golang新建一个结构，但返回一个接口

Question

Here are golang code, the func newXXX return an interface, but why it does not return a struct

type _ABitOfEverythingServer struct {
    v map[string]*examples.ABitOfEverything
    m sync.Mutex
}

type ABitOfEverythingServer interface {
    examples.ABitOfEverythingServiceServer  // interface
    examples.StreamServiceServer            // interface
}

func newABitOfEverythingServer() ABitOfEverythingServer { 
//<-why not return _ABitOfEverythingServer, is it a good way?
    return &_ABitOfEverythingServer{
        v: make(map[string]*examples.ABitOfEverything),
    }
}

Answer 1

First, you need to learn the basics of Go. Take A Tour of Go.

Simplifying a complicated example,

package main

import "fmt"

type S struct{ F int }

type I interface{}

func newI() I {
    return &S{F: 42}
}

func main() {
    i := newI()
    s := i.(*S)
    f := s.F
    fmt.Println(f)
}

Playground: https://play.golang.org/p/tHbTZHEQZ-L

Output:

42

The newI function returns a value of interface type I, which contains a value of concrete type *S.