匿名字段作为指针或嵌入

Question

What is the different between anonymous field as pointer or anonymous field as usual. Consider, how I embed Foo into Bar struct.

Look at the following code snippet:

First with anonymous field as pointer

package main

import (
    "fmt"
)

type Foo struct{}

func (*Foo) Run() {
    fmt.Println("Hello")
}

type Bar struct {
    *Foo
}

func main() {
    bar := new(Bar)
    bar.Run() 
}

and second anonymous field as usual:

package main

import (
    "fmt"
)

type Foo struct{}

func (*Foo) Run() {
    fmt.Println("Hello")
}

type Bar struct {
    Foo
}

func main() {
    bar := new(Bar)
    bar.Run()
}

What is different between them?

Update: I pick up this sample from revel webframework, how they extend a custom controller. Look at this code snippet

type App struct {
    *revel.Controller
}

why do revel use pointer to embed controller struct. What is the sense of it?

Answer 1

Given that Foo is an empty struct, embedding it in Bar will make no change to to the memory footprint of Bar.

However, embedding a pointer to Foo will change Bar; it will add a pointer value, initialized to nil. If the only purpose of Foo is to have methods, then using a pointer is superfluous.

Of course, were you to add some fields into Foo, then, in the pointer case, you'd also need to add bar.Foo = new(Foo) into main, or else you'll have problems.

The advantage of using a pointer would be that you could share the same Foo instance between multiple instances of Bar, or else you could keep it nil until you really need it.

The disadvantage would be that it would be slightly more trouble, and slightly less efficient, if there's always a 1-to-1 Foo-to-Bar mapping.