Golang function pointer as a part of a struct

Question

I have the following code:

type FWriter struct {
    WriteF func(p []byte) (n int,err error)
}

func (self *FWriter) Write(p []byte) (n int, err error) {
    return self.WriteF(p)
}

func MyWriteFunction(p []byte) (n int, err error) { 
    // this function implements the Writer interface but is not named "Write"
    fmt.Print("%v",p)
    return len(p),nil
}

MyFWriter := new(FWriter)
MyFWriter.WriteF = MyWriteFunction
// I want to use MyWriteFunction with io.Copy
io.Copy(MyFWriter,os.Stdin)

What I am trying to do is to create a Writer interface to wrap MyWriteFunction because it is not named "Write" and I can't use it with anything that requires a "Writer" interface.

This code won't work as it complains:

method MyWriterFunction is not an expression, must be called

What am I doing wrong here? How can I set WriteF to be MyWriteFunction?

Note: I simplified this problem as much as I can and in reality I have a struct which has MyWriteFunction AND a normal Write function so it gets a little bit complicated... (Also if there is a better way to solve this problem of mine then I'll be glad to hear it!)

Thanks!!

EDIT: I have notice my typo and fixed it (MyWriterFunction --> MyWriteFunction).

I think I over-simplified the question in a manner that mislead you of my original intent. Following the Anonymous comment and peterSO kind comments I have re-created the error to better demonstrate my problem:

package main

import (
    "fmt"
    "io"
    "strings"
)

type ProxyWrite interface {
    Write(p []byte) (n int, err error)
    SpecialWrite(p []byte) (n int, err error)
}

type Implementer struct {
    counter int
}

func (self Implementer) Write(p []byte) (n int, err error) {
    fmt.Print("Normal write: %v", p)
    return len(p),nil
}

func (self Implementer) SpecialWrite(p []byte) (n int, err error) {
    fmt.Print("Normal write: %v\n", p)
    fmt.Println("And something else")
    self.counter += 1
    return len(p),nil
}


type WriteFunc func(p []byte) (n int, err error)

func (wf WriteFunc) Write(p []byte) (n int, err error) {
    return wf(p)
}

func main() {
    Proxies := make(map[int]ProxyWrite,2)
    Proxies[1] = new(Implementer)
    Proxies[2] = new(Implementer)

    /* runs and uses the Write method normally */
    io.Copy(Proxies[1], strings.NewReader("Hello world"))
    /* gets ./main.go:45: method Proxies[1].SpecialWrite is not an expression, must be called */
    io.Copy(WriteFunc(Proxies[1].SpecialWrite), strings.NewReader("Hello world"))
}

I hope it clarifies what I meant to present on the first attempt.

Any thoughts?

What version of go are you using? Methods as values don't appear until 1.1. And this code works fine on the playground... play.golang.org/p/AmRi_fujPw — Paul Hankin
– Paul Hankin, Commented Dec 22, 2013 at 16:18

Paul Hankin · Accepted Answer · 2013-12-22 13:04:14Z

19

There's a typo in your code, but wrapping the func into a struct is unnecessary anyway. Instead, you can just define a WriteFunc type that wraps a function, and that you can define a Write method on. Here's a full example.

package main

import (
    "fmt"
    "io"
    "strings"
)

type WriteFunc func(p []byte) (n int, err error)

func (wf WriteFunc) Write(p []byte) (n int, err error) {
    return wf(p)
}

func myWrite(p []byte) (n int, err error) {
    fmt.Print("%v", p)
    return len(p), nil
}

func main() {
    io.Copy(WriteFunc(myWrite), strings.NewReader("Hello world"))
}

answered Dec 22, 2013 at 13:04

Paul Hankin

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1 Comment

user340495 Over a year ago

Hi, this answer is very nice and works well. I have edited the question to better explain what I originally meant. if you would mind taking a look that would be awesome. thanks!

peterSO · Accepted Answer · 2013-12-22 12:22:21Z

7

Fix MyWriterFunction/MyWriteFunction typo. For example,

package main

import (
    "fmt"
    "io"
    "os"
)

type FWriter struct {
    WriteF func(p []byte) (n int, err error)
}

func (self *FWriter) Write(p []byte) (n int, err error) {
    return self.WriteF(p)
}

func MyWriteFunction(p []byte) (n int, err error) {
    // this function implements the Writer interface but is not named "Write"
    fmt.Print("%v", p)
    return len(p), nil
}

func main() {
    MyFWriter := new(FWriter)
    MyFWriter.WriteF = MyWriteFunction
    // I want to use MyWriteFunction with io.Copy
    io.Copy(MyFWriter, os.Stdin)
}

answered Dec 22, 2013 at 12:22

peterSO

168k32 gold badges303 silver badges293 bronze badges

1 Comment

user340495 Over a year ago

hey, Thanks for the kind answer, it indeed fixed the presented problem, but I have now fixed my question to better describe the problem, Thanks for the kind answer.

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