Meaning of a struct with embedded anonymous interface?

Question

sort package:

type Interface interface {
    Len() int
    Less(i, j int) bool
    Swap(i, j int)
}

...

type reverse struct {
    Interface
}
         


        

Answer 1

Ok, the accepted answer helped me understand, but I decided to post an explanation which I think suits better my way of thinking.

The "Effective Go" has example of interfaces having embedded other interfaces:

// ReadWriter is the interface that combines the Reader and Writer interfaces.
type ReadWriter interface {
    Reader
    Writer
}

and a struct having embedded other structs:

// ReadWriter stores pointers to a Reader and a Writer.
// It implements io.ReadWriter.
type ReadWriter struct {
    *Reader  // *bufio.Reader
    *Writer  // *bufio.Writer
}

But there is no mention of a struct having embedded an interface. I was confused seeing this in sort package:

type Interface interface {
    Len() int
    Less(i, j int) bool
    Swap(i, j int)
}

...

type reverse struct {
    Interface
}

But the idea is simple. It's almost the same as:

type reverse struct {
    IntSlice  // IntSlice struct attaches the methods of Interface to []int, sorting in increasing order
}

methods of IntSlice being promoted to reverse.

And this:

type reverse struct {
    Interface
}

means that sort.reverse can embed any struct that implements interface sort.Interface and whatever methods that interface has, they will be promoted to reverse.

sort.Interface has method Less(i, j int) bool which now can be overridden:

// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
    return r.Interface.Less(j, i)
}

---

My confusion in understanding

type reverse struct {
    Interface
}

was that I thought that a struct always has fixed structure, i.e. fixed number of fields of fixed types.

But the following proves me wrong:

package main

import "fmt"

// some interface
type Stringer interface {
    String() string
}

// a struct that implements Stringer interface
type Struct1 struct {
    field1 string
}

func (s Struct1) String() string {
    return s.field1
}


// another struct that implements Stringer interface, but has a different set of fields
type Struct2 struct {
    field1 []string
    dummy bool
}

func (s Struct2) String() string {
    return fmt.Sprintf("%v, %v", s.field1, s.dummy)
}


// container that can embedd any struct which implements Stringer interface
type StringerContainer struct {
    Stringer
}


func main() {
    // the following prints: This is Struct1
    fmt.Println(StringerContainer{Struct1{"This is Struct1"}})
    // the following prints: [This is Struct1], true
    fmt.Println(StringerContainer{Struct2{[]string{"This", "is", "Struct1"}, true}})
    // the following does not compile:
    // cannot use "This is a type that does not implement Stringer" (type string)
    // as type Stringer in field value:
    // string does not implement Stringer (missing String method)
    fmt.Println(StringerContainer{"This is a type that does not implement Stringer"})
}