStruct struct {
    is *InnerStruct
}

If I print out an instance of OuterStruct with %+v, then the inner struct is not expanded like it would be if it weren't a pointer. I understand and accept why that is, but because I don't have any chance of recursion, I'd like to get the full thing printed out.

It occurred to me that I might be able to implement the Formatter interface on a pointer receiver of InnerStruct in order to get the behavior I want, and indeed, the following does exactly what I want, at least for the cases I had (perhaps there are ways in which it could go horribly wrong; please tell me!):

func (is *InnerStruct) Format(s fmt.State, verb rune) {
    switch verb {
    case 'v':
        if s.Flag('+') {
            fmt.Fprintf(s, "&%+v", *is)
        } else if s.Flag('#') {
            fmt.Fprintf(s, "&%#v", *is)
        } else {
            fmt.Fprintf(s, "&%v", *is)
        }
    case 's':
        fmt.Fprintf(s, "&%s", *is)
    }
}

But when I run go vet on the code, it complains:

./thing.go:341: Fprintf format %s has arg *s of wrong type thing.InnerStruct

Is there a better way to get at the default string representation of a struct that avoids this issue with vet?

FWIW, I tried go-spew, and got (almost) the exact output I wanted, so I'll probably just go down that route, but I'd still like to know what, if anything, could be done to satisfy vet.

Answer 1

You have duplicate names s *InnerStruct and s fmt.State. go vet complains, that you have format verb s instead of v: fmt.Fprintf(s, "&%s", *s).

Fix your typos. For example,

package main

import (
    "fmt"
)

type InnerStruct struct {
    whatever bool
}

type OuterStruct struct {
    is *InnerStruct
}

func (s *InnerStruct) Format(f fmt.State, verb rune) {
    switch verb {
    case 'v':
        if f.Flag('+') {
            fmt.Fprintf(f, "&%+v", *s)
        } else if f.Flag('#') {
            fmt.Fprintf(f, "&%#v", *s)
        } else {
            fmt.Fprintf(f, "&%v", *s)
        }
    case 's':
        fmt.Fprintf(f, "&%v", *s)
    }
}

func main() {}

Playground: https://play.golang.org/p/B2pAus3kEwt