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

257

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
5

answered Jul 17 '14 at 16:56

OneOfOne

95,033
20
184
185

7

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
2

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
2

The first proposal leads to name space collisions. Instead of "Proxy" you have to use "ConfigurationProxy" to avoid collisions. And if you do it correctly, you will notice that this gets more annoying with every nesting level. – ceving Mar 03 '23 at 11:54

score 173 · Answer 2 · answered Sep 07 '16 at 09:12

173

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

5,479
3
29
33

24

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

1,129
8
21

3

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

score 12 · Answer 4 · answered Jan 29 '15 at 14:52

12

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

125
1
5

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 11 · Answer 5 · answered Jan 18 '19 at 10:14

11

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 Pruis

1,073
8
9

Great example +1. For clarity you could add `fmt.Println(c.Val, c.Proxy.Address, c.Proxy.Port)`. – Sep 19 '20 at 02:24
I'd like to add using `new()` only makes sense if you want a pointer. Otherwise just do `var c Configuration`. – Ferdy Pruis Sep 01 '22 at 19:13

score 9 · Answer 6 · 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 7 · 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 1 · 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",
        },
    }

}

score 0 · Answer 10 · answered Jan 27 '23 at 15:58

When your configuration is something global, you can do it this way:

package main

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

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

How to initialize a nested struct?

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