使用new初始化嵌套结构

Question

This is my Go code. Also available at Go Playground

package main

import "fmt"

type App struct {
    OneHandler *OneHandler
    TwoHandler *TwoHandler
}
type OneHandler struct {
}
type TwoHandler struct {
    NestedTwoHandler *NestedTwoHandler
}
type NestedTwoHandler struct {
    NestedNestedTwoHandler *NestedNestedTwoHandler
}
type NestedNestedTwoHandler struct {
}

func main() {
    app := &App{
        OneHandler: new(OneHandler),
        TwoHandler: new(TwoHandler),
    }

    fmt.Println(*app)
    fmt.Println(*app.OneHandler)
    fmt.Println(*app.TwoHandler)
}

Its output is

{0x19583c 0x1040c128}
{}
{<nil>}

Why is NestedTwoHandler nil? I was expecting it to be {some_pointer_location} with NestedNestedTwoHandler being {}. How can I create an empty deep nested struct using new?

Answer 1

new(TwoHandler) is creating a new instance of struct TwoHandler. All its fields will have zero values. For a pointer type, this is nil, so that is what NestedTwoHandler will be unless you specify it.

new only zeroes memory, so if you want to initialize anything, you need to use something else such as a composite literal:

    TwoHandler: &TwoHandler{new(NestedTwoHandler)},

This creates a pointer to a TwoHandler struct with the only field set to a new NestedTwoHandler. Note that TwoHandler.NesterTwoHandler.NestedNestedTwoHandler will be nil as we're using new again, so it remains the zero value.

You can keep initializing fields using literals:

    TwoHandler: &TwoHandler{&NestedTwoHandler{new(NestedNestedTwoHandler)}}

You can read more details about allocating with new.