如何将interface {}转换为bytes.Buffer

Question

How to convert interface{} to bytes.Buffer?

Minimal example

package main

import (
    "bytes"
    "fmt"
)

func ToJson5(any interface{}) string {
    if any == nil {
        return `''`
    }
    switch any.(type) {
    case bytes.Buffer: // return as is
        return any.(bytes.Buffer).String()
    // other types works fine
    }
    return ``
}

func main() {
    x := bytes.Buffer{}
    fmt.Println(ToJson5(x))
}

The errors was:

main.go:14: cannot call pointer method on any.(bytes.Buffer)
main.go:14: cannot take the address of any.(bytes.Buffer)

When changed to bytes.Buffer{} (that I think less correct), the error was:

main.go:13: bytes.Buffer literal (type bytes.Buffer) is not a type
main.go:14: bytes.Buffer literal is not a type

Answer 1

You can use a Short variable declaration in the Type switch to have the typed value in the case branches:

switch v := any.(type) {
case bytes.Buffer: // return as is
    return v.String() // Here v is of type bytes.Buffer
}

Try it on the Go Playground.

Quoting form the spec:

The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared at the beginning of the implicit block in each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.