来自流的字符串，用于多种对象类型

Question

I'm used to Java, and setting first steps in google go. I have a tree of objects with child objects etc... This tree is recursively dumped to an io.Writer. Output might be huge, so I don't want to create a string for each object, and concatenate the result in memory..

For debugging purposes, i want to fmt.Printf parts of this tree. Thus, I want to create a generic String() function on each object in which calls the ToStream function, returning the result as a string. In Java, this is easy: create the method on the base class. How do I do this in GO, without creating a custom String method for each kind of object.

See the code for what I want, specifically the line marked ERROR

package main

import (
"io"
"fmt"
"bytes"
)

//Base is an interface for bulk output
type Base interface {
    ToStream(io.Writer)
}

//Impl1 has interface Base
type Impl1 struct{
    stuff int
}

func (Impl1) ToStream(w io.Writer) {
    fmt.Fprintf(w, "A lot of stuff")
}

//Impl2 has interface Base
type Impl2 struct{
    otherstuff int
}

func (Impl2) ToStream(w io.Writer) {
    fmt.Fprintf(w, "A lot of other stuff")
}

//I want to convert any base to a sting for debug output
//This should happen by the ToStream method

func (v Base) String() string {//ERROR here: Invalid receiver type Base (Base is an interface type)
//func (v Impl1) String() string {//This works, but requires re-implementation for every struct Impl1,Impl2,...
    var buffer bytes.Buffer
    v.ToStream(&buffer)
    return string(buffer.Bytes())
}

func main(){
    aBase:= new(Impl1)
    fmt.Printf("%s
",aBase)
}

Answer 1

You can wrap around a Base to add the necessary String() function. Here is one approach:

type StreamerToStringer struct {
    Base
}

func (s StreamerToStringer) String() string {
    var buffer bytes.Buffer
    s.Base.ToStream(&buffer)
    return string(buffer.Bytes())
}

With this, you can augment any Base instance so it has a String() method.

func main() {
    aBase1 := StreamerToStringer{new(Impl1)}
    aBase2 := StreamerToStringer{new(Impl2)}
    fmt.Printf("%s
", aBase1)
    fmt.Printf("%s
", aBase2)

    // These wrapped values still support ToStream().
    var buffer bytes.Buffer
    aBase1.ToStream(&buffer)
    fmt.Println(buffer.Bytes())
}