How to initialize a nested struct?

Question

I cannot figure out how to initialize a nested struct. Find an example here: http://play.golang.org/p/NL6VXdHrjh

package main

type Configuration struct {
    Val   string
    Proxy struct {
        Address string
        Port    string
    }
}

func main() {

    c := &Configuration{
        Val: "test",
        Proxy: {
            Address: "addr",
            Port:    "80",
        }
    }

}

Just learning go and had exactly the same question. You can omit element types for arrays and maps but not for nested structs. Illogical and inconvenient. Can someone explain why? — Peter Dotchev, Mar 24 '18 at 06:16

score 230 · Accepted Answer · edited Dec 09 '19 at 04:36

230

Well, any specific reason to not make Proxy its own struct?

Anyway you have 2 options:

The proper way, simply move proxy to its own struct, for example:

type Configuration struct {
    Val string
    Proxy Proxy
}

type Proxy struct {
    Address string
    Port    string
}

func main() {

    c := &Configuration{
        Val: "test",
        Proxy: Proxy{
            Address: "addr",
            Port:    "port",
        },
    }
    fmt.Println(c)
    fmt.Println(c.Proxy.Address)
}

The less proper and ugly way but still works:

c := &Configuration{
    Val: "test",
    Proxy: struct {
        Address string
        Port    string
    }{
        Address: "addr",
        Port:    "80",
    },
}

edited Dec 09 '19 at 04:36

gwvandesteeg

15
4

answered Jul 17 '14 at 16:56

OneOfOne

88,915
19
172
173

5

In the second method, can we avoid the repetitive struct definition? – Gaurav Ojha Jan 30 '17 at 06:46
1

@GauravOjha not all the way, but something like https://play.golang.org/p/n24BD3NlIR – OneOfOne Jan 30 '17 at 15:07
I think using an embedded type is more apt for is-a relationship. – crackerplace Jul 15 '17 at 20:53
1

as pointed out below by @sepehr you can access the variables directly using the dot notation. – snassr Jan 19 '18 at 18:32
1

What if the proxy has a field with struct as type? How to initialize them inside another nested struct? – kucinghitam Dec 20 '19 at 10:50
@GauravOjha I think I found the more improper way to do it without defining any type. https://play.golang.org/p/_mHcB0pBtep – proximab Aug 15 '20 at 20:11

score 140 · Answer 2 · answered Sep 07 '16 at 09:12

140

If you don't want to go with separate struct definition for nested struct and you don't like second method suggested by @OneOfOne you can use this third method:

package main
import "fmt"
type Configuration struct {
    Val   string
    Proxy struct {
        Address string
        Port    string
    }
}

func main() {
    c := &Configuration{
        Val: "test",
    }

    c.Proxy.Address = `127.0.0.1`
    c.Proxy.Port = `8080`
}

You can check it here: https://play.golang.org/p/WoSYCxzCF2

answered Sep 07 '16 at 09:12

sepehr

4,989
3
27
33

19

Wow, an actual answer to the question of how to initialize **nested** structs. – Max Nov 17 '16 at 16:46
1

This is actually pretty good, but would've been better if we could do it in one line! – Gaurav Ojha Jan 30 '17 at 06:47
1

I was looking for a way where you wouldn't need to do `c.Proxy.Address = \`127.0.0.1\`` `c.Proxy.Port = \`8080\`` Is there a way to initialize those values during `&Configuration{}` assignment? – Matheus Felipe May 23 '17 at 18:18
1

@MatheusFelipe you can but then you have to define `Proxy` as its own struct, see first method in the answer by @OneOfOne – sepehr May 24 '17 at 02:30
1

What when Proxy is an array, `Proxy []struct{}` ? – Ishmeet Sep 27 '21 at 12:52
@Ishmeet both methods from the accepted answer work with slices. – sepehr Sep 29 '21 at 00:26
@sepehr is there an example with slices ? – Ishmeet Sep 29 '21 at 09:29
@Ishmeet [this is](https://go.dev/play/p/nMcDlUtBUmX) an example when Proxy is a slice. – sepehr Jan 11 '22 at 22:42

score 16 · Answer 3 · answered Jul 17 '14 at 16:55

16

Define your Proxy struct separately, outside of Configuration, like this:

type Proxy struct {
    Address string
    Port    string
}

type Configuration struct {
    Val string
    P   Proxy
}

c := &Configuration{
    Val: "test",
    P: Proxy{
        Address: "addr",
        Port:    "80",
    },
}

See http://play.golang.org/p/7PELCVsQIc

answered Jul 17 '14 at 16:55

Vitor De Mario

1,079
9
21

3

What if `P Proxy` is an array? – Ertuğrul Altınboğa Jun 23 '20 at 09:26

score 11 · Answer 4 · answered Jan 29 '15 at 14:52

11

You have this option also:

type Configuration struct {
        Val string
        Proxy
}

type Proxy struct {
        Address string
        Port    string
}

func main() {
        c := &Configuration{"test", Proxy{"addr", "port"}}
        fmt.Println(c)
}

answered Jan 29 '15 at 14:52

Jose

117
1
4

Yes or the same with the second type of initializers [field:value](https://play.golang.org/p/tLmJKBEm-Vs) – Pierrick HYMBERT Feb 27 '18 at 14:52
What if `Proxy` is an array? – Ertuğrul Altınboğa Jun 23 '20 at 09:27
@ErtuğrulAltınboğa in this case you can do an `append(pxyArr, Proxy{"addr", "port"})` – Shadoweb Jul 27 '21 at 16:51

score 9 · Answer 5 · answered Jan 23 '16 at 14:04

One gotcha arises when you want to instantiate a public type defined in an external package and that type embeds other types that are private.

Example:

package animals

type otherProps{
  Name string
  Width int
}

type Duck{
  Weight int
  otherProps
}

How do you instantiate a Duck in your own program? Here's the best I could come up with:

package main

import "github.com/someone/animals"

func main(){
  var duck animals.Duck
  // Can't instantiate a duck with something.Duck{Weight: 2, Name: "Henry"} because `Name` is part of the private type `otherProps`
  duck.Weight = 2
  duck.Width = 30
  duck.Name = "Henry"
}

For those how forgot like me, name your struct attributes with capital letters otherwise, you will face `cannot refer to unexported field or method ` error. — tagaism, Jun 17 '20 at 17:45

score 9 · Answer 6 · answered Jan 18 '19 at 10:14

9

You also could allocate using new and initialize all fields by hand

package main

type Configuration struct {
    Val   string
    Proxy struct {
        Address string
        Port    string
    }
}

func main() {
    c := new(Configuration)
    c.Val = "test"
    c.Proxy.Address = "addr"
    c.Proxy.Port = "80"
}

See in playground: https://play.golang.org/p/sFH_-HawO_M

answered Jan 18 '19 at 10:14

Ferdy

846
6
7

Great example +1. For clarity you could add `fmt.Println(c.Val, c.Proxy.Address, c.Proxy.Port)`. – Sep 19 '20 at 02:24

score 6 · Answer 7 · answered Jun 06 '18 at 07:09

You need to redefine the unnamed struct during &Configuration{}

package main

import "fmt"

type Configuration struct {
    Val   string
    Proxy struct {
        Address string
        Port    string
    }
}

func main() {

    c := &Configuration{
        Val: "test",
        Proxy: struct {
            Address string
            Port    string
        }{
            Address: "127.0.0.1",
            Port:    "8080",
        },
    }
    fmt.Println(c)
}

https://play.golang.org/p/Fv5QYylFGAY

score 2 · Answer 8 · answered May 16 '18 at 07:07

You can define a struct and create its object in another struct like i have done below:

package main

import "fmt"

type Address struct {
    streetNumber int
    streetName   string
    zipCode      int
}

type Person struct {
    name    string
    age     int
    address Address
}

func main() {
    var p Person
    p.name = "Vipin"
    p.age = 30
    p.address = Address{
        streetName:   "Krishna Pura",
        streetNumber: 14,
        zipCode:      475110,
    }
    fmt.Println("Name: ", p.name)
    fmt.Println("Age: ", p.age)
    fmt.Println("StreetName: ", p.address.streetName)
    fmt.Println("StreeNumber: ", p.address.streetNumber)
}

Hope it helped you :)

score 0 · Answer 9 · answered Oct 09 '20 at 07:39

package main

type    Proxy struct {
        Address string
        Port    string
    }

type Configuration struct {
    Proxy
    Val   string

}

func main() {

    c := &Configuration{
        Val: "test",
        Proxy: Proxy {
            Address: "addr",
            Port:    "80",
        },
    }

}

How to initialize a nested struct?

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