Golang problems with referencing arrays of sub structures

Question

I am having a problem figuring out how to reference elements of a sub structure.

See: http://play.golang.org/p/pamS_ZY01s

Given something like following.... How do you reference data in the room struct? I have tried fmt.Println(*n.Homes[0].Rooms[0].Size), but that does not work.

Begin Code example

package main

import (
    "fmt"
)

type Neighborhood struct {
    Name  string
    Homes *[]Home
}

type Home struct {
    Color string
    Rooms *[]Room
}

type Room struct {
    Size string
}

func main() {
    var n Neighborhood
    var h1 Home
    var r1 Room

    n.Name = "Mountain Village"
    h1.Color = "Blue"
    r1.Size = "200 sq feet"

    // Initiaize Array of Homes
    homeslice := make([]Home, 0)
    n.Homes = &homeslice

    roomslice := make([]Room, 0)
    h1.Rooms = &roomslice

    *h1.Rooms = append(*h1.Rooms, r1)
    *n.Homes = append(*n.Homes, h1)

    fmt.Println(n)
    fmt.Println(*n.Homes)

}

Toni Cárdenas · Accepted Answer · 2015-01-23 23:13:44Z

4

First, *[]Home is really wasteful. A slice is a three worded struct under the hood, one of them being itself a pointer to an array. You are introducing a double indirection there. This article on data structures in Go is very useful.

Now, because of this indirection, you need to put the dereference operator * in every pointer-to-slice expression. Like this:

fmt.Println((*(*n.Homes)[0].Rooms)[0].Size)

But, really, just take out the pointers.

edited Jan 23, 2015 at 23:13

answered Jan 23, 2015 at 22:30

Toni Cárdenas

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3 Comments

jordan2175 Over a year ago

Thanks, the reason for the pointer is because I will be marshal-ing the data in to JSON and that is how Andrew Gerrand from Google told me to do it so I do not have empty elements showing up in the JSON data.

Toni Cárdenas Over a year ago

And that's true for non-reference Go types, e. g. for mapping JSON objects to structs, or JSON numbers to ints. But slices themselves can be nil, represented as null. play.golang.org/p/psN_WIcCo4

jordan2175 Over a year ago

Great. I now understand Andrew's comments better... Thanks for all of your feedback. This really helps me.

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