在Golang中转换组合类型

Question

I've been reading about type alias and composed structs in Golang. I want to be able to have two structs which are structurally identical but can be easily converted between each other.

I have a parent structure defined as:

type User struct {
    Email    string `json:"email"`
    Password string `json:"password"`
}

And a composed struct defined as:

type PublicUser struct {
    *User
}

I would expect that if I define a User:

a := User{
        Email:    "[e-mail]",
        Password: "1234",
    }

I could then perform the following type conversion:

b := (a).(PublicUser)

But it fails with an invalid type assertion:

invalid type assertion: a.(PublicUser) (non-interface type User on left)

How can I convert between structurally similar types in Go?

https://play.golang.org/p/I0VqrflOfXU

Answer 1

Type assertions in Go let you tap into an interface's concrete type, not into structs:

A type assertion provides access to an interface value's underlying concrete value.
https://tour.golang.org/methods/15

However, with slight modifications, this code works and probably behaves as you would expect:

package main

import (
    "fmt"
)

type User struct {
    Email    string `json:"email"`
    Password string `json:"password"`
}

type PublicUser User

func main() {
    a := User{
        Email:    "[e-mail]",
        Password: "1234",
    }
    fmt.Printf("%#v
", a)
    // out: User{Email:"[e-mail]", Password:"1234"}

    b := PublicUser(a)
    fmt.Printf("%#v", b)
    // out PublicUser{Email:"[e-mail]", Password:"1234"}
}

Here, PublicUser is a redefinition of the User type; most importantly, it is a standalone type, shares the fields, but not the method set of User (https://golang.org/ref/spec#Type_definitions).

Then, you can simply use the PublicUser type constructor, as you might have been doing similarly with string/[]byte conversions: foo := []byte("foobar").

If, on the other hand, you were to use an actual type alias (type PublicUser = User) your output will list User as type for both instances: PublicUser is only a new name for the old thing, not a new type.