进入嵌入式自定义界面

Question

I'm trying to understand Go's interfaces and embedding.

What I'm trying to do here is create my own custom writer and reader where each one implements either io.Writer or io.Reader

Now I want to take these custom structures that implement basic interfaces and embed them into another custom struct that implements Read/Write/Close. The below code is what I have so far but when I run it I get the following error

cannot use test (type MyReadWriteCloser) as type io.Writer in argument to fmt.Fprintf: MyReadWriteCloser does not implement io.Writer (missing Write method)

I thought when you embed a structure within another structure you also get the methods of the embedded structures. Can someone tell me what I'm missing?

package main

import (
"fmt"
"io"
)

type MyWriter struct {
    w io.Writer
}

func (m MyWriter) Write(b []byte) (n int, err error) {
    // encrypt b and write to underlying writer
    m.w.Write(b)
    return
}

type MyReader struct {
    r io.Reader
}

func (m MyReader) Read(b []byte) (n int, err error) {
    // decrypt b
    m.r.Read(b)
    return
}

type MyReadWriteCloser struct {
    MyWriter
    MyReader
}

func (m MyReadWriteCloser) Close() error {
    return nil
}

func main() {
    fmt.Println("main start")

    r, w := io.Pipe()

    test := MyReadWriteCloser{
        MyWriter{w},
        MyReader{r},
    }
    fmt.Fprintf(test, "hello world
")
} 

Answer 1

Your syntax is wrong so the interfaces/types are not actually being imbedded.

type MyWriter struct {
    w io.Writer
}

should be

type MyWriter struct {
    io.Writer
}

And based on my understanding a proper embed leaves you no reason to define a method like;

func (m MyWriter) Write(b []byte) (n int, err error) {
    m.w.Write(b)
    return
}

because the body would just be m.Write(b) so that just doesn't make sense. If you embed a type the embedding type will be able to call methods from the embedded type directly (it's the nearest thing to inheritance, it looks the same to a casual reader). I don't remember the resolution rules if there are conflicts between method or property names in embedded types or the embedding type an embedded type but a good rule of thumb would be to avoid embedding types that will cause naming conflicts because the behavior will not be obvious.

EDIT: Example of how to override a method in the underlying type and invoke the 'base' (i quote cause some gophers might be offended by that word choice) method from inside it;

func (m MyWriter) Write(b []byte) (n int, err error) {
     b = EncodeBFromMethodThatsInScopeHere(b)
     return m.Writer.Write(b)
}

Basically, if you want to explicitly use the embedded types method you can refer to it as a property of the type using the embedded types type as if it were a property name.